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Feasibility study of using Cross-Laminated Timber core for the UBC Tall Wood Building Moudgil, Manu 2017

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FEASIBILITY STUDY OF USING CROSS-LAMINATED TIMBER CORE  FOR THE UBC TALL WOOD BUILDING by  Manu Moudgil   Bachelor of Technology, Punjab Technical University, 2014   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF   MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Civil Engineering)   THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   August 2017  © Manu Moudgil, 2017 ii Abstract Mass-timber has gained popularity in the construction of mid-rise buildings in the last decade. The innovation of constructing tall buildings with mass-timber can be seen in the student residence at Brock Commons built in 2016 at the University of British Columbia. It is the world’s tallest timber hybrid building with 18 stories and 53 meters’ height above the ground level. The building has 17 stories of mass-timber superstructure resting on a concrete podium with two concrete cores that act as a lateral force resisting system for earthquake and wind forces. The mass-timber superstructure of 17 stories took ten weeks whereas the concrete cores were built in fourteen weeks. There could have been a substantial reduction in the project timeline leading to cost savings, if mass-timber was used for the cores. The motivation for concrete cores was driven by the sole purpose of easier approval procedure.  The objective of this thesis was to evaluate the possibility to design the Brock Commons building using mass-timber cores. First, the procedure for the approvals for tall timber buildings by understanding the code compliance for Brock Commons is discussed. Then, the actual building with concrete cores is modeled, with the model being calibrated with the results from the structural engineers of record. These concrete cores are then replaced by the same configuration using Cross Laminated Timber (CLT) cores to investigate the structural feasibility of Brock Commons with a mass-timber core. The results presented herein show that Brock Commons with CLT core having the same dimensions and configuration is unstable under seismic loading for Vancouver, BC, as specified by National Building of Canada 2015. However, when the configuration and thickness of CLT cores are changed, the structure can meet the seismic performance criteria as per the code.   iii Lay Summary Mass-timber is a suitable material for construction of tall buildings, owing to its aesthetic, environmental and structural benefits over conventional materials. Cross Laminated Timber (CLT) is formed by gluing three or more lumber layers crosswise. Brock Commons at the University of British Columbia is currently the tallest timber hybrid building in the world. The building utilizes a concrete ground floor to resist vertical loads from the mass-timber superstructure and two concrete elevator cores to resist seismic and wind forces. This thesis discusses the approval procedure for Brock Commons and presents a numerical model of the building to investigate the viability of using CLT cores in place of the concrete cores.  iv Preface The material from chapters 3 and 4 of this thesis was presented at following conferences: Tannert, T., Moudgil, M. (2017) “Structural Design, Approval and Monitoring of UBC Tall Wood Building” Proceedings of ASCE Conference 2017, Denver, CO, USA, 1-4 April (7 Pages) Poirier, E., Moudgil, M., Fallahi, A., Staub-French, S., Tannert, T., (2016) “Design and Construction of a 53-meter-Tall Timber Building at University of British Columbia” Proceedings of World Conference on Timber Engineering, Vienna, Austria, 22-25 August (10 pages) Chapters 4 and 5 of this thesis are developed by the author of this thesis under the direct supervision of Dr. Thomas Tannert. The author of this thesis is accountable for the literature review, structural modeling, analyzing and inferring the results.  v Table of Contents Abstract .......................................................................................................................................... ii Lay Summary ............................................................................................................................... iii Preface ........................................................................................................................................... iv Table of Contents ...........................................................................................................................v List of Tables ................................................................................................................................ ix List of Figures .................................................................................................................................x Acknowledgements .................................................................................................................... xiii Dedication ................................................................................................................................... xiv Chapter 1: Introduction ................................................................................................................1 1.1 UBC Tall Wood Building ............................................................................................... 1 1.2 Objectives ....................................................................................................................... 4 1.3 Research Methodology ................................................................................................... 5 1.4 Scope ............................................................................................................................... 6 1.5 Thesis Outline ................................................................................................................. 6 Chapter 2: Literature Review .......................................................................................................7 2.1 Rationale for using Wood in Construction ..................................................................... 7 2.1.1 Engineered Wood Products......................................................................................... 7 2.1.2 Population Increase ..................................................................................................... 8 2.1.3 Sustainability............................................................................................................... 9 2.1.4 Sustainably Managed Forests ................................................................................... 10 2.1.5 Fire Resistance .......................................................................................................... 11 vi 2.2 History of Tall Wood Buildings ................................................................................... 12 2.2.1 Early Use of Wood as Construction Material ........................................................... 12 2.2.2 Tall Wood Buildings before the 19th Century ........................................................... 12 2.2.3 Tall Timber Buildings in Canada in the early 20th Century ..................................... 14 2.2.4 Modern Mid-Rise Hybrid Wood Buildings .............................................................. 15 2.2.5 Mass-timber Buildings at the University of British Columbia ................................. 18 2.3 Future Tall Timber and Hybrid Buildings .................................................................... 21 2.3.1 North American Tall Wood Buildings ...................................................................... 22 2.3.2 European Tall Wood Buildings ................................................................................ 24 2.4 Proposed Concepts for Tall Timber Buildings ............................................................. 26 2.4.1 Creative Resource and Energy Efficiency (CREE) .................................................. 27 2.4.2 The FFTT System ..................................................................................................... 28 2.4.3 SOM Timber Tower Project ..................................................................................... 29 2.4.4 Wooden Empire State Building ................................................................................ 30 2.5 Literature Review Conclusion ...................................................................................... 31 Chapter 3: Regulatory Framework of the UBC TWB .............................................................32 3.1 History of Canadian Code Framework ......................................................................... 32 3.2 2005 NBCC- An Objective-Based Building Code ....................................................... 33 3.3 British Columbia’s Regulatory Framework .................................................................. 35 3.3.1 Overview ................................................................................................................... 35 3.3.2 Alternative Solutions in BCBC 2012 ........................................................................ 36 3.4 Site Specific Regulation for UBC Brock Commons..................................................... 37 3.4.1 Overview ................................................................................................................... 37 3.4.2 Authorities Having Jurisdiction (AHJ) ..................................................................... 38 vii 3.4.3 Site Specific Regulation ............................................................................................ 39 3.4.4 Acceptable Solutions in SSR .................................................................................... 41 3.4.5 Alternate Solutions in SSR ....................................................................................... 42 3.4.6 Discussion about TWB approval process ................................................................. 46 3.5 Future Mass-timber Building in Canadian Code .......................................................... 47 Chapter 4: Structural System of UBC Tall Wood Building ....................................................49 4.1 Overview ....................................................................................................................... 49 4.2 Structural Materials ....................................................................................................... 50 4.2.1 Concrete .................................................................................................................... 50 4.2.2 Mass timber ............................................................................................................... 53 4.3 Connections................................................................................................................... 55 4.3.1 Wood to Wood Connection ...................................................................................... 55 4.3.2 Wood to Steel Connections ....................................................................................... 59 4.3.3 Steel to Concrete Connections .................................................................................. 60 4.4 Discussion on TWB design ........................................................................................... 61 Chapter 5: Numerical Model ......................................................................................................62 5.1 Overview ....................................................................................................................... 62 5.2 Response Spectrum Analysis (RSA) ............................................................................ 65 5.3 Material Properties ........................................................................................................ 68 5.4 Loads & Load Combinations ........................................................................................ 70 5.4.1 Gravity Loads............................................................................................................ 70 5.4.2 Lateral Loads ............................................................................................................ 71 5.4.3 Load Combinations ................................................................................................... 72 5.5 Results ........................................................................................................................... 72 viii 5.5.1 Modal Results ........................................................................................................... 73 5.5.2 Inter-storey Drift ....................................................................................................... 79 5.5.3 Base Shear ................................................................................................................. 80 5.6 Discussion on proposed changes to TWB .................................................................... 81 5.7 Further benefits of using CLT cores in TWB ............................................................... 82 Chapter 6: Conclusions ...............................................................................................................84 Bibliography .................................................................................................................................85 ix List of Tables Table 2-1: Fire Resistance Rating of CLT .................................................................................... 12 Table 2-2: Modern Tall Wood Buildings Globally[24] ................................................................ 15 Table 2-3: Upcoming Tall Wood Buildings Globally [24] ........................................................... 21 Table 3-1: Construction classification in NBCC 1941 [46].......................................................... 32 Table 3-2: List of Expert Panelists for UBC TWB SSR ............................................................... 40 Table 3-3: Alternative Solutions proposed ................................................................................... 43 Table 4-1: Concrete Components of Brock Commons ................................................................. 52 Table 4-2: Use of Engineered Timber in Brock Commons .......................................................... 54 Table 4-3: Drag Strap Properties .................................................................................................. 60 Table 5-1: Properties for GLT and PSL (Source: CSA-086 (2014) and Weyerhaeuser PSL Guide)....................................................................................................................................................... 68 Table 5-2: Modelling Parameters for CLT (Structurlam Crosslam Guide) .................................. 70 Table 5-3: Load distribution in 3D model .................................................................................... 71 Table 5-4: Load Combinations in NBCC 2010 ............................................................................ 72 Table 5-5: Calibrated Modal Results ............................................................................................ 74 Table 5-6: Comparative Results of three models .......................................................................... 74 Table 5-7: Seismic Weight............................................................................................................ 81 Table 5-8: Comparative Base shear .............................................................................................. 81   x List of Figures Figure 1-1: UBC Tall Wood Building (Credits- Aston Ostry Architects) ...................................... 1 Figure 1-2: Construction Schedule for Concrete: (1) 4th January 2016; (2) 8th March 2016; (3) 18th April 2016; (4) 30th May 2016 (Source: Urban One Builders) .............................................. 3 Figure 1-3: Construction Schedule for Mass-timber and envelope: (1) 6th June 2016; (2) 28th June 2016; (3) 18th July 2016; (4) 10th August 2016 (Source: Urban One Builders) ........................... 4 Figure 1-4: Research Methodology ................................................................................................ 5 Figure 2-1: Manufacturing of CLT panels (Source: Structure Magazine) ..................................... 8 Figure 2-2: Population Increase in Canada (Adapted from Statistics Canada [10]) ....................... 9 Figure 2-3: Advantages of Wood over Concrete & Steel [13] ..................................................... 10 Figure 2-4: Sustainably managed forests across the world [15] ................................................... 11 Figure 2-5: Ancient Pagodas: (1) Yingxian Pagoda [19]; (2) Horyu-Ji Pagoda [20] ................... 13 Figure 2-6: Timber Buildings in Vancouver [22]: (1) The Landing; (2) The Leckie ................... 14 Figure 2-7: Modern Tall Wood Buildings Globally: (1) LCT One (Credits: Architekten Hermann Kaufmann); (2) WIDC (Credits: MGA) ;(3)Forte (Credits: Lend Lease); (4) Stadthaus (Credits: Waugh Thistleton Architects ); (5) TREET (Credits: SWESCO); (6)3-D model of TREET [28] 17 Figure 2-8: Mass-timber in UBC: (1) ESB at UBC (Picture Courtesy- Perkins+Will); (2) SAIL (Picture Courtesy- Adera); (3) BRDF (Picture Courtesy-www.projectservices.ubc.ca); (4) New Sub (Picture Courtesy-www.wesbridge.com); (5) Transit Shelter (Picture Courtesy-publicdesign.ca); (6) UBCO F&WC (Picture Courtesy-www.kelownadailycourier.ca) .............. 20 Figure 2-9: Future Tall Wood Buildings in North America: (1) 475 West 18th, New York, US (Courtesy: SHoP Architects); (2) Framework, Portland, US (Credits: Lever Architecture); (3) Origine, Montreal, Canada (Courtesy: Nordic Structures); (4) Arbora, Quebec City, Canada (Courtesy: Nordic Structures) ....................................................................................................... 23 xi Figure 2-10: Future Tall Timber Buildings in Europe: (1) Hyperion, Bordeaux, France (Courtesy: Jean-Paul Viguier et Associes); (2) Hoho, Vienna, Austria (Courtesy: Rüdiger Lainer + Partner Architekten); (3) HAUT, Amsterdam, Netherland (Courtesy: Team V Architecture) ................. 25 Figure 2-11: Proposed Buildings: Baobab, Paris, France (Courtesy: MGA); Wooden Skyscraper, Stockholm, Sweden (Courtesy: C.F. Møller Architects); Four-20 Storey Wooden Apartment Building, Stockholm, Sweden (Courtesy: Tham & Videgård Architects) .................................... 26 Figure 2-12: CREE Timber Concrete Composite System: (1) Leverage for Building services; (2) Structural solution (Credits: CREE by Rhomberg) ...................................................................... 27 Figure 2-13: Options for FFTT system (Source: The Case of Tall Wood) .................................. 28 Figure 2-14: SOM Timber Tower Project [52] ............................................................................. 29 Figure 2-15: Conceptual Design of Empire State Building in Wood [53] ................................... 30 Figure 3-1: Tyco Window Sprinklers[69]..................................................................................... 44 Figure 4-1: Hybrid Configuration of Brock Commons (Source: Fast+Epp) ................................ 49 Figure 4-2: Concrete elements in TWB (Source: Fast+Epp) ........................................................ 51 Figure 4-3: Foundations in TWB (Source: Fast+Epp) .................................................................. 51 Figure 4-4: CLT panel arrangement in Levels 2-18 (Source: Naturally Wood) ........................... 53 Figure 4-5: Typical CLT slab (Adapted from GHL Consultants) ................................................ 54 Figure 4-6: Column Configuration: (1) Standalone Column; (2) Column within Partition walls 55 Figure 4-7: CLT to CLT panel connection: (1) Panel to Panel (Adapted from CLT handbook); (2) Brock Commons CLT to CLT (Source-UrbanOne) ..................................................................... 56 Figure 4-8: Connection for Wood Column with Transfer Slab: (1) Transfer Slab to Column (Credits-CadMakers); (2) Site picture transfer slab to column (Source-UrbanOne) .................... 56 Figure 4-9: Column to Column with CLT panel connection (Source: Aston Ostry Architects) .. 58 Figure 4-10: Cotter Pin in HSS (Source: Aston Ostry Architects) ............................................... 58 Figure 4-11: Drag Strap and Chord in Brock Commons (Source: UrbanOne) ............................. 59 Figure 4-12: Drag Strap Properties Description ........................................................................... 60 xii Figure 4-13: Connections with concrete (Adapted from Fast+Epp) ............................................. 61 Figure 5-1: Plan view of proposed system .................................................................................... 62 Figure 5-2: Methodology for Research ......................................................................................... 63 Figure 5-3: 3D view of Base Model with concrete cores in ETABS............................................ 64 Figure 5-4: UHS for Vancouver NBCC 2015 ............................................................................... 66 Figure 5-5: Concrete Cores: (1) in ETABS; (2) in Brock Commons (Source: UrbanOne) .......... 69 Figure 5-6: Typical floor loading pattern from Storey 2-18 ......................................................... 71 Figure 5-7: Modal shapes for concrete model: (1) Mode 1=1.99s; (2) Mode 2= 1.85s; (3) Mode 3=1.32s .......................................................................................................................................... 75 Figure 5-8: Modal shapes for CLT model: (1) Mode 1=2.13s; (2) Mode 2= 1.87; (3) Mode 3=1.76....................................................................................................................................................... 76 Figure 5-9: Modal shapes for refined model: (1) Mode 1=1.85s; (2) Mode 2= 1.71; (3) Mode 3=1.50 ........................................................................................................................................... 76 Figure 5-10: Mass Participation in X-Direction ........................................................................... 77 Figure 5-11: Mass Participation in Y-Direction ........................................................................... 78 Figure 5-12: Mass Participation in Z-Direction ............................................................................ 78 Figure 5-13: Drift Percentage in X-Axis ...................................................................................... 79 Figure 5-14: Drift Percentage in Y-Axis ...................................................................................... 80 Figure 5-15: Potential Environmental Benefit from proposed system ......................................... 83   xiii Acknowledgements Firstly, I would like to express my deep gratitude to my supervisor, Dr. Thomas Tannert, for allowing to me to work on this research project. The endless encouragement and support have aided me to develop my research skills. I would also like to acknowledge Dr. Sherly-Staub-French as the second reader of this thesis. Secondly, I would like to thank my fellow research group members, especially Cristiano, Kuldeep, Hercend, and Shahnewaz, for sharing their knowledge. I would also like to thank Gurvinder, a fellow masters’ student, for being my writing companion, a source of energy and Mohammed for helping me start off with new software.  Furthermore, I would like to thank Robert Jackson from Fast+Epp structural engineers for discussing the results of the actual structural model of the building. Also, thanks to GHL Consultants and Urban One for their transparency for sharing the building plans and reports. I am thankful to my Vancouver housemates, Saravanan and Gursimran, for providing a home not just a place. Special thanks to Saravanan for his delicious cooking recipes, which eventually motivated me to learn cooking. I would like to thank Tejinder and Archana Dhami for providing a home away from home. My great thanks to Chirashu, who supported me with love, inspiration, and encouragement during my entire adulthood and childhood. Finally, and most importantly, I am extremely grateful to my father, Kamal Moudgil, and my mother, Shashi Moudgil, for their unconditional love, assistance, and encouragement. I would like to thank my brother, Yogi, who has always been my long-distance support system. Also, my beloved friend from home, Vipul for his constant backing and support. xiv Dedication  To my beloved father and mother   1 Chapter 1: Introduction 1.1 UBC Tall Wood Building Brock Commons, also known as the University of British Columbia’s (UBC) Tall Wood Building (TWB), is an 18-storey student residence building as shown in Figure 1-1. It houses 404 beds for upper year undergraduate and graduate students. TWB is a 53m high hybrid building with a concrete podium and two concrete cores for elevator and staircases. The superstructure is made from combustible engineered mass timber. TWB is a vital fragment of the future Brock Commons mixed-use hub by providing beds and amenity spaces for students.  Figure 1-1: UBC Tall Wood Building (Credits- Aston Ostry Architects) 2 Brock Commons is one of the structures chosen among three tall wood buildings in Canada for the Tall Wood demonstration project initiated in 2013 and is funded by Natural Resources Canada (NRCan) [1]. The design and pre-construction phase began in early 2015 and spanned over nine months. The construction for TWB started in November 2015, and the structure including the envelope was completed in mid-August 2016 [2]. The floor slabs in the superstructure utilize prefabricated Cross Laminated Timber (CLT) panels with a non-structural concrete topping. Most columns are made of Glued-Laminated Timber (GLT) columns whereas columns made of Parallel Strand Lumber (PSL) is used at lower levels. This mass-timber superstructure rests on a cast-in place reinforced concrete transfer slab which transfers the gravity loads to foundations through concrete columns in the first floor.  The concrete cores in TWB act as a lateral force resisting system against seismic and wind forces. As the decision to work with concrete cores in TWB was taken in the early stages of the design process by stakeholders. The use of mass-timber in the lateral resisting system was ruled out, owing to the regulatory guidelines. However, the construction process of TWB displayed a significant difference in the timeline for concrete cores and 17 stories of mass-timber superstructure. The concrete cores were cast-in place reinforced concrete and were built earlier, which was then wrapped around by the mass-timber superstructure and pre-fabricated envelope panels. The construction process of concrete cores spanned over more than 12 weeks whereas the 17 stories of superstructure including the envelope took just ten weeks as shown in Figure 1-2 and Figure 1-3 respectively. The construction of TWB with mass-timber cores could have undoubtedly decreased the schedule leading to cost savings and environmental benefits. This notion motivated the author to investigate the possibility of the viability of mass-timber cores in place of concrete cores. 3  Figure 1-2: Construction Schedule for Concrete: (1) 4th January 2016; (2) 8th March 2016; (3) 18th April 2016; (4) 30th May 2016 (Source: Urban One Builders) 4  Figure 1-3: Construction Schedule for Mass-timber and envelope: (1) 6th June 2016; (2) 28th June 2016; (3) 18th July 2016; (4) 10th August 2016 (Source: Urban One Builders) 1.2 Objectives  The first research objective is to study the regulatory and approval process of alternate solutions for tall timber hybrid buildings at hand of the approval procedure of TWB. The second and main objective of this research is to determine the structural feasibility of TWB with CLT core in place of the concrete core from a seismic perspective in Vancouver.   5 1.3 Research Methodology The author documented the design and pre-construction phase of TWB to explore the challenges. The project stakeholders were interviewed and the obstacles faced were documented. One of the core aspects according to all stakeholders was to get the building permit [3]. Thus, the approval procedure of TWB is explained before investigating structural alternatives with CLT cores. The numerical part is carried out by developing a Finite Element Model (FEM) for the original building with concrete cores and mass-timber superstructure. The model is developed using the actual floor plans and materials. A linear dynamic analysis is performed on this base model, and the model is validated by calibrating the modal results with the structural engineers working on the project.  Subsequently, the core material is changed to investigate the seismic performance of CLT cores with the same configuration and dimensions. However, the results show that TWB with such a CLT doesn’t meet the seismic performance requirements. Hence, a modified system is proposed with CLT core and additional L-shaped perimeter shear walls with increased thickness. This refined system performs well according to the design codes in Vancouver. The research methodology is summarized in Figure 1-4.  Figure 1-4: Research Methodology 6 1.4 Scope The research encompasses the performance of the lateral system without considering any change in the gravity resisting system, i.e., the floor slabs. Hence, the modified structural system is not analyzed for gravity loads. The proposed model captures only linear behavior and is analyzed for a linear dynamic analysis in the form of Response Spectrum Analysis. The analysis captures the ultimate limit state and does not assess the effects of the dynamic wind forces for the serviceability state. Hence, the models developed are checked for peak design loads, which could have caused the collapse of the structure but not for the long-term effects of the loadings. Also, the nonlinear behavior of the structural elements and connections for the mass-timber elements including the CLT core is outside of the scope of the thesis.  1.5 Thesis Outline The thesis contains six chapters. Chapter 2 of this thesis provides the literature review for using mass-timber for tall timber buildings and shows some of the tall timber hybrid structures around the world. It also includes some of the upcoming tall timber buildings and other proposed concepts related to tall timber buildings. Chapter 3 provides an overview of the regulatory context of tall wood buildings by going through a Site-Specific Regulation with the structural system opted for the Brock Commons. Chapter 4 describes the structural details of TWB with a detailed discussion of the connections and the structural material used for the construction. Chapter 5 summarizes the results from a numerical model for TWB for the concrete benchmark and then compares the results the CLT core solution. Also, the results from a viable CLT core solution is discussed in terms of the design performance requirements. Lastly, Chapter 6 concludes the thesis with the summary of the findings and further recommendations for the proposed solution. 7 Chapter 2: Literature Review 2.1 Rationale for using Wood in Construction  Unlike steel or concrete, wood is a natural and renewable material with low carbon footprint. Some factors which favor the use of mass-timber in tall buildings are discussed in the following. 2.1.1 Engineered Wood Products Mass-timber products are different from traditional sawn lumber. Mass-timber panels made of Cross-laminated timber (CLT) can have a length more than 16m and can be prefabricated easily [4]. The strength and stiffness properties of mass-timber per unit mass are comparable to steel and concrete. This lower weight of timber results in lighter structures, thus reducing the seismic demands. For instance, Bridport House in the UK had required low weight due to the site sensitive circumstances [5]. The usage of mass-timber overcame the hindrance. CLT is manufactured by stacking layers of lumber cross-wise and then gluing them together as shown in Figure 2-1. The individual layers generally vary from 10mm to 80mm thickness, thus providing panel thickness up to 500mm. It is not mandatory to have individual thicknesses same in a panel. However, the layer thickness of a panel is driven by the cost. The length and width can be maximum up to 16.50m and 4m respectively, which however depends on the manufacturer’s tools and machines [6]. Timber, owing to its anisotropic nature, exhibits different strengths when loaded in different orientations. When loaded perpendicular to the grain, it is weaker than when loaded in parallel to grain direction. The cross stacking of lumber in CLT panels overcomes this weakness of timber by acting as two-way span as in the case of concrete, providing enough strength and stability [7]. 8  Figure 2-1: Manufacturing of CLT panels (Source: Structure Magazine) Glue-laminated timber (GLT) is manufactured by gluing layers of wood laminations using durable adhesives. It can be widely used in beams, rafters, trusses, columns and other engineering applications. Parallel Strand Lumber (PSL) is formed by gluing the wood strands together under high pressure [8]. It can be used as columns or beams as well as headers and lintel beams. 2.1.2 Population Increase According to United Nations Department of Economic and Social Affairs/Population Division, the world’s population was recorded 7.3 billion in mid-2015. This number is expected to increase at a rate of 1.2 % (83 million) annually [9]. Also, from a Canadian context, Canada’s population on July 1, 2015, was estimated to be 35.8 million with an increase of 0.9% from the previous year. This increase is largest among G8 countries [10]. According to the world fact book, 82% of Canada’s population lives in urban areas, and urbanization in Canada is increasing at a rate of 1.2% annually [11]. Figure 2-2 shows the population growth in Canada during the last century. This increasing trend of urbanization is accompanied by the construction of tall buildings. Building tall with a renewable material like wood is the most sustainable approach to meet the housing needs of escalating population. 9  Figure 2-2: Population Increase in Canada (Adapted from Statistics Canada [10]) 2.1.3 Sustainability The natural potential of wood to capture and store carbon-dioxide during its whole lifecycle is known as carbon sequestration. This characteristic of wood makes it a sustainable construction material compared to other conventional building materials, namely concrete and steel. In the United States, buildings contribute approximately 47% of greenhouse gas emissions which is even more than 33% of transportation greenhouse gas emissions [12]. Figure 2-3 shows the advantages of wood over concrete and steel with lower energy usage, lower greenhouse gas emission, and reduced air pollution, as well as lower solid waste creation and environmental resource impact. CLT or GLT can sequester approximately 1640kg CO2 per ton. These benefits of wood over other construction materials make it a prudent choice for multi-storey commercial and residential buildings. The increased trend of green building ratings worldwide has favored the use of mass-timber to upsurge energy efficiency and building performance. 10  Figure 2-3: Advantages of Wood over Concrete & Steel [13] 2.1.4 Sustainably Managed Forests According to Canada’s forest annual report 2014, Canada has 348-million-hectare forest land. It represents 9% of world’s forests and contains almost 47 billion m3 of wood. In 2012, about 0.3% (148 million m3) of total wood was harvested whereas a much higher percentage was destroyed by forest fires [14]. The percentage of deforestation in Canada remained zero for 20 years. Deforestation can be understood as the conversion of forest land to non-forest land for agricultural, industrial or residential purposes.  Major third parties in Canada which sustainably manage the forests operations are the Canadian Standard Association Sustainable Forest Management (CSA SFM), the Forest Stewardship Council (FSC) and the Sustainable Forestry Initiative (SFI). If a tree is cut in a sustainably managed forest, other trees of same species are planted right away. Figure 2-4 shows the graphical representation of certified sustainable forests across the world. It is evident that Canada has the maximum quantity of sustainably managed forests [15]. With this natural treasure of managed 11 forests, it is judicious for the Canadian construction industry to use timber as the primary building material in mid-rise and high-rise buildings.  Figure 2-4: Sustainably managed forests across the world [15] 2.1.5 Fire Resistance Fire is one of the main concerns when using timber as construction material. Mass timber, however, behaves differently than small-dimensional lumber when exposed to fire. Mass-timber can be protected from fire using Encapsulation or Charring [16]. In Encapsulation, mass-timber is covered by protective coverings. Gypsum boards are usually used to add the fire resistance in this method. Whereas in Charring, the structural elements are provided with an added thickness called a char layer to encounter the fire. This added thickness acts as a sacrificial layer and thus saving the structural integrity of the element by burning itself. Small scale, as well as full-scale fire tests, have been performed on CLT panels at FPInnovations [17]. A Fire Resistive Rating 12 (FRR) up to 2 hours, as required by most codes, was achieved using encapsulation for 3-ply CLT whereas an FRR of 3 hours was obtained for 7-ply CLT using charring. Some of the significant findings of testing the fire resistance of these assemblies are summarized in Table 2-1. Table 2-1: Fire Resistance Rating of CLT CLT Configuration Thickness (mm) Fire Resistance (Minutes) CLT Gypsum Board Wall 3-Ply+2 GB 114 2*12.7 106 5-Ply 105 - 57 5-Ply 175 - 113 Floor 3-Ply+1 GB 105 1*15.9 86 5-Ply 175 - 96 5-Ply+1 GB 175 1*15.9 124 7-Ply 245 - 178  2.2 History of Tall Wood Buildings 2.2.1 Early Use of Wood as Construction Material  Timber was the only naturally available building material available at hand for construction of houses before the Stone Age. The use of wood in construction has evolved from log construction to wood-frame construction, and more lately to mass-timber construction. Wood is a durable building material. The use of timber as supports or roof beams can be traced back to the 7th century in Greece [18]. Innovation and industrial advancements in wood cutting tools improved the use of wood products.  2.2.2 Tall Wood Buildings before the 19th Century Japan and China have ancient tall wood temples which are taller than contemporary buildings. Some of these temples or pagodas are about a millennium old. Yingxian Pagoda (Figure 2-5-1) in 13 China was built in 1056 A.D. with a 67m height. It is erected in a post and beam structure to take advantage of compression strength of wood [19]. Horyu-Ji Pagoda (Figure 2-5-2) in Japan was built in 711 A.D. with a 32.5m height. It has performed well in a high-seismic zone mainly because of its unconnected floors which allow independent movement of respective floors [20].   Figure 2-5: Ancient Pagodas: (1) Yingxian Pagoda [19]; (2) Horyu-Ji Pagoda [20] Barsana Monastery in Romania with a height of 56m was built in the 1700s, and an ancient church named Urnes Stavkirke, constructed in 1130 in Norway are more examples of tall ancient timber structures that are still standing [13]. While these pagodas do not qualify as tall timber buildings since the Council on Tall Buildings and Urban Habitat (CTBUH) [21] defines a “tall timber building” when 50% of the height of the structure is occupied by usable floor area. However, the lifespan of these pagodas and churches epitomizes the durability of wood in tall structures. 14 2.2.3 Tall Timber Buildings in Canada in the early 20th Century  The use of brick and wood post and beam construction was prevalent in Canada in beginning of the 20th century. A total of 129 buildings were built using this method in Toronto. Out of these, around 62 buildings are more than five stories tall. In Vancouver, there are around 50 buildings with timber as their main structural component out of which six buildings are taller than seven stories [22]. A combined total of 25 wood buildings, which are 7-9 stories can be found in Vancouver and Toronto. The Landing (Figure 2-6-1) in Vancouver, B.C, Canada was constructed in 1905. It is a brick and beam building with eight stories with a floor area of 16,000m2. Leckie Building (Figure 2-6-2) was built in 1908 as a seven stories warehouse with unreinforced masonry on the exterior and exposed heavy timber interior. However, these buildings underwent seismic retrofitting and renovation throughout their service life. These buildings are used as a mixed-use, such that they have commercial retail stores on the ground floor with housing units on the upper floors.  Figure 2-6: Timber Buildings in Vancouver [22]: (1) The Landing; (2) The Leckie 15 2.2.4 Modern Mid-Rise Hybrid Wood Buildings A tall timber building as per CTBUH [21] must have vertical and lateral structural elements made up of timber while connections can be of steel or any other material. It means a building is a timber building only if 85% of structural material comprises of timber [23]. Buildings utilizing a concrete core as Lateral Load Resisting System (LFRS) or steel beams for added ductility are considered a hybrid building. Most of the modern buildings built across the world till 2015 and listed in Table 2-2 are hybrid buildings. Europe, as an early adopter of mass timber, specifically CLT, houses most of these mid-rise timber hybrid buildings. Some of these hybrid buildings are briefly explained with their structural system and main features. Table 2-2: Modern Tall Wood Buildings Globally[24] Name Location Year Stories Limnologen Växjö, Sweden 2009 8 Stadthaus London, U.K. 2010 8 Bridport House London, U.K. 2010 8 Holz8 Bad Aibling, Germany 2011 8 E-3 Berlin, Germany 2011 7 Forte Melbourne, Australia 2012 10 Lifecycle Tower One Dornbirn, Austria 2012 8 Pentagon II Oslo, Norway 2013 8 Wagramerstrasse Vienna, Austria 2013 7 Cenni di Cambiamento Milan, Italy 2013 9 Maison de I’Inde Paris, France 2013 7 Panorama Glustinelli Triste, Italy 2013 7 TREET Bergen, Norway 2015 14 Strandparken Stockholm, Sweden 2014 8 WIDC B.C, Canada 2014 8 Contrailminada Liedo, Spain 2014 8 St. Die-des-Vosges St. Die-des-Vosges, France 2014 8 Trafalgar Place London, U.K. 2015 10 Banyan Wharf London, U.K. 2015 10 Dalston Lane London, U.K. 2015 10 Shoreditch London, U.K. 2015 10 16 The Life Cycle Tower ONE (LCT ONE), in Figure 2-7-1, is eight stories commercial office tower. It is a hybrid timber construction with concrete foundations and a concrete central core with GLT columns and hybrid slabs. These hybrid slabs span up to 9m. Most of the assembly was prefabricated so that the building envelope was erected at the rate of one storey a day. Also, the building meets the passive house standard [25].  Currently, the tallest pure timber building in Canada is the 29.3m tall Wood Innovation and Design Center (WIDC) (Figure 2-7-2) in Prince George, BC. It consists of six stories including a mezzanine. The structural concept was the dry construction which eliminated the use of concrete above the foundation level except for the mechanical penthouse. Diverse wood species and mass-timber products are utilized in the WIDC for structural and aesthetic purposes [26]. CLT floor panels are combined with GLTcolumns. The exterior cladding is made up of charred Western Red Cedar siding. CLT floors (3-ply and 5-ply) are connected using shear connectors whereas self-tapping screws are used to join CLT panels. Pitzl connections are used for connecting GLT beams to columns [27]. Forte (Figure 2-7-3), is the first tall mass-timber residential building in Australia. It is 32.2m tall and has ten stories with only the first floor and foundation made of concrete. The structure utilizes 5-ply CLT panels for load bearing walls and slabs. Prefabricated modules reduced the construction time from 14 months to 11 months when compared to its concrete equivalent [27].  “Stadthaus” or Murray Grove (Figure 2-7-4) was one of first tall wood buildings in the world. It is a nine stories residential building completed in 2010 in London. CLT is used in a honeycomb pattern around the structural core, as a main structural material with a concrete podium. The superstructure was completed within 27 working days which reduced the total construction time.  17  Figure 2-7: Modern Tall Wood Buildings Globally: (1) LCT One (Credits: Architekten Hermann Kaufmann); (2) WIDC (Credits: MGA) ;(3)Forte (Credits: Lend Lease); (4) Stadthaus (Credits: Waugh Thistleton Architects ); (5) TREET (Credits: SWESCO); (6)3-D model of TREET [28] Figure 2-7-5 shows TREET, a 14-storey building in Bergen, Norway. It is the tallest contemporary timber building in the world with a height of 45m above concrete foundation level. It consists of load carrying GLT trusses with prefabricated building modules to reduce the construction time. 18 CLT is used for the elevator shaft, internal walls, and staircases. The singularity of this building is the introduction of strengthened GLT “power stories” shown in Figure 2-7-6. This power storey is located after every four levels that carry a prefabricated concrete slab on top and behaves as a base for the three levels above it [28]. The diagonal GLT elements are connected using slotted steel plates using dowels. Charring method is used as fire resistive method exposing the timber for aesthetic purposes [29]. 2.2.5 Mass-timber Buildings at the University of British Columbia UBC has always advocated the use of sustainable materials on both its Vancouver as well as Okanagan campus. Mass-timber has been used for structural and aesthetic purposes on some small-scale structures, such as in transit shelters, or more lately, in Brock Commons as the main structural material. Some of UBC’s academic and the residential buildings with the use of mass-timber are discussed in the following section. The Earth Sciences Building (ESB) (Figure 2-8-1) integrates the use of prefabricated CLT panels and GLT in the construction of the main structural system. However, the south wing of the building which houses the laboratories and offices is constructed with concrete whereas wood is a main structural component in the north wing. A timber-concrete composite system with 89mm thick mass-timber panels covered with 100mm of concrete topping connected using HBV system is used as a floor. The use of Sherpa connectors in wood beams to columns joints reduced the timeline of the project. Also, diagonal GLT bracing along with the steel knife plates ensured the controlled resistance for shear loads and acted as Seismic Force Resisting System (SFRS). The HSK system is used for the free- floating staircase [30]. 19 Sail (Figure 2-8-2) is a two six-storey residential wood-frame building with a total of 15,500m2 of building area. The entry pavilion is made of GLT columns and beams whereas LVL is used in the floor system. The usage of engineered joists and beams can be seen in the interior structure. Douglas-Fir is used for the mass-timber elements as well as the wood frame [31]. The Bioenergy Research & Demonstration Facility (BRDF) (Figure 2-8-3) displays an ingenious use of mass-timber in GLT moment frames and CLT panels for structural and architectural purposes. These GLT moment frames are connected to post base using steel box connectors.  CLT is used for the roof, floor, bearing and nonbearing walls as well as in suspended staircase [31]. The new Student Union Building (SUB) (Figure 2-8-4), with a gross floor area 18950m2, is a five-floor hybrid building with the use of steel, concrete, and timber. Wood is used in the central atrium space for aesthetics as well as structural purposes. GLT columns are used in the exterior curtain walls which extend from the first floor to the central atrium space whereas CLT panels are used to form a saw-tooth structure in skylight roof [32] The small-scale structural use of mass-timber in University Boulevard transit shelters (Figure 2-8-5) with the use of steel columns describes a creative way of the use of glulam. GLT is used in a pentagonal pattern (flipped structure) connected with self-tapping screws to hold up glass above them [31]. CLT is used in the two-storey Fitness & Wellness Center in UBC, Okanagan (UBC F&WC) (Figure 2-8-6) by forming grid type structure in a curvy manner. Also, CLT is used in the column-to-beam moment connections. Two-way spanning ability of CLT is well utilized in roof and floor decks, which span more than 6m because of the light weight of CLT and exceptional structural performance of CLT [33]. 20 Other noticeable UBC mass-timber buildings include the Center of Interactive Research on Sustainability (CIRS), the Indoor Baseball Facility and Engineering Design Center (EDC).   Figure 2-8: Mass-timber in UBC: (1) ESB at UBC (Picture Courtesy- Perkins+Will); (2) SAIL (Picture Courtesy- Adera); (3) BRDF (Picture Courtesy-www.projectservices.ubc.ca); (4) New Sub (Picture Courtesy-www.wesbridge.com); (5) Transit Shelter (Picture Courtesy-publicdesign.ca); (6) UBCO F&WC (Picture Courtesy-www.kelownadailycourier.ca)    21 2.3 Future Tall Timber and Hybrid Buildings The future of building with tall timber in North America is possible with monetary support from the government as well as growing sustainability ratings. Canadian and US governments are supporting the developers with the help of industry and research partners. The trend of mass-timber in residential and commercial buildings escalating quickly. Table 2-3 summarizes some of the future tall wood buildings.  Table 2-3: Upcoming Tall Wood Buildings Globally [24] Name Location Stories Wood City Helsinki, Finland 8 Abrora Montreal, Canada 8 Carbon 12 Portland, USA 8 Framework/Beneficial Bank Portland, USA 12 475 West 18th New York, USA 10 HoHo Vienna, Austria 24 Origine Quebec City, Canada 13 Hypérion Bordeaux, France 18  NRCan introduced the Tall Wood Demonstration Initiative for the development of wood in taller buildings in 2013. The Canadian Wood Council (CWC) issued an Expression of Interest (EOI) for builders, contractors, and design firms to showcase a new approach to build tall with wood. A funding of CAD$5 million was allotted to two other selected projects (Abrora and Origine ) in addition to UBC Brock Commons [34]. Similarly, in 2014, US Tall Wood Building Prize Competition was announced by United States Department of Agriculture (USDA) in collaboration with Binational Softwood Lumber Council and Softwood Lumber Board. Two projects named 475 West 18th (New York City) and Framework (Portland) are supported with a funding US$1.5 million [35].  22 2.3.1 North American Tall Wood Buildings 475 West 18th is ten stories residential condo with ground floor for commercial use located in New York City, USA. This 36.6m tall structure will have two- and three-bedroom units. The method of construction adopted is post and beam. GLT columns and beams are used with CLT shear walls and floors. The one-way CLT floor system including a non-structural concrete topping is proposed to be 305mm thick. CLT shear walls, as well as CLT core, provide the lateral resistance. The timber elements except the CLT shear walls are proposed to be exposed with an increased thickness to accommodate a char layer. Interior of the building with exposed timber is shown in Figure 2-9-1. CLT core and shear walls are designed to have two-hour FRR using encapsulation. GLT beams and CLT floors are designed to meet two-hour FRR whereas GLT columns are designed for three-hour FRR using charring method [35]. Framework (Figure 2-9-2) is 12 stories, 39.6m tall mixed-use building with residential apartments as well as retail offices. It will also house a “Tall Wood Exhibition” on the ground floor. The structure is designed with GLT columns of double height with CLT floor system and cores. As Portland is in a seismically active zone, special considerations have been taken care of. CLT cores are arranged in a honeycomb pattern with the use of post-tensioning (Pres-lam) and rocking technology of New Zealand [35]. This system allows having a ductile failure rather than brittle by yielding of steel using dissipating devices.  Origine, in Figure 2-9-3, is a 13 stories housing condo with 41m height and has 92 residential units is being constructed in Quebec city, Canada [36]. Post and beam methodology is used here with GLT columns. CLT is employed in floors as well as walls. The building rests on a concrete podium. The CLT in cores and stair shaft act as LFRS. NBCC requires two-hour FRR for the combustible structural core. A full-scale fire test is performed to convince the Authorities Having 23 Jurisdiction (AHJ) for the approval process for the wood core. Fire test was conducted on a mass-timber shaft to show that a fire caused in any adjacent apartment of the core would have no or little effect on core [37].  Arbora Complex (Figure 2-9-4) is three-8 stories mass-timber buildings with a floor area of 55,515m2 to be constructed in Montreal, Canada. It has a total of 434 units in the form of townhouses, condos and rental units. 80% of the structure is built with CLT in this massive residential project whereas 20% of GLT is used [38]. 175mm of 5-ply CLT is used with dimensions of 2.4m by 6.1m. A high suitability rating is anticipated for the building. This building has around 40% area dedicated towards green space.   Figure 2-9: Future Tall Wood Buildings in North America: (1) 475 West 18th, New York, US (Courtesy: SHoP Architects); (2) Framework, Portland, US (Credits: Lever Architecture); (3) Origine, Montreal, Canada (Courtesy: Nordic Structures); (4) Arbora, Quebec City, Canada (Courtesy: Nordic Structures) 24 The growth of mass-timber buildings is evident in the United States. Carbon 12 in Portland and T3 in Minneapolis are in the construction phase. Carbon 12 is an eight storey tall mass-timber building with a height of 26m, constructed using prefabricated GLT and CLT panels [39] whereas, T3 is a mixed use amenity space with seven stories of mass-timber structure [40]. Another 80 stories tall wood concept in Chicago, named River Beech tower, proposed by Perkins+Wills and Thornton Tomasetti engineers, is planned to have around 300 duplex units. The timber elements are arranged in a diagonal manner on the exterior side which also acts as LFRS and GFRS in combination with internal cross-bracing. This grouping assists to distribute the loads evenly [41]. 2.3.2 European Tall Wood Buildings Europe is leading in tall hybrid structures because the regulatory approvals require the use of sustainable building materials. The introduction of sustainability ratings has immensely favored, directly or indirectly, the use of mass-timber in construction. The number of stories of North American wood buildings is lower compared to buildings in Europe. Some of these future buildings are described in this section. Hyperion, in Figure 2-10-1, is 18 stories mixed-use tall wood tower. It is planned to be built with a height of 57m in Bordeaux, France. The name Hyperion itself refers to the tallest tree on earth (Californian Redwood) and is expected to be completed in 2019. Jean-Paul Viguier et Associes have collaborated with Effage and Woodeum for this tall wood pilot project in France [42]. The structure comprises of CLT floors, LVL and GLT substructure installed in post and beam method, lumber facade with steel and glass. Also, green balconies make this structure look remarkable. Rüdiger Lainer + Partner Architekten ZT from Austria have proposed the tallest building named Hoho in Vienna, Austria. It will be made up of 75% of timber with concrete core for housing 25 stairs and elevators [43]. The gross floor area is around 50,000m2 which would include rental, commercial use. With a height of 84m, HoHo will have 24 floors as in Figure 2-10-2. “Haute Couture” or HAUT (Figure 2-10-3) is a planned 21 stories timber building in Amsterdam, Netherland. In collaboration with ARUP, Team V Architecture, Lingotto, Nicole Maarsen and Nederlandse Energie Maatschappij has proposed a 73m tall 21 stories residential tower [44]. It has a lot of variations in the floor layouts. CLT is intended to be used thus sequestering over 3 million Kg of carbon. HAUT is anticipated to have high sustainability rating for Building Research Establishment Environmental Assessment Method (BREEAM).  Figure 2-10: Future Tall Timber Buildings in Europe: (1) Hyperion, Bordeaux, France (Courtesy: Jean-Paul Viguier et Associes); (2) Hoho, Vienna, Austria (Courtesy: Rüdiger Lainer + Partner Architekten); (3) HAUT, Amsterdam, Netherland (Courtesy: Team V Architecture) Other concepts were not achieved because of monetary or regulatory reasons. A 35 stories wooden mixed-use building named Baobab was proposed by MGA in collaboration with a Paris-based structural engineering firm DVVD in Paris under the Réinventer Paris competition see Figure 2-11-1 [45]. The proposed structural material is CLT which will act as a sustainable as well as carbon sequestrating material [46]. 26 For the HSB Stockholm 2023 architectural competition, C.F. Møller Architects and Dinell Johansson proposed a 34 stories timber tower shown in Figure 2-11-2. The building comprises of wooden columns and beams with a concrete core to inhabit the elevator within [47]. All the interior finishes including ceiling, window frames and walls are planned to be of wood [48].  Tham & Videgård Architects from Stockholm drew up plans for constructing four towers of 20 stories height out of mass-timber in Stockholm. This proposed development houses 240 apartments built out of solid wood. Its proposed site is along with a six stories structure. Hence a careful study on the orientation of buildings has been done which can be seen in Figure 2-11-3. “Swedish solid wood” will be used for the load bearing assembly as well as for the envelope and interior finishes [49].  Figure 2-11: Proposed Buildings: Baobab, Paris, France (Courtesy: MGA); Wooden Skyscraper, Stockholm, Sweden (Courtesy: C.F. Møller Architects); Four-20 Storey Wooden Apartment Building, Stockholm, Sweden (Courtesy: Tham & Videgård Architects) 2.4 Proposed Concepts for Tall Timber Buildings The revolution of mass-timber has inspired designers, architects and construction industry to build taller buildings with wood. However, building tall with timber alone is challenging for design 27 considerations as well as for approvals of the building. The solution is to proceed with hybridization. Some proposed concepts are discussed in the following sections. 2.4.1 Creative Resource and Energy Efficiency (CREE)  A hybrid timber-concrete concept for stories up to 30 is proposed by Rhomberg group under Creative Resource & Energy Efficiency (CREE) company. This notion utilizes the advantage of Timber Concrete Composites (TCC) slabs for longer spans and for overcoming the fire resistance, acoustic and building services problems as shown in Figure 2-12-1. The proposed concept is designed with a concrete foundation and concrete podium with “unenclosed double GLT beams and glulam-concrete hybrid floor slabs” as in Figure 2-12-2. Also, it is approximately 30% lighter than Reinforced Concrete structures (RCC), thus reducing the seismic demands. CREE emphasizes on the prefabrication of the composites and installing them at the site, thus reducing the construction timeline.   Figure 2-12: CREE Timber Concrete Composite System: (1) Leverage for Building services; (2) Structural solution (Credits: CREE by Rhomberg) 28 2.4.2 The FFTT System Finding the Forest Through the Trees (FFTT) system is a hybrid steel-timber concept based on balloon framing of timber panels. A “strong-column weak beam” approach is followed in which ductile behavior of steel undergoes inelastic deformation before the brittle failure of timber elements. Buildings with 12, 20 and 30 stories were analyzed with options shown in Figure 2-13, which are proposed to be in a seismically active region Vancouver, B.C., Canada. Mass-timber shear walls and core and columns are provided to be a part of LFRS as well as Gravity Force Resisting System (GFRS) in each option [50]. However, Fairhurst (2012), found out that wind governed the system as it goes higher compared to the earthquake loadings in the dynamic analysis using nonlinear behavior models. The FFTT system with 22 stories and above does not meet the serviceability criteria under wind loading [51].   Figure 2-13: Options for FFTT system (Source: The Case of Tall Wood) 29 2.4.3 SOM Timber Tower Project  This project investigated a timber hybrid proxy of “Dewitt-Chestnut Apartments” in Chicago with same height and stories. The idea proposes a “Concrete Jointed Timber Frame” structural solution [52]. The main objective is to develop a sustainable and a cost-competitive structural solution compared to concrete. Timber floors, columns, and shear walls add up to 70% of entire building whereas the rest 30% is concrete substructure including foundations. A similar type of foundations, belled caissons, as for the benchmark was proposed whereas incorporated floor system consists of CLT panels. GLT columns are proposed at the perimeter of the building with timber shear walls and “reinforced concrete spandrel beams” as shown in floor layout in Figure 2-14. The lateral resistance and resistance for overturning was achieved by the action of shear walls coupled with concrete beams to act as a cantilever. “Timber Tower Research Project” by Skidmore, Owings & Merrill (SOM), LLP also looked into the architectural designs, sustainability by calculating carbon sequestration and into building services [52].  Figure 2-14: SOM Timber Tower Project [52] 30 2.4.4 Wooden Empire State Building The Wooden Empire State Building is a case study like the SOM Timber Tower. Metsä Wood with collaboration with MGA and Equilibrium came out with a visionary concept of turning out the original Empire State Building out of wood with the identical height and similar structure [53]. A modular structure using “Kerto LVL” wood panels is used in this 102-storey 443m high structure. The design comprises of wood columns up to six stories high with moment connections. These columns are then connected by “box beams” along the short axis of the building. Four pre-tensioned cables run within the beams to tie the structure together. The use of prefabricated LVL panels up to 25m long with a width of 2.4m in the long axis of the building makes the conceptual construction fast and economical [53]. Charring is used for fire protection as these panels have low charring rate up to 0.7mm per minute. The conceptual design is illustrated in Figure 2-15.  Figure 2-15: Conceptual Design of Empire State Building in Wood [53] 31 2.5 Literature Review Conclusion The use of mass-timber in construction for tall and mid-rise buildings is advantageous over the traditional building materials. Historical wood buildings across the globe validate the durability of timber in construction. The abundant amount of sustainably managed forests in Canada favours the notion of utilizing wood in construction to meet environmental and urbanisation goals.  With the advancement of innovative building materials, engineered wood products have proven to be a viable alternative in mid-rise timber buildings. Much research has been done on proposing tall timber building concepts but no such building taller than 14 stories haves been constructed to date. This is primarily due to regulatory or monetary concerns. This research focusses on investigating the possibility of a pure tall timber building by using the structural layout of TWB as a benchmark and understanding the challenges faced during the TWB approval process.  32 Chapter 3: Regulatory Framework of the UBC TWB 3.1 History of Canadian Code Framework  The British North American Act (BNA) established the first formal requirements for building construction in Canada after the Confederation (July 1, 1867). Canada was divided into four provinces, namely, Nova Scotia, Ontario, New Brunswick and Quebec. In this act, the responsibility of building construction regulations was attributed to the Provinces and various municipalities [54]. It created a mixture of construction methods and techniques within Canada as well as in the same Province. In late 1930’s, National Research Council (NRC) was established. This organization’s concern was to enact a set of specific and uniform regulations for construction in Canada. By that time, United States had already adopted a model code for a uniform construction across the states. The first National Building Code of Canada (NBCC) was published in 1941. The construction typologies were separated into 5 groupings according to occupancy (A, B, C, D, and E) and further divided into “Maximum Permissible Areas”- Table 1 (NBCC 1941) and “Maximum Permissible Heights”- Table 2 (NBCC 1941) [54]. These classifications are summarized in Table 3-1. Table 3-1: Construction classification in NBCC 1941 [46] Construction Type Height (m) Area (m2) Fire Resistive 13.71 to Unlimited 930 to Unlimited Heavy Timber 16.75 to 22.85 700 to 1400 Masonry and Frame 10.65 to 16.75 465 to 700 Wood Frame 6 to 10.65 275 to 465 Unprotected Metal 1 Storey Unlimited 33 Any change in the building area and height in the subsequent updated versions of NBCC was based on various components, for instance- fire department access, fire egress, fire spread in and around the surroundings of a building. Building area and height were increased in the subsequent versions with the advances in technology such as structural protections and materials, automatic sprinkler systems and higher fire ratings. It can be simply understood by the fact that usage of automatic sprinklers raised the building area percentage two-fold from the first edition in 1941 to 1990 and then again for the 1995 and 2005 versions [55].  The NBCC model building code was restructured with innovation and the advancement of the construction industry. Since the first edition in 1941, NBCC has undergone 14 updates and many changes, yet some of the regulations regarding building height and building area remain unchanged. New terms were included; for instance, a clear classification of combustible and noncombustible building types in 1960 NBCC and a sheer distinction between protected and unprotected combustible and noncombustible construction in 1965 NBCC [54]. 3.2 2005 NBCC- An Objective-Based Building Code An objective based National building code was proposed in the mid-90s to fulfill the need of clearer scope, requirements, and practice of new materials and techniques. With the assistance of NRC, Canadian Commission on Building and Fire Codes (CCBFC) projected the implementation of the objective-based code in 2005. In this version, each requirement is tied to a specific objective, with a detailed explanation [56]. The main purpose of adoption of objective-based code was to remove the barriers for innovative proposals. Prior to 2005, the building code consisted of certain rules named as “prescriptive or acceptable solutions.” However, after the implementation of the 2005 objective-based NBCC, another way 34 for code compliance was through “alternative solutions.” In this concept, building performance of alternative solutions should exceed or at least equal the corresponding specification of the objectives and functional statements of an acceptable solution. NBCC’s objectives are related to safety, health, accessibility, and efficiency. Some provinces may adopt additional objectives. Also, for a detailed understanding of the quantitative meaning of acceptable solutions, intent statements were also provided. 2005 NBCC is divided into 3 partitions, namely Division A, Division B and Division C [57].  • Division A underlies the conditions which are essential for code compliance and contains the objectives as well as functional statements which must be achieved during the application of the code. An objective expresses the code’s intention whereas functional statements decode these objectives into working terms. In Division A, there is an explanation of what needs to be done but how it can be achieved is explained in Division B. • Division B consists of acceptable solutions with references to objectives and functional statements in Division A. Every acceptable solution is directly linked to one or more objectives and the corresponding functional statements. Acceptable solutions achieve building performance through the prescriptive requirements. • Division C consists of the administrative provisions and varies for different provinces and territories authorities so that they can add or remove provisions as per their specifications.  The biggest change in the 2005 objective-based code is the introduction of alternate solutions. Any design, material, technology or design which varies from acceptable solutions in Division B is considered as an alternative solution. These alternative solutions are performance-based design provisions which exceed or equal the code benchmark, i.e., the acceptable solutions.  35 3.3 British Columbia’s Regulatory Framework 3.3.1 Overview  Before abiding the current three-tier organization of code regulations, i.e., federal, provincial and municipal, British Columbia followed the building code under local bylaws [58]. In September 1973, British Columbia’s government adopted NBCC 1970 through a municipal act. This act gave the power to establish regulations for the building code to the provincial government. In 1977, the minister responsible for housing was given the authority to approve building code. More recently in 2015, the minister responsible for housing was given the discrete responsibility to set building requirements through the Building Act [58]. This act gave accountability of alteration and supervision of the BC Building Code to the Building and Safety Standards Branch (BSSB). British Columbia adopts the NBCC after determining the clauses for division C and some other amendments. After the objective based NBCC 2005, BC adopted BCBC 2006 in which up to four stories of wood-frame buildings were allowed. BCBC 2012 was based on the 2010 objective-based code. After BCBC 2012, BC became the first province to allow the construction of six-storey wood frame buildings with a specific area limit.  BCBC 2012 is similarly organized into three parts namely, Divisions A, B and C [59]. This code is a provincial directive which establishes minimum criteria for Safety (OS), Health (OH), Accessibility (OA), Fire, Structural Safety (OP) and Energy and Water efficiency of buildings. Division A represents the compliance, objectives and functional statements whereas Division B outlines the acceptable solutions for different building characteristics [59].  36 3.3.2 Alternative Solutions in BCBC 2012 Buildings within the scope of BCBC follow the regular process as per the code. But buildings outside the scope must go through an alternate solution approval process. This alternative solution concept formulates a performance-based design which equals or exceeds the acceptable code limits. BCBC in Article 1.2.1.1., Part 1, Division A  states that “compliance with the code shall be achieved by complying with applicable acceptable solutions in Division B or using Alternative Solutions that will achieve at least the minimum level of performance required by Division B in the areas defined by the objectives and function statements attributed to the applicable Acceptable Solutions [59].” An alternate solution for a different location may differ but have the following general elements to be addressed [60]. • Overview of the project and applicable objectives and functional statements • Proposed Alternative solutions • Demonstration of solutions with explanation of all the assumptions and requirements • Detailed analysis and reports BC introduced a Building Act in 2015 which aimed to create a consistent, competent and innovative construction regulatory system. This act supported the idea of accepting innovative proposals which do not conform to BCBC. This act has three main aspects. First, to set guidelines and accept innovative proposals, second, to conduct technical reviews to ensure an acceptable level of safety, and finally enacting a Site-Specific Regulation (SSR) after the permit approval. This act enables the construction of TWN [61]. 37 3.4 Site Specific Regulation for UBC Brock Commons 3.4.1 Overview The UBC Vancouver campus is located on the west coast of British Columbia in Vancouver, so the applicable provincial codes were BCBC 2012 and BC Fire Code 2012. Prior to Building Act 2015 in British Columbia, UBC used its own building regulation regulated by UBC’s Chief Building Officer (CBO). The 2015 act required the UBC board of governors to review the UBC by-laws and amend them if they did not comply with BCBC [61]. UBC Brock Commons’ building height, material, and area were outside the scope of the above listed provincial codes. Hence, getting approvals was one of the biggest challenges for construction of TWB, considering its tight schedule timeline. Hence, the building had to comply with a site-specific regulation, namely the UBC Tall Wood Building Regulation (TWR).  With the addition to the above regulatory outline, buildings to be constructed at UBC must comply with UBC Policy#92 (Land Use and Permitting), a set of land use rules for development and building on the UBC Vancouver Campus which include: • The UBC Vancouver Campus Plan, a three-volume document which lays out the Governance Requirements with respects to development and building on campus • The UBC Land Use Plan, which “provides a vision and goals for future development, broad land use considerations and objectives for more detailed planning.”  • The UBC Development and Building Regulations which ensure that “projects proposed on the UBC Vancouver Site are consistent with the UBC Land Use Plan, Neighborhood Plans, and Vancouver Campus Plan and the intent thereof.”  38 Some other sustainability parameters as well energy efficiency parameters in the construction of a building at UBC should be considered. All the UBC academic, as well as residential buildings, are required to have a minimum of Gold Leadership in Energy and Environmental Design (LEED) certification or similar standards [62]. LEED is a green building certification which is accepted worldwide. There is a total of possible 126 points which a building can achieve by these five chief aspects in LEED [63]: • Sustainable Sites • Efficiency related to water • Energy & Atmosphere,  • Materials & Resources, and  • Indoor Quality  The usage of timber certainly helped in increasing the overall points by enhancing the indoor air quality, decreasing wastage of raw materials due to prefabrication and using renewable materials. Also, acquiring the locally sourced wood from sustainably managed forests for TWB helped to achieve LEED Gold certification [64]. Brock Commons was also required to fulfill the standards for ASHRAE 90.1-2010 [65]. This specification provides the minimum requirements and the ways to compliance for energy efficient buildings during its whole life-cycle. 3.4.2 Authorities Having Jurisdiction (AHJ) AHJs are the regulatory agencies which are responsible for the approvals of the building. It can be local, regional or provincial. A Site-Specific Regulation (SSR) was developed for the student residence at Brock Commons due to the implication of the BC Building Act in 2015. SSR was designed by the provincial and local AHJ’s, namely the UBC CBO and BC’s BSSB with the help 39 of third party code consultants, namely GHL Consultants Ltd. These code consultants were brought int the design team in the initial design phase to facilitate the process. It was then authorized by the Building Standards and Safety Act and sanctioned by the Ministry of Natural Gas Development and Minister Responsible for Housing as well as the UBC board of governors [66]. These AHJs were involved in the project at early design stage which helped the stakeholders to access the different design challenges and provisions to make the cumbersome SSR process easier.  3.4.3 Site Specific Regulation An SSR is specific for a site which means it cannot be used as a precedent for any tall timber building in future, even on the UBC campus without going again through a similar procedure. This is the reason that SSR developed for WIDC was not applicable. The owner involved the design team from the commencement of the project due to its peculiarity. This integrated design team included architects, structural engineer, code consultant, construction manager, mechanical and electrical engineer, and Virtual and Design Construction (VDC) integrators. This helped in arriving at a consensus for the design which was possible for construction.  The Brock Commons’ SSR approval process involved a panel of experts which focused on two specific aspects of the building: the structural safety and the fire safety. The members of both the panels are listed in Table 3-2. These experts challenged the design and advised on various design issues, which were suitably addressed by the project’s structural engineers, architects, and other stakeholders.  40 Table 3-2: List of Expert Panelists for UBC TWB SSR Fire Expert Panel Structural Expert Panel Gage-Babcock and Associates UBC Forestry Building Code Appeal Board Chair Gage-Babcock and Associates Sereca Fire Forest Innovation Investments City of Vancouver Fire Chief StructureCraft Builders City of Surrey Fire Chief City of Vancouver Forestry Innovation Investment Equilibrium Consulting National Research Council of Canada Wood Science and Technology Centre 3 Representatives from BSSB 3 Representatives from BSSB Vancouver Building Policy Engineer   Office of Mcfarlane Biggar architects + designers Read Jones Christoffersen  The SSR process necessitates a thorough peer review process. A Canadian firm, Read Jones Christoffersen (RJC) and an international company, Merz Kley Partner (MKP) AG were chosen as third party reviewers for the structural design. These peer-reviewers with expertise in wood construction submitted a final report discussing the design issues to the expert panel. RJC’s report focused on the design problems with the hybrid system and the code requirements. They also reviewed the lateral design, gravity design, fire design and other design considerations like long term differential settlement, and progressive collapse [67]. However, MKP reviewed the component-level ultimate and serviceability limit states for CLT panels as well as the columns. They also helped to revise the column to column connection [67]. An additional feature which helped the design team to validate their solution was to build a full-scale mock-up. This mock-up consisted of two stories having 3 bays by 3 bays with the L-shaped concrete core. Different structural connections were investigated along with the prefabricated 41 envelope system. This mock-up helped confirm the column to column connection design based on its constructability. Also, it assisted in finalizing a more aesthetic envelope system [68]. 3.4.4 Acceptable Solutions in SSR The uniqueness of the approval process led the owner to hire an experienced code consultant who helped them prepare the SSR. The building had to be approved through the alternate solution process due to its 53m height and engineered wood as a construction material. In BCBC 2012, building height is limited to six stories wood-frame structures. The use of engineered wood like CLT or GLT for tall wood buildings is still outside the scope of the code.  Brock Commons is 18 stories student residence; the major occupancy is Group C (residential) whereas minor occupancy is A-2 (assembly) due to the amenity spaces located on the 1st and 18th floors. The building had to be compliant with Division B, Article 3.2.2.23 “Group A, Division 2, Any Height, Any Area, Sprinklered” as well as Article 3.2.2.47, “Group C, Any Height, Any Area, Sprinklered” and the SSR. The permitted combustible construction materials according to the SSR are [66]: • CLT slabs or floors not less than 150mm with four layers of 15.9mm gypsum boards. • Columns with minimum dimensions of 265mm by 215mm. Protective coverings of four layers of 16mm gypsum on stand-alone columns and three layers for all other columns. These protections on the combustible structural materials ensure a 2-hour fire protection. The acceptable solutions for “Fire Safety in High Buildings” under Division B, subsection 3.2.6 were applied whereas the SSR includes some measures to enhance the mechanical design: 42 • The pressurization of exit stair shaft to create a positive differential pressure of 12 Pascal within exit stair during the activation of fire-alarm within 1 minute. • In case university’s water supply is breached during a fire or earthquake, the backup water supply of 5,000 gallons is provided onsite for sprinklers to be operational for 30 minutes. • Sprinkler protection of exterior CLT canopies in accordance with NFPA 13- 2013. • The automatic sprinkler system will be monitored to provide signals to Fire Department. • Fire alarms and fire department access, emergency lighting and power, spatial separations and exposure protection, exits, washroom requirements and provisions for persons with disabilities according to the code. 3.4.5 Alternate Solutions in SSR The design team has eight months to get the approval permit for TWB. Due to this tight schedule, the design team tried to adhere to tried and tested solutions. Some of them were: • Involving AHJ at the inception of the design process. • Building a full-scale mock up to validate the structural concept. • Having an integrated design team at the outset of the design phase. Even with the stakeholders adhering to tested solutions for facilitating the approval procedure, it took around eight months for the designing and approval process [2]. To further simplify the approval process, the decision for the concrete core was taken. The noncombustible concrete cores are a standard feature in high-rise buildings. Though the building is fully encapsulated (except 18th floor), the core provided two-fold advantages. These cores acted as LFRS for lateral forces and provided an escape route during fire events. Also, concrete serves as a noncombustible material in the exit shafts. This notion strengthened the fact of having safe exit during any fire 43 event. On the other hand, the same building with a mass-timber core could have delayed the permit process. The alternative solutions are acknowledged as a part of applicable building code when they are accepted and approved by AHJ. For Brock Commons, with the above stated acceptable solutions in TWR, three alternate solutions were proposed in the SSR as are summarized in Table 3-3: Table 3-3: Alternative Solutions proposed Alternate Solution Why Acceptable solution BCBC Sprinkler Protected Glazing System Exposed GLT columns at 18th Floor Sentence 3.1.7.1. (1) as in Division A, Sentence 1.2.1.1. (1). Suspended Wood Ceiling Decorative suspended wood grille ceiling in the 1st storey and the 18th storey Division A, Article 1.2.1.1 Electromagnetic Locking Devices at Doors Opening of secured doors in case of fire Sentence 3.4.6.16. (4) as in the objective and functional statements  Sprinkler Protected Glazing (SPG) System:  All the glued laminated columns are covered or encapsulated by protective coverings of type X gypsum boards except on the 18th floor of the building. This exposed timber will be part of a student lounge. As no combustible material is allowed to be exposed, so to fulfill the code objective of 2-hour fire resistance rating, this alternate solution is proposed.  BCBC sentence 3.1.7.1 (1) explains the techniques to govern the fire resistance ratings. This relates the functional statements [F03-OS1.2] and [F03-OP1.2]. OS narrates the safety of occupants in case of fire event whereas OP recounts to the damage of building in fire event from 44 the point of origin of the fire. The exposed timber was allowed in the 18th storey only by compartmentalizing from the rest of building by 2-hour fire separations achieved as follows: • A glazed separation of 6mm is provided using tempered glass in aluminum or steel frames between the lounge space and the public corridor.• An SPG system using Tyco window sprinklers (Figure 3-1) for 2-hour fire separation. These Tyco sprinklers bulb shaped quick response sprinklers in accordance with the requirements of National Fire Protection Association (NFPA-2013 edition) [69].  • These sprinklers are spaced at 25 mm from the surface of glazing conforming to the research done at Nation Research Council of Canada. • Doors and the glazing are protected by water curtains on both sides using sprinklers.  Figure 3-1: Tyco Window Sprinklers[69] Suspended Wood Ceiling An exposed wood ceiling is provided on the 18th as well as the 1st floor of UBC Brock Commons for decorative purposes. But as per Division B sentence 3.1.5.10(3), all combustible ceiling must not have a Flame Spread Rating (FSR) of more than 25. This flame spread is the surface burning property of any combustible material and is measured in FSR with a comparison to other materials. This can be found in NBCC Division B, Appendix D, Section D-3 for all the interior finishing materials [70]. This phenomenon is linked to pre-flashover and post-flashover stages of fire. In the pre-flashover stage, code restrains the construction material to be of noncombustible 45 nature to reduce the amount of burning fuel. Thus, this curbs the probability of fire to increase and reach its flashover point. Whereas, after the flashover stage, code objective is the safety of occupants and any damage to the building by the collapse of the building under gravity loads. Therefore, in this alternate solution, both the stages must be taken care of. In addition to [F03-OS1.2] and [F03-OP1.2], which checks the safety of occupants and the safety of damage of building, this alternate solution had to exceed F02 objective, too. F02 functional statement narrates “To limit the severity and the effects of fire or explosions” during any fire event. So, a suspended wood grille is provided to exceed the acceptable solutions: • To tackle the pre-flashover stage and protection of wood ceilings, two rows of sprinklers are provided above and below the wood ceiling. This hurdles the fire spread along the interior ceiling and exceeds the minimum FSR defined in code. • Also, to reduce the amount of combustible fuel, wood treated with fire-retardant is used in the grille. This fire-retardant treatment is done before the installation of these ceilings. Electromagnetic Locking Devices at Doors In accordance with the acceptable terms of code, a delay of 15 seconds is allowed in case of non-emergency state whereas free exits need to open without any delay in the event of a fire emergency. BCBC Division B, Sentence 3.3.1.13(2) explains that the exit doors should be readily accessible in any case of urgency. This describes the objective code OS3.7; “In the case of any emergency like a fire, the safety of any occupant living in or adjacent to the building triggered due to hazards caused by the delay from moving to a safe place.” The functional statements associated with this objective F10 and F81 clarifies the intent of “judicious movement of persons to a safe location during an emergency” and “curtail the risk of damage, tampering, malfunction, 46 interference, lack of use or misuse.” Also, code requires the exit doors to be opened without any special keys or devices. Being a student residence with a student collegium space at the 1st storey, only the residents should have access to upper stories. This needs the use of special accessing keys or devices to their apartments. But during any emergency, for the free movement, these doors should open. Hence, while abiding most of the prescriptive performance measures in sentence 3.4.6.16 (4), this alternate solution proposes: • Magnetic locks with a fire alarm should unlock the doors in case of activation of alarms. • Appropriate bold signage at doors to make the people comprehend that to open the door in case of emergency pull the alarm to trigger it. The use of combustible mass-timber used in the superstructure was addressed by the Tall Wood Regulation, and these alternate solutions focused on the exposed timber on the 18th floor.  3.4.6 Discussion about TWB approval process BC’s 2015 Building Act has undoubtedly assisted in the construction of UBC TWB. Still, it took around eight months for approvals through a lengthy SSR procedure. The early engagement of the stakeholders in the form of an integrated team certainly assisted the design process. The choice of having a noncombustible exit route in the form of concrete cores accelerated the approval process. The idea of building a mock-up helped to validate the conceptual design and assisted in choosing the best selection based on constructability, economic and technical requirements. Also, allowing a third party Virtual Design Construction (VDC) integrator from the inception of the project aided to quicken the design and pre-construction phase [2].  47 3.5 Future Mass-timber Building in Canadian Code Stringent rules in Canada for working with tall wood have stalled innovation until BCBC underwent a change to allow 6-storey light-frame wood construction through an extensive process in 2012. The main perceived concern with wood is fire. Hence, there should be enough credible information on fire behavior and fire risks in the tall timber buildings before proposing any change to building codes. The construction phase is the most susceptible to fire because of the welding and other fire-related works going on at the site with no sprinklers installed. Appropriate construction-site safety protocols and manuals should be mandatory to avoid construction fires. Public awareness about the safety and reliability of structural and fire performance of mass-timber structures will reduce the general misconception of considering its behavior similar as of traditional light-wood framing. These knowledge gaps should be filled by engineers, fire experts, and designers by organizing free workshops or events to make them acutely aware of the benefits of mass timber. The risk of fire should be studied thoroughly using fire software as well as fire test. Also, fire resisting procedures like encapsulation and charring with the functioning of sprinklers should be thoroughly elucidated. Explanatory fire tests can validate the performance of timber building. For instance, Origine in Quebec City had to undergo a full-scale CLT shaft fire tests to demonstrate its feasibility [37].  However, NBCC 2015 has permitted the use of combustible timber core in wood frame buildings up to six stories [37]. Even the International and Canadian timber design codes have started addressing and accommodating mass-timber as a construction material. In 2015, the International Building Code (IBC) had approved the use of CLT in certain categories of construction by quoting ANSI /APA PRG 320 -2012 (Standard for Performance-Rated Cross-Laminated Timber) [71]. The CSA-086 2016 supplement has included the use of CLT in gravity and lateral resisting 48 systems. Also, the design of connections for CLT assemblies is covered [72]. However, the use of CLT as shear walls is limited to platform-type construction.  Timber elevator shafts or cores can be possible in tall wood buildings with enough testing and monitoring of existing buildings with timber cores. Only full-scale fire tests for tall timber cores can validate their performance under fire events. There is a need for component and system level testing for pure tall timber buildings to understand the structural performance. The connections between timber components need to be thoroughly investigated and tested for the ductility. Also, full-scale shake table experiments are required to justify their seismic performance. Thus, more comprehensive technical investigation, in terms of fire resistance and structural performance, is needed before any code approvals for timber cores in tall buildings.  49 Chapter 4: Structural System of UBC Tall Wood Building 4.1 Overview After Ponderosa Commons and Orchard Commons, Brock Commons is the third housing development in the planned five “mixed-uses and hubs” for student housing at UBC’s Vancouver campus. The height of the TWB is 54.81m at the top of the parapet, which exceeds the allowable height (53m) for mixed-use hubs as described in UBC Campus plan [62]. This 18-stories student residence houses 404 student beds with 272 single bed units and 33 quad bedrooms with shared kitchen and bathroom. A student lounge with exposed wood is located on the 18th floor. The building has a total floor area of 840m2 with dimensions 15m by 56m. Brock Commons demonstrates a hybrid configuration with a combination of different structural material as shown in Figure 4-1.   Figure 4-1: Hybrid Configuration of Brock Commons (Source: Fast+Epp) 50 This hybridization takes the benefit of attributes of different materials used, thus overcoming each material’s weakness. For instance, the lighter weight of wood, the ductility of steel and strength of concrete combined can lower the seismic demands, increase the energy dissipation and provide a stable structure respectively. The superstructure in Brock Commons is built in mass timber, but the LFRS is made of concrete. The connections are made of steel. The roof consists of metal decking supported over steel channels. The envelope for the superstructure is a prefabricated panel system in which steel stud and fiberglass batt insulation assembly are used with rainproof wood laminate cladding system [73]. The use of each structural material, as well as the connections, are explained in following sections. 4.2 Structural Materials 4.2.1 Concrete Concrete with a characteristic strength of 35MPa is used in most structural concrete components including foundations, columns, transfer slab and core as shown in Figure 4-2. Concrete with a strength of 32MPa is used in non-structural topping over CLT floor and 30MPa for other architectural concrete. The concrete work is performed in accordance with CSA A23.1 [74]. These concrete cores act as vertical SFRS and transfer the lateral loads from floor and roof diaphragms to the foundations. These cores have openings for doors, windows, and elevators. These openings in the shear walls made the core acts as a coupled shear wall due to the coupling action developed in the lintel beams connecting the two walls above the openings. Bending moments and horizontal shear, together with axial tension and compression, is resisted by distinct wall elements resulted from coupling action. The capacity design philosophy is used in Brock Commons for yielding of Reinforced Concrete Coupled shear wall core. It is achieved by flexural 51 hinging at the bottom of coupled walls. The shear and flexural inelastic deformations of the coupled beams permit the structure to distort inelastically through plastic rotations at the base of walls and inelastic chord rotations in the coupling beams [75]. Table 4-1 illustrates the use of concrete in the building.  Figure 4-2: Concrete elements in TWB (Source: Fast+Epp) The foundations which include the raft foundation, spread footings as well as the strip footings are cast-in-place concrete as shown in Figure 4-3.   Figure 4-3: Foundations in TWB (Source: Fast+Epp) 52 Table 4-1: Concrete Components of Brock Commons Component Use Number Size (mm) Foundation Spread Footings for Independent Columns 30 2800*2800*700 Raft Foundation for Core and adjacent Columns 2 1600 Raft Foundation for Water Tank 1 300 Strip Footing for non-load bearing walls - 450*250 Below Perimeter wall - 600*300 Strip footing for masonary wall - 900*300 Wall Perimeter of Building and Water Tank - 250 Transfer Slab Bears gravity load from above 17 floors - 600 Laid over general column Grid 5m by 5m -  Columns Transfers load from transfer slab to foundation 42 500*500 2 350*850 Concrete Masonary Cantilever Walls outside of building - 190 Non-Load Bearing Concrete Masonary Units -  Core Houses staircases and elevator shaft 2 450  Topping For Acoustical and Fire Protection of CLT panels - 40  The lateral resistance against wind and earthquakes is provided by the two reinforced concrete cores which contain the staircases and elevator. The allowable and ultimate bearing capacity of the soil is 400KPa and 800KPa respectively. These cores are anchored to soil using four 64mm diameter soil anchors of 1250KN tension capacity. The superstructure is supported by a reinforced concrete slab to transfer the gravity loads of level 2-18 to the concrete columns and finally to foundations. These concrete columns are 5m in height providing a greater head room for the ground floor. The concrete masonry units are used in non-load bearing walls and on the ground floor for partition walls. Also, concrete is also used in retaining wall on the sides of building with a thickness of 300mm and 200mm. A 40mm non-structural concrete topping over the CLT slabs is provided for acoustic insulation ranging between 52 to 54 Sound Transmission Class (STC). 53 4.2.2 Mass timber Mass-timber in TWB included CLT floors, GLT columns and PSL columns. These mass-timber components of the superstructure were prefabricated. Prefabrication became one of the leading reasons to complete the 17 stories of mass-timber structure within just ten weeks including the envelope [76]. The total volume of CLT used is 1973m3. A 5-ply CLT of 169mm thickness is used as slabs or floor in level 2-18 with 29 panels on each floor. These CLT panels were prefabricated and pre-drilled with the openings reducing the construction time. Figure 4-4 shows the arrangement of these various panels [3].   Figure 4-4: CLT panel arrangement in Levels 2-18 (Source: Naturally Wood) The CLT panel layup is such that the outer layers are Machine Stressed Lumber whereas inner layers are no1/no.2 SPF as per Structurlam Crosslam Design Guide [78]. The “E series CLT” is used in Brock Commons with minor adjustments. The floor assembly is such that these slabs are immediately covered by a layer of 15.9mm gypsum board. Then two layers of gypsum board are again provided after a steel hat track. These steel hat tracks are provided for providing room for electrical and other fittings. The typical layout of CLT slab is shown in Figure 4-5. These CLT panels form the rigid diaphragms for the lateral forces and transfer them to vertical SFRS. Also, a 3-ply panel of 105mm thickness is used in the canopy at the ground level.  54 The usage of timber in TWB is summarized in Table 4-2. The work with mass-timber is done in accordance with CSA 086-2014 [77] as well as SSR.  Table 4-2: Use of Engineered Timber in Brock Commons Timber Use Number of material used Size (mm) Grade CLT Slabs for Level 2-18; Laid out in different panel lengths of 6m, 8m, 10m & 12m 29 panels/floor (Total-464) 5 layered 169  6m-2/floor (Total-32) Long -32 1650f-1.5E 8m-19/floor (Total- 304) Cross -35 No. 1/2 10m-2/floor (Total- 32) Long -35 No. 1/2 12m-6/floor (Total- 96) Cross -35 No. 1/2  Long -32 1650f-1.5E GLT L 2-10 60/floor (Total-480) 265*265 D-Fir 16c-E L 10-17 78/floor (Total-624) 265*215 L 18 48  L 5-10 18/floor (Total-90) 265*265 PSL L 2-5 18/floor (Total-54) 265*265 PSL 2.2E   Figure 4-5: Typical CLT slab (Adapted from GHL Consultants) A total of 260m3 of GLT and PSL is used in columns as the vertical load bearing elements of the mass-timber superstructure. PSL columns with a size of 215mm by 265mm are on the lower floors for augmenting the compressive strength around the core. The GLT columns are of size 265mm by 265mm from level 2 to level 10, and the size decreases to 265mm by 215mm from level 10 to level 18 due to a reduction in the gravity loads. However, for the fire protection purposes, these 55 columns are encapsulated by layers of drywall. The stand-alone columns (Figure 4-6-1) are encapsulated with four layers of Type X gypsum board of 15.9mm thickness whereas columns within partition walls (Figure 4-6-2) are encapsulated with three coverings of Type X gypsum board to achieve a fire resistance up to 2 hours.   Figure 4-6: Column Configuration: (1) Standalone Column; (2) Column within Partition walls  4.3 Connections Timber behaves as a brittle material when loaded in tension or shear. These brittle failures are avoided by providing energy dissipating mechanism in the form of steel connections. They provide necessary stability, stiffness, ductility as well as strength. 4.3.1 Wood to Wood Connection The single surface spline method is utilized to connect CLT panels [7]. The ease of fabricating on the site makes it a suitable method. However, it provides a single shear plane between CLT panels as shown in Figure 4-7-1. The CLT panels are connected using Douglas-Fir plywood splines. These plywood splines are fastened with nails with a 4mm diameter and 60mm length with a spacing of 100mm (Level 2-16) and 64mm (Level 17&18). Also, partially threaded screws are also used at a spacing of 600mm throughout with a diameter of 8mm and length 120mm, as shown 56 in Figure 4-7-2. These screws are provided with washer heads for a rigid connection. A minute gap between CLT panels is also provided for tolerances for expansion.  Figure 4-7: CLT to CLT panel connection: (1) Panel to Panel (Adapted from CLT handbook); (2) Brock Commons CLT to CLT (Source-UrbanOne) The columns on the second level are joined with the transfer slab using a Hollow Steel Section (HSS) of diameter 127mm and a thickness of 13mm. This HSS is anchored in small pedestal over transfer slab using 4-19mm diameter Hot Dipped Galvanized (HDG) anchor bolts. The upper part of HSS is connected to the column with 4-threaded glued rods with a diameter 16mm and length 140mm with a steel plate of dimensions of the column (265x265mm) and thickness of 25mm. A schematic illustration of this connection is shown in Figure 4-8.  Figure 4-8: Connection for Wood Column with Transfer Slab: (1) Transfer Slab to Column (Credits-CadMakers); (2) Site picture transfer slab to column (Source-UrbanOne) 57 In the levels 3 to 17, where GLT columns support CLT panels, this connection varies. It transfers both vertical gravity loads as well as panel shear loads. Also, it allows CLT panels to act as a rigid diaphragm to resist and transfer lateral loads. There are three steel plates which play a major role in this connection. The column from upper level has the first steel plate (Steel Plate-1 in Figure 4-9) of thickness 29mm with four corner holes for 16mm diameter epoxy threaded rods which are drilled and tapped in plate 1 (16mm deep). Two central holes with 12mm diameter are provided to hold HSS in position. With the change in dimensions for the columns (265x265mm or 265x215mm), the dimensions of the plate also change accordingly. The second plate with the bolts (Steel Plate 2), typically is responsible for pin supporting the CLT panels in the connection. This plate has a thickness of 1.58mm with four corner holes of 19mm diameter. 16mm diameter bolts are drilled and tapped in two holes on each side which hold the CLT panels in position on either side. However, the general size of this plate is 300x300mm on the typical column size (265x265mm) but reduces by 250x300mm on upper-level columns (265x215mm). The second plate has the space for accommodating the HSS. The third plate (Steel Plate 3) placed on the column coming from the lower level has 29mm thickness. This plate accommodates the four holes for epoxied threaded rods for columns (16mm diameter) as well four holes for bolts for the CLT panels. This plate has similar functioning as plate 1. A 13mm diameter drain hole for excessive glue left over from the epoxy for threaded rods is also provided. A shim plate of thickness 1.58mm is provided for accounting the tolerances between these plates and HSS. The HSS consists of two rounded stub sections. One with a thickness of 13mm and diameter 127mm and inner with a thickness of 6.4mm and diameter of 89mm. The section is upheld by two 12mm diameter A325 anchor bolts which are tightened in the inner section with a welded steel plate of diameter 76mm and thickness 9mm.  58  Figure 4-9: Column to Column with CLT panel connection (Source: Aston Ostry Architects) From the lower side, the outer section is upheld using the same dimension bolts but with a welded steel plate of diameter 100mm and thickness of 9mm. The HSS has a slotted hole of 16 mm diameter and 38mm length for the erection bolts of diameter 12mm with cotter pin as shown in Figure 4-10. This cotter pin restrains the rotation of the column rotation about a vertical axis and provides a “hanging action” in the case of disproportionate collapse [79]. It prevents further collapse of the structure even if one column is removed or broken accidentally. In effect, this hangs the column from the floor above and detaches the rest of structure.  Figure 4-10: Cotter Pin in HSS (Source: Aston Ostry Architects) 59 4.3.2 Wood to Steel Connections CLT slabs adjacent to the concrete core are supported by ledger angles of size 203mm by 152mm by 13mm. These angles are screwed to CLT with Strong Drive Screws (SDS) with 6.4mm diameter and 89mm length. The spacing of the screws from level 2-16 is 250mm but on level 17, and 18 is 100mm. These angles were welded during the time of the construction of concrete cores. Another important connection is the drag straps attached to the CLT panels. These drag straps are necessary for transferring the lateral forces from the rigid diagram to concrete core. These drag straps are attached under the concrete topping to CLT panels and are then anchored into the concrete core using a drag strap face plate (Figure 4-11-1). The drag straps on the outer edge of CLT slab is called as a chord as shown in Figure 4-11-2. However, they provide the same function same as the drag straps of transferring the load to adjacent CLT panel.   Figure 4-11: Drag Strap and Chord in Brock Commons (Source: UrbanOne) These drag straps and chords are connected to CLT with two rows of 6.4mm diameter and length of 89mm with different orientations. The drag straps and the chords have different thickness and lengths, but the same width with various spacing as shown in Figure 4-12 and the properties listed in Table 4-3. 60  Figure 4-12: Drag Strap Properties Description Table 4-3: Drag Strap Properties Properties Drag Strap in N-S Drag Strap in E-W Chords on slab edge L2- L16 L17 & 18 L2- L 16 L17 & 18 L2- L16 L17 & 18 Spacing (mm) 250 150 50 50 50 50 Length (mm) 7200 7200 1500 3000 1000 2000 Width (mm) 100 100 100 100 100 100 Thickness (mm) 6.4 12.5 6.4 12.5 6.4 12.5 Row Spacing (mm) 50 50 50 50 50 50  4.3.3 Steel to Concrete Connections The transfer of vertical shear from the CLT panels to the concrete core is done by providing ledger angles. The core ledger angles are connected using a steel plate of dimensions 300x250x9.5mm with the concrete using four headed studs with diameter 16mm and 125mm length (225mm gauge). For developing tension capacity, the drag straps have a base plate welded to their concrete core facing side as shown in Figure 4-13. These base plates are connected to the concrete core using threaded bar rods. From level 2-16, 2-20T with a minimum embedment length of 800mm is used whereas on level 17, and 18, 2-25T with an embedment length of 950mm are used. This embedment length also changes the thickness of the base plates. 61  Figure 4-13: Connections with concrete (Adapted from Fast+Epp) For level 2-16, a thick base plate already welded to drag straps have dimensions 260x125x25mm with 32mm diameter holes, is then connected to the concrete core with a steel washer plate of dimensions 40x40x10mm with 22mm diameter hole. For level 17 & 18), the welded thick plate has dimensions of 260x125x38mm with a hole of 38mm diameter which is attached to the concrete core using a steel washer plate of dimensions 50x50x10mm with a diameter of 27mm. Also, the CLT canopy at the ground floor is held up using tension steel rods. 4.4 Discussion on TWB design This chapter briefs the design concept and the materials used for the construction of TWB in detail. The connections between various structural components explain the innovation. The structural layout of TWB is explained before modeling it in a Finite Element software in the next chapter. 62 Chapter 5: Numerical Model 5.1 Overview This research investigates into the possibility of using CLT core in place of the concrete core in UBC TWB. It includes the comparative results from three numerical finite element (FEM) models. Firstly, a base 3D structural model of the actual building with concrete cores and mass-timber superstructure is developed. This model is validated by comparing the modal results with the actual concrete model by the structural engineers (Fast+Epp). Secondly, the concrete cores are replaced by CLT cores with the same dimensions and configuration without any change in the other elements or the layout of the building. The results, with the same dimension CLT cores, demonstrates that a system with the same thickness (450mm) and configuration is not suitable for the design requirements and structural performance. Hence, finally, a modified structure is proposed with supplementary L- shaped perimeter shear walls having a 3m length along with the CLT cores as shown in Figure 5-1. The thickness of the core and shear walls in the proposed structure is 500mm.  Figure 5-1: Plan view of proposed system  63 All 3D structural models were developed using ETABS software [80], a powerful tool for the structural analysis of multi-storey buildings, suitable for the analysis performed in this thesis. Figure 5-2 illustrates the 3D view of three structural models. Firstly, the concrete model, represents the original building with concrete cores. Secondly, in the CLT model, all the other constraints are kept same, but the concrete core is changed to CLT core by changing the material used for the core. In the third model, Refined CLT core or the proposed model, additional L-shaped shear walls having 3m length are added on the perimeter along with the CLT core with an increased thickness of 500mm.  Figure 5-2: Methodology for Research Figure 5-3 shows the FEM model of the concrete core building. The building components are modeled as per the original building as explained in Chapter 4. 64  Figure 5-3: 3D view of Base Model with concrete cores in ETABS The following assumptions were made: • The foundations are not modeled. Hence, the soil structure interaction is neglected. Hence, the base of the supports is assumed to be fixed. • NBCC 2010 is used for load combination patterns, but the seismic loads are taken according to NBCC 2015. A Uniform Hazard Spectra (UHS) confirming NBCC 2015 is used as an input for RSA.  • Only the structural components taking part in the seismic analysis are modeled. Hence, the CLT canopies at ground floor at the entrance of Brock Commons are not modeled. • In the original building, there are steel channels on the roof. For simplification, the roof is modeled with a 3-ply CLT with a thickness of 105mm. 65 • The research is limited to the seismic analysis. Hence, the floor diaphragms are not checked against the gravity loads. Hence, no gravity design has been performed.  • CLT floors are modeled as flat plate concrete slabs by adjusting its stiffness and mass. Therefore, the connections between slab to slab were assumed to be rigid. Also, the connections between vertical columns are also assumed to be rigid, as they are expected to take vertical loads only. • The force modification factors for the platform type CLT shear walls are 2 and 1.5 according to CSA 2016 supplement. However, in this investigation, the core walls are balloon-framed. Hence, the Rd and Ro factors are taken as 1 and 1.3 as suggested in CSA-086. The CLT core connections are assumed to be rigid as well.  5.2 Response Spectrum Analysis (RSA) NBCC outlines three methods for seismic analysis in Part 4 of Division B. Firstly, a linear static analysis is known as the Equivalent Static method is used for regular structures which deals with the fundamental period of the structure. Secondly, Response spectrum analysis (RSA) can be used for taller and irregular structures. In this method, a design spectrum for a location, where the structure is to be erected, is used as an input. The peak values of modal contributions are calculated, and structure’s response is computed after combining these modal results. Thirdly, Time History analysis involves different time histories of ground motions correspondingly similar to the ground motion of a particular site. Time history dynamic analysis can be done as linear as well as in nonlinear method. A nonlinear analysis gives more accurate results than a linear model because it incorporates the inelastic behavior of structure [81]. As the investigation performed for the feasibility of CLT cores in place of concrete cores is, preliminary, so a linear dynamic analysis in the form of modal Response Spectrum Analysis (RSA) is implemented in this thesis.  66 RSA involves an input of acceleration design spectrum to obtain the time periods (modal frequencies), modal shapes and their corresponding contribution [81]. UBC Brock Commons is located in Vancouver. The UHS (Figure 5-4) based on the probability of exceedance 2% in 50 years (return period-2475 years) with a 5% damping for Vancouver is used. The peak ground acceleration is 0.375g. The building is on Site class C. So, as per Table 4.1.8.4 NBCC 2010, the acceleration based site coefficient (Fa), as well as velocity based site coefficient (Fv), are 1 [82]. The importance factor (Ie) is 1 for the building.  Figure 5-4: UHS for Vancouver NBCC 2015 There are three methods to combine the modal responses obtained from the modal analysis. Absolute Sum (ABSSUM) sums up the maximum responses obtained without considering the algebraic sign. This is very unlikely to happen as the peak response in each modal case occurs at 67 different time periods. Hence, it is a conservative method as it gives an upper-bound to the calculated value [81]. If y is the modal parameter (displacement, velocity or acceleration), ABSSUM rule can be written by equation 5-1:  𝑦 = ∑|𝑦𝑖|𝑚𝑎𝑥𝑛𝑖=1 5-1 Square Root of the Sum of Squares (SRSS) rule is a better statistical approach than ABSSUM because it squares, sums and then gives the value of total peak response by taking the square root of modal peak responses. The SRSS method is well suited for well separated modal frequencies [81]. It has the same drawback of giving a positive result. If y is a modal parameter, SRSS can be written by equation 5-2:  𝑦 = √∑ 𝑦𝑖2𝑚𝑎𝑥𝑛𝑖=1 5-2 Complete Quadratic Combination (CQC) is the simplification of the SRSS method. This method is well suited for closely spaced modal frequencies [81]. This method of calculating modal parameters can be written by equation 5-3:  𝑦 = √∑ 𝑦𝑖2 +  ∑ ∑ 𝜌𝑖𝑗𝑛𝑗=1𝑦𝑖𝑦𝑗𝑛𝑖=1𝑛𝑖=1 5-3  The value of ρij can be calculated using various methods and is generally within a value of 0 to 1 [81]. For, combining the modal results in the analysis, CQC is used. 68 5.3 Material Properties  ETABS offers in built material properties for concrete, steel, aluminium, masonry, and rebar. Timber is not yet included, so user-defined properties are assigned according to the specifications of the manufacturer.  The columns in the 3D model except for the columns at ground floor are made of GLT and PSL. The properties of GLT are taken from CSA-086 (2014) [77] whereas Weyerhaeuser design guide is used for properties of PSL [83] and are summarized in Table 5-1. Table 5-1: Properties for GLT and PSL (Source: CSA-086 (2014) and Weyerhaeuser PSL Guide) Parameters (MPa) Glulam PSL Modulus of Elasticity 12,400 15,170 Shear Modulus 530 950 Compression (Parallel to grain) 20.4 20 Tension (Parallel to grain) 30.2 20 Shear (Perpendicular to grain) 2 2  The concrete is used in ground floor columns, floor slabs, transfer slab and concrete core for the initial model required for calibrating results with structural engineers. Transfer slab and concrete core are modeled as thin shell elements. The concrete core has a same plan as the actual building. Also, the elevator openings in the core are also provided in the 3-D model (Figure 5-5-1) as in the case of actual building as shown in Figure 5-5-2.  69  Figure 5-5: Concrete Cores: (1) in ETABS; (2) in Brock Commons (Source: UrbanOne) The characteristic strength of concrete is 35MPa for concrete elements with a modulus of elasticity of 24,850MPa. The concrete structural elements including the core are proportioned accordingly to the Clause 21 in Design of Concrete Structure A23.3-14 [74]. This proportioning of the concrete elements is necessary to account for the inelastic response of structure in case of any seismic event. The gross section of area (Ag) and gross moment of inertia (Ig) are multiplied by a reduction factor 𝜀𝜔 according to equation 5-4 to get the cracked properties: 𝜀𝜔 = 0.6 +𝑃𝑎𝐴𝑔 ∗ 𝑓𝑐≤ 1 5-4 Where Pa is the axial force resulting from factored dead and live load using earthquake load factors, Ag is the gross section of the area and fc is the characteristic strength of concrete.  The floor slabs in the building are made of CLT with a 40mm non-structural concrete topping. These floor slabs are modeled as a flat plate concrete slab and are assigned rigid diaphragm. This is done by calculating the total seismic weight of the CLT, columns and the envelope. These floor 70 slabs are modeled with a thickness of 189mm with concrete as the material. This is done because two-way CLT slab span behaves similarly to the flat plate concrete slab [84]. CLT is modeled as thin shell elements for its usage in the core wall and additional shear walls. The properties of CLT are taken from the Structurlam’s design guide [4] and are summarized in Table 5-2. Table 5-2: Modelling Parameters for CLT (Structurlam Crosslam Guide) Parameters (MPa) 1 2 3 Modulus of Elasticity 4000 8000 500 Shear Modulus 600 500 100 Poison’s Ratio 0.07 0.35 0.35  5.4 Loads & Load Combinations 5.4.1 Gravity Loads Gravity loads on the building are determined according to NBCC 2010 and include the dead and live loads on each floor. Also, dead and snow loads on the roof are included in gravity loads. The dead load applied includes the superimposed dead loads which include the dead loads of materials as well as the partition walls. The typical loading pattern on each floor is shown in Figure 5-6 and is summarized in Table 5-3. However, the roof loading is uniform with no live load but dead and snow loads as summarized in Table 5-3. 71  Figure 5-6: Typical floor loading pattern from Storey 2-18 Table 5-3: Load distribution in 3D model Load Assignment Dead (kPa) Live (kPa) Snow (kPa) Corridor (L2-18) 1.25 4.8 - Rooms (L2-18) 2.25 1.9 - Roof 1 - 1.84  5.4.2 Lateral Loads The horizontal loads or the lateral load includes the earthquake and wind loads acting on the structure. These loads are dependent on the geographic location. Article 4.1.8.11 in NBCC 2010 describes the minimum lateral force on the structure by Equation 5-5 [82]:  𝑉 =𝑆(𝑇𝑎)𝑀𝑣𝐼𝑒𝑊𝑅𝑑𝑅𝑜 5-5 Where S(Ta) = Spectral acceleration value from UHS at the fundamental period of the building; Mv = Factor accounting higher mode effects; Ie = Importance factor; W = Weight of the building; Rd= Ductility Factor in the SFRS, and Ro = Over strength Factor for SFRS.  72 The SFRS in the 3-D model with concrete core walls is partially coupled shear wall in N-S Direction so, Rd and Ro factors are 3.5 and 1.7 respectively. Whereas in E-W direction, the SFRS is a concrete shear wall, so Rd and Ro factors are 3.6 and 1.7 respectively. According to NBCC Table 4.1.8.9, the Rd and Ro factors for CLT balloon framed core walls are not described in NBCC yet. Hence, the Rd and Ro for CLT core walls is taken as 1 and 1.3 [72].  5.4.3 Load Combinations As the gravity loads and lateral loads act on the structure simultaneously, the load combinations are necessary to evaluate the performance of the building. These load cases are multiplied with a specified amplification factor to obtain the combined effect on the building.  NBCC 2010 defines the load combinations as summarized in Table 5-4 for Ultimate Limit State. These combinations have been used in the structural model to access the structural behavior. Table 5-4: Load Combinations in NBCC 2010 Combination 1 1.4D Combination 2 1.25D+1.5L+0.5S 0.9D+1.5L+0.4W Combination 3 1.25D+1.5S+0.5L 0.9D+1.5S+0.4W Combination 4 1.25D+1.4W+0.5L 0.9D+1.4W+0.5S Combination 5 1D+1Ex+0.5L+0.25S 1D+1Ey+0.5L+0.25S  5.5 Results This research provides the comparative results for three models. Firstly, the concrete model, which is the actual building with concrete cores. Secondly, a CLT core with same dimensions and configuration as the concrete core. Lastly, with CLT core walls and additional shear walls 500mm 73 thick. The results are stated in terms of modal response, inter-storey drifts, and base shear. Modal response of a structure obtained from a dynamic seismic analysis describes the behavior of the building under earthquake loading. Modal results are necessary to comprehend the shape and the frequency of the structure when the seismic loads will amplify. Inter-storey drift limit is one of the necessary restraint expressed in most of the building codes. On the other side, base shear is a virtue of the quantification of the earthquake force experienced by the structure at its base.  5.5.1 Modal Results The results of the modal analysis describe the shape of the building and its corresponding frequency or time period when subjected to any shaking during an earthquake. It also narrates the direction of seismic mass participation in each mode. The natural or fundamental time period is the time required to complete one full oscillation. As the time period of a building is inversely proportional to the stiffness and directly proportional to the mass of the building. Hence, buildings with heavier mass and smaller stiffness (flexible) have higher period compared to lower mass and rigid structures [84]. The concrete model is validated for the first three modes with structural engineers, and the results are summarized in Table 5-5Table 5-5:. This calibration is done by modeling the TWB with same material properties for the lateral as well as gravity resisting system in ETABS under same loading patterns.  The time periods for three modes for concrete are calculated on the basis of cracked sections. This is done because the gross properties of the reinforced concrete do not reflect its real behavior due to the action of longitudinal reinforcement [85]. An increase of 40% in the time period is observed due to the cracking because of a decrease in stiffness. 74 Table 5-5: Calibrated Modal Results Mode Structural Engineer Research 1 1.99 1.99 2 1.65 1.85 3 1.35 1.32  By changing concrete to CLT, the period increases because of the decrease in stiffness. Hence, first three modes of CLT model shows an increase in the period. In the proposed model, the total mass increases with the shear walls added on the perimeter. However, the percentage of increase in stiffness is more than the percentage increase in mass. This leads to a lower time period. These results are summarized in Table 5-6Table 5-6:. Only first three modes are studied because their mass participation is higher than all other modes. As these modes have a higher time period or lower frequency, these structure is most likely to exhibit these behavior during any seismic excitation. Table 5-6: Comparative Results of three models Mode Concrete CLT Refined 1 1.99 2.13 1.85 2 1.85 1.87 1.71 3 1.32 1.76 1.50  When a structure is subjected to an earthquake shaking, it can undergo oscillation in the form of translational, rotational or their combination about three major axes, i.e., X, Y, and Z. However, an integral attribute of the modal behavior of a regular building with a symmetrical geometry is having its first two translational and third rotational mode [85]. If a building has lower torsional 75 stiffness it can undergo huge displacements causing damage or total collapse of the structure. Hence, it is a prudent engineering judgement to avoid torsion in first two modes [85]. Figure 5-7 shows the first three modes of the 3D model of TWB with concrete cores. The first mode for the concrete model is pure translational mode towards X-axis with a time period of 1.99s. As the building dimension is longer on the X-axis, the structure behaves translationally in the same direction for its first mode or the fundamental mode. Similarly, the second mode with a time period of 1.85s exhibits pure translational behavior in Y-axis. The third mode is a pure rotational mode with a time period of 1.32s. This behavior is typical for symmetrical buildings.  Figure 5-7: Modal shapes for concrete model: (1) Mode 1=1.99s; (2) Mode 2= 1.85s; (3) Mode 3=1.32s As illustrated in Figure 5-8, when the concrete cores are replaced by CLT cores with same dimensions and configuration, the structure starts behaving with significant torsion in its fundamental mode (2.13s). This is because this structure is not stiff enough to have a translational behavior in its major direction. The second mode with 1.87s behaves translationally in Y-axis; the third mode behaves in a combination of torsion and translation with 1.76s. 76  Figure 5-8: Modal shapes for CLT model: (1) Mode 1=2.13s; (2) Mode 2= 1.87; (3) Mode 3=1.76 In the proposed model, Figure 5-9, the thickness of the CLT core is increased to 500mm. Also, L-shaped perimeter walls are added on all the four edges of the building. This changes the geometry of the structure and makes it stiffer, which eventually decreases the period to 1.85s. The second mode (1.71s) is translational and the third mode (1.50s) has dominant torsion.  Figure 5-9: Modal shapes for refined model: (1) Mode 1=1.85s; (2) Mode 2= 1.71; (3) Mode 3=1.50 77 Mass participation factor is a function of modal shape, building’s mass distribution and the direction of seismic excitation. This percentage determines the contribution of a specific mode to the total response of the structure [86]. Figure 5-10 shows that both the concrete and the refined models have more than 60% of mass-participation in X-direction in the first mode. On the other hand, CLT model has distributed translational motion between first and third mode.   Figure 5-10: Mass Participation in X-Direction Figure 5-11 illustrates the mass participation in Y-axis. All three models performed translationally in Y-axis in the second mode because of the shorter direction. Figure 5-12 shows that about 30% of torsion is present in CLT model in the first mode, thus making it unstable. The proposed model has more than 60% in torsion in the third mode making the torsion a dominant third mode.  78  Figure 5-11: Mass Participation in Y-Direction  Figure 5-12: Mass Participation in Z-Direction 79 5.5.2 Inter-storey Drift Table 4.1.2.1 in NBCC 2010 limits the maximum deflection of a building according to its importance factor. The importance factor (𝐼𝑒) for TWB is 1 and the maximum permissible allowed drift according to NBCC is 2.5% of the storey height [82]. This limit is independent of the material used for the building. The inter-storey drifts obtained from the linear dynamic analysis (∆𝑥𝑒) are amplified by Rd and Ro as shown in equation 5-6 to obtain the realistic inter-storey drifts (∆𝑥). Inter-storey drifts are expressed in terms of storey height: ∆𝑥= ∆𝑥𝑒𝑅𝑑𝑅𝑜𝐼𝑒 5-6 Figure 5-13 shows the drift percentage in X-axis. It is observed that the concrete model has lower inter-storey drifts as compared to both other models.  Figure 5-13: Drift Percentage in X-Axis 80 Figure 5-14 suggests that the refined model exhibits lower inter-storey drift than CLT model in Y-axis. Hence, this refined solution has lesser inter-storey displacement under seismic loads.  Figure 5-14: Drift Percentage in Y-Axis Although all three models have inter-storey drift percentage under than maximum limit prescribed in NBCC. However, the proposed model behaves closely to that of the concrete model. 5.5.3 Base Shear The amount of lateral force acting at the base of a structure is termed as base shear. Base shear is proportional to the weight and stiffness of the structure. CLT is six times lighter than reinforced concrete. Hence, using CLT cores in place of concrete cores decreases the seismic demand on the building. Table 5-7 shows the seismic weight of the structure and the cores for all three models.  81 Table 5-7: Seismic Weight Weight Concrete (kN) CLT (kN) Refined (kN) Core + Shear walls 37750 11200 15000 Whole Building 135800 104375 110550  The results show that the concrete cores represent the 28% of the total seismic weight of the structure whereas the accumulative weight for CLT core as well as additional shear walls in the proposed system combines for almost 14% of the weight. Also, the proposed system is approximately 20% lighter than the original TWB. This decrease in weight is reflected in the base shear of three models in Table 5-8.  The base shear values of the concrete model are calculated using the un-cracked properties [87]. The model with additional shear walls has approximately 35% less base shear in X-direction and 40% less base shear in Y-Direction compared to the concrete model. Table 5-8: Comparative Base shear Base Shear Concrete (kN) CLT (kN) Refined (kN) X-Direction 14650 5,400 9,780 Y-Direction 20110 9,370 11,750  5.6 Discussion on proposed changes to TWB The results presented from the linear dynamic analysis in the form of RSA clearly show that when CLT core with same dimensions as of concrete core is used, the system becomes unstable in the first mode (having dominant torsion). The first mode has the highest contribution to the total response of the structure because of a higher percentage of seismic mass participation and lower 82 frequency. The structural performance is enhanced with suitable placement of additional shear walls on the edges of the building. The increased thickness of the CLT core and assignment of additional CLT shear walls on the perimeter of the building enables the structure to behave stiffer. Also, the results presented fulfills the design criteria for Canadian code. The inter-storey drift, one of the fundamental parameter for design performance is substantially reduced in the minor direction. The total weight of the structure decreases by eliminating concrete from the cores and substituting it with CLT and additional CLT shear walls. A considerable reduction in base shear due to the lighter weight of CLT core is observed. 5.7 Further benefits of using CLT cores in TWB Along with the structural performance, the proposed solution for using CLT cores in place of concrete could have a significant influence on the schedule and thus, on the total project cost. The reduced mass could have reduced the dimensions of raft foundations under the cores. As the construction of a reinforced concrete shear wall needs formwork, reinforcing, pouring and then curing for almost a month. This can be eliminated by the technique of prefabrication in the case of engineered timber shear walls. The construction of TWB validates this notion by completing 17 stories of the superstructure, and the prefabricated envelope in less than ten weeks.   Also, the proposed model would have a considerable influence on the overall sustainability parameters. The proposed solution has approximately 42% potential carbon benefit than TWB and as shown in Figure 5-15.  83  Figure 5-15: Potential Environmental Benefit from proposed system  84 Chapter 6: Conclusions This thesis discusses the Brock Commons Building at UBC. The building’s approval procedure is explained to understand the regulatory procedure for the construction of a unique project with unconventional materials. The structural system of TWB which forms the basis of the numerical modeling is described. The research comprises of calibration of the actual building model with the structural engineers, then substituting the concrete cores with CLT cores and further refining the structural system to obtain a feasible solution for seismic design.  The results demonstrate that CLT cores with the same configuration and dimensions as the original concrete cores leads to torsional behavior in the fundamental mode and higher storey drifts. Thus, TWB a CLT core with same dimensions of the concrete core is not feasible. However, if the wall thickness of the CLT cores is increased and additional CLT shear walls are used, then a pure mass-timber building fulfills the performance criteria for linear dynamic seismic analysis according to NBCC. The main advantages of such a solution are the reduced construction time and additional environmental benefits. The results presented in this thesis are based on linear seismic analyses only. Nonlinear time history as well as a dynamic wind analyses are required for the comprehensive understanding of the proposed structural system. Modeling the CLT core as a rigid single entity without any steel connections was a chosen in this research. The ductility of the structure will increase when appropriate models for the steel connections are used, buts stiffness will decrease. Therefore, additional research for suitable CLT connections as well as base of the core to the concrete connections and subsequent numerical analyses are needed. Increasing the core stiffness with the pre-stressed technology for joining the CLT panels should be studied.  85 Bibliography [1] Natural Resources Canada, “2013 Tall Wood Structure Demonstration Projects- A request for an Expression Of Interest (EOI),” 2013. [Online]. Available: http://www.cwcdemoproject.ca/en/faqs.php#. [Accessed: 06-Apr-2016]. [2] A. 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