UBC Faculty Research and Publications

Effects of Basins during Subduction Earthquakes on the Collapse Fragility of Existing Tall Steel Buildings Molina Hutt, C; Marafi, N.; Berman, J.; Eberhard, M. Mar 29, 2018

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Eleventh U.S. National Conference on Earthquake EngineeringIntegrating Science, Engineering & PolicyJune 25-29, 2018Los Angeles, CaliforniaEffects of Basins during Subduction Earthquakes on the CollapseFragility of Existing Tall Steel BuildingsC. Molina Hutt1, N. Marafi2, J. Berman3 and M. Eberhard4ABSTRACTSedimentary basins tend to increase the intensity of earthquake ground motions at long periodsand the resulting damage in tall structures. However, the effects of basin amplification on spectralacceleration are not directly included in the seismic hazard maps used in US building codes. Thisissue is particularly important in the Pacific Northwest, which has several deep basins and thepotential for large-magnitude subduction earthquakes that can dominate the seismic hazard at longperiods. This study aims to evaluate the impact of deep basins and long-duration shaking on theresponse of existing tall steel moment-frames by means of a 50-story archetype office buildingdesigned per the 1973 Uniform Building Code. Suites of ground motions, developed to investigatethe impacts of magnitude 9 Cascadia subduction zone earthquakes on the Pacific Northwest, areused to evaluate the impact of basin effects by comparing seismic demands for records insideversus outside a basin. The study also isolates the effect of duration by using spectrally equivalentmotions to the inside and outside basin sets from shorter duration ground motions recorded duringcrustal earthquakes. A suite of ground motions consistent with the 2475 year return period UHS inSeattle is also considered in order to study the impact of ground motions whose spectra match theUHS versus ground motions consistent with large-magnitude subduction earthquakes. Anincremental dynamic analysis approach is carried out for each ground motion set in order toevaluate impacts on the expected collapse fragility of the archetype building. The results indicatethat for the archetype 50-story 1970s steel moment frame building: (i) inside basin motions aremore likely to cause collapse than the outside basin motion, (ii) duration effects have negligibleimpact on the relative collapse capacity, and (iii) UHS motions are the least likely to causecollapse.1Assistant Professor, Dept. of Civil Engineering, University of British Columbia, Vancouver, BC Canada V6T 1Z4carlos.molinahutt@civil.ubc.ca2Researcher, 3Associate Professor, 4Professor , Dept. of Civil and Environmental Engineering, University ofWashington, Seattle, WA 98195Molina Hutt C., Marafi N., Berman J. and Eberhard M. Effects of Basins during Subduction Earthquakes on theCollapse Fragility of Existing Tall Steel Buildings. Proceedings of the 11th National Conference in EarthquakeEngineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018.IntroductionSedimentary basins tend to increase the intensity of earthquake ground motions at long periodsand the resulting damage in tall structures. However, the effects of basin amplification onspectral acceleration are not directly included in the seismic hazard maps used in US buildingcodes. Several GMPEs recognize that these amplifications are largest at long periods, whichaffect the response of tall structures. This issue is particularly important in the Pacific Northwest,which has several deep basins and the potential for large-magnitude subduction earthquakes thatcan dominate the seismic hazard at long periods. Furthermore, conventional seismic designprocedures do not account for the effects of ground motion duration, associated with largemagnitude earthquakes, which are also known to affect structural response.This study aims to evaluate the impact of deep basins and long-duration shaking on theresponse of existing tall steel moment-frame buildings. Specifically, the study considers steelmoment-frame buildings constructed during the late 1960's through mid-1990's, with the type ofwelded connections that experienced sudden brittle fractures during the 1994 Northridgeearthquake. These buildings constitute a significant proportion of tall buildings in Seattle (andother cities on the US West Coast) and were designed following an equivalent lateral forceprocedure based on the first mode translation response, without capacity design principles thatprotect against story mechanisms, and lower base-shear strengths than those specified in modernbuilding codes.Suites of ground motions developed to investigate the impacts of magnitude 9 Cascadiasubduction zone earthquakes on the Pacific Northwest, are used in this study. These records areused to evaluate the impact of basin effects on tall buildings by comparing seismic demands forinside and outside basin ground motions suites. A 50-story steel moment-resisting frame officebuilding designed per the 1973 Uniform Building Code is used as an archetype for this study.The building is intended to represent the state of design and construction practice for tallbuildings from the mid-1970s to the mid-1980s. The deformation demands for the building areevaluated with nonlinear dynamic analysis. An incremental dynamic analysis approach is carriedout to evaluate impact of basin and duration effects on the expected collapse fragility of thesebuildings. The study also isolates the effect of duration by using spectrally equivalent motions tothe inside and outside basin sets from shorter duration ground motions recorded during crustalearthquakes. A ground motion suite consistent with the 2475 year return period UHS in Seattle isalso considered to study the collapse fragility associated with ground motions whose spectramatch the UHS versus ground motions consistent with large-magnitude subduction earthquakes.Archetype Tall Steel Moment-Frame BuildingSteel moment-frame buildings represent a significant proportion of the existing tall buildingstock in Seattle. Figure 1 shows the construction material for buildings greater than 20-storiesbuilt in Seattle between 1960 and 1990 [1]. While the data in Figure 1 does not specify the lateralresisting system type, a review of existing building drawings suggests that steel moment-frameswere the most prevalent lateral resisting system type for these tall buildings in pre 1990sconstruction. This observation coincides with a review of existing pre-1990s tall buildings inother cities on the US west coast, such as San Francisco [2].Figure 1: Construction material of tall buildings built between 1960 and 1990 in Seattle.Based on this information, a 50-story steel moment-frame archetype building is selectedfor this study. The archetype building is designed in accordance with the provisions of UBC1973 [3] and the SEAOC Bluebook of 1973 [4], which was commonly employed to supplementminimum design requirements. The building occupancy is that of a commercial office, with twolevels for mechanical equipment, one at mid-height, and one at the top floor. The frames aremade up of built-up box columns (denoted R in Figure 2), wide flange beams, and welded beam-to-column connections. Typical story heights and beam spans are 3.8 m and 8.5 m, respectively.In the 1970s, it was customary to have moment connections in all beam-to-column intersections,as illustrated in Figure 2a. The resulting section sizes for a typical frame are shown in Figure 2b.The archetype building has a total building dead load of 784,220 kN. The design wind andseismic base shears are equal to 1.80% and 1.96% of total building dead load, respectively. Thewind and seismic drift limits used in design are 0.0025 and 0.005 respectively. For a detaileddescription of the design method and assumptions please refer to [5].There are many requirements in modern design standards, which were not considered inthe designs of the 1970s, that drastically improve seismic performance, including: i) responsespectrum analysis method as opposed to equivalent lateral force procedure based on the firstmode translation response; ii) consideration of lateral forces acting simultaneously in bothbuilding directions; iii) consideration of accidental torsion; iv) minimum base shear requirements(scaling of forces and displacements); v) p-delta effects (scaling of forces and displacements); vi)consideration of vertical and horizontal irregularities; vii) strong column weak beamconsideration; viii) panel zone consideration; ix) capacity design principles; and x) prequalifiedseismic connection details. In addition to these design deficiencies, a critical detail expected tonegatively affect the seismic performance of the archetype building considered in this study isthat of fracture prone-moment connections. The switch in the weld process that led to welds withvery low toughness, as evidenced by fractures observed in the 1994 Northridge earthquake, tookplace in the mid-1960s [6]. Therefore, it is assumed that that fracture-prone pre-Northridgemoment connections are common in designs from the 1970s.In order to conduct nonlinear dynamic analysis of the archetype building, finite elementmodels capable of capturing the response of all structural elements that significantly contributeto the strength and stiffness of the system are developed. A 2D numerical model of arepresentative frame, as illustrated in Figure 2, is developed in LS-DYNA [7]. The first (T1),second (T2) and third (T3) modes of the representative 2D frame are 5.48, 2.12 and 1.26 seconds,respectively.Stories: 20-29 Stories: 30-39 Stories: 40-49 Stories: ≥ 50Key structural elements include beams, columns and panel zones. Beams are modelled aslumped plasticity beam elements following recommendations in [8], which propose empiricalrelationships for modelling steel beams, based on a large database of experimental tests. Toaccount for fracture in the moment connections, a plastic rotation threshold at which fracture isset to occur in the connections is introduced according to ASCE 41 recommendations [9]. Theimpact of introducing the plastic rotation threshold at fracture is observed by comparing thehysteretic response for a post- versus pre-Northridge sample moment connection, illustrated inFigure 2c versus 2d.Columns are modelled as lumped plasticity beam elements with yield surfaces capable ofcapturing interactions between bending moment and axial force following the recommendationsin [10], calibrated based on experimental tests of tubular steel columns in [11], which account forgreater rates of degradation in the moment-rotation response of columns as a function of axialload-to-capacity ratios. A sample moment-rotation response of a column subject to an appliedaxial load-to-capacity ratio of 0.3 is illustrated in Figure 2e. Panel zones are modeled using theKrawinkler model as outlined in [12], which incorporates an assembly of rigid links androtational springs to represent the true dimensions of the panel zone. A sample shear force-deformation response of a panel zone is illustrated in Figure 2f. Where possible, analyticalcomponent models are verified against experimental data as illustrated in Figures 2c, 2e and 2f.Ground Motion Suite DevelopmentThe Cascadia subduction zone is capable of producing Mw 8–9 earthquakes along the plateinterface (known as interface earthquakes) and deeper Mw 6–7 earthquakes within the slab(intraslab earthquakes). Geologic and historic evidence indicates that the Cascadia region hasbeen subjected to subduction-zone interface earthquakes up to Mw 9 [14, 15] and that future greatearthquakes are inevitable. Suites of ground motions developed in [16] to investigate the impactsof Mw 9 Cascadia subduction zone earthquakes on the Pacific Northwest are used in this study.Because recordings from the Cascadia subduction zone are not available, [16] usesground motion recordings from basins in Japan that have similar properties as the Puget Lowlandbasin. In this study, only records from the Konsen basin, located in Eastern Hokkaido, are usedin the evaluation. Ground motion recordings from two large magnitude subduction interfaceearthquakes (2003 Mw 8.3 Tokachi-Oki earthquake, 2011 Mw 9.0 Tohoku earthquake) and fourlower-magnitude (Mw 6.7-7.9) subduction interface earthquakes are used. These records are usedto evaluate the impact of basin effects on tall buildings by comparing seismic demands for aninside and outside basin suite of ground motions.Large magnitude subduction earthquakes tend to have long durations. To isolates theeffect of duration, spectrally equivalent motions to the inside and outside basin sets aredeveloped from shorter duration ground motions recorded during crustal earthquakes. Thesespectrally equivalent records are selected from the NGA-West-2 Database to match the Konsenmotion spectra between 0.5s to 10s. Linearly scaled records are selected to match the targetspectrum using a maximum scaled factor of 5, records with PGA more than 0.05g and closestdistances greater than 5 km.Figure 2: Archetype steel moment-frame building (a) plan and (b) elevation. Sample hystereticresponse of key structural components: (c, d) beams, (e) columns and (f) panel zones.-6,000-5,000-4,000-3,000-2,000-1,00001,0002,0003,0004,0005,0006,000-0.075 -0.05 -0.025 0 0.025 0.05 0.075Moment(kN-m)Rotation (rad)ASCE 41-13 BackboneNumerical SimulationMy = 3742 kN-mθp = 0.016 radθpc = 0.113 radΛ = 0.835 radW36x150θp-fract = 0.0045 rad-2,000-1,500-1,000-50005001,0001,5002,000-0.010 -0.005 0.000 0.005 0.010Moment(kN-m)Rotation (rad)Experimental DataNumerical SimulationColumn:H300x300x16x25(mm)Beam:H600x200x11x17(mm)Specimen:TS9900b-TCLRB1γy = 0.0012 radγp = 0.0048 radMy = 850 kN-mMp = 1050 kN-m-6,000-5,000-4,000-3,000-2,000-1,00001,0002,0003,0004,0005,0006,000-0.075 -0.05 -0.025 0 0.025 0.05 0.075Moment(kN-m)Rotation (rad)Experimental DataNumerical SimuationMy = 3742 kN-mθp = 0.016 radθpc = 0.113 radΛ = 0.835 radSpecimen: BD-0024-C2RW36x150-500-400-300-200-1000100200300400500-0.15 -0.10 -0.05 0.00 0.05 0.10 0.15Moment(kN-m)Rotation (rad)Experimental DataNumerical SimulationMy = 240 kN-mθp = 0.021 radθpc = 0.147 radΛ = 0.743 radPn = 3,365 kNALR = 0.3Specimen: S1703Section: 200x200x17x17(mm)(a)(b)(c)(d)(e)(f)The effects of the shape of the response spectrum are not typically considered inconventional design. However, recent studies have shown that spectral shape influences collapseprobabilities for structures [16, 17, 18]. A suite of ground motions consistent with the 2475 yearreturn period UHS in Seattle is also developed using similar criteria to that of the spectrallyequivalent motion suites. This suite enables a comparison between the collapse fragilityassociated with ground motions whose spectra match the UHS versus ground motions consistentwith large magnitude subduction earthquakes.The Inside Basin (IB) and its NGA equivalent (NGA IB) suites consist of 6 bi-directionalground motions. The Outside Basin (OB) and its NGA equivalent (NGA OB) suites consist of 9bi-directional ground motions. The UHS suite consists of 7 unidirectional ground motions.Individual record spectra and the average ground motion spectra for all suites are illustrated inFigure 3. The selected ground motions are input at the base of the analytical model, which isassumed to be fixed at its base. A damping ratio of 2.5% is assumed in the analysis [13]. (a) (b)(c) (d)(e) (f)Figure 3: Individual record and average suite spectra for all ground motion suites (a though e)and comparison of average suite spectra (f) 2 4 6 8 10SpectralAcceleration(g)Period (seconds)IB0.000.250.500.751.001.251.500 2 4 6 8 10SpectralAcceleration(g)Period (seconds)OB0.000.250.500.751.001.251.500.00 2.00 4.00 6.00 8.00 10.00SpectralAcceleration(g)Period (seconds)NGA IB0.000.250.500.751.001.251.500.00 2.00 4.00 6.00 8.00 10.00SpectralAcceleration(g)Period (seconds)NGA OB0.000.250.500.751.001.251.500.00 2.00 4.00 6.00 8.00 10.00SpectralAcceleration(g)Period (seconds)UHS0.010.101.0010.000.01 0.1 1 10SpectralAcceleration(g)Period (seconds)IBOBNGA IBNGA OBUHSIncremental Dynamic Analysis ResultsAn incremental dynamic analysis approach is carried out for all ground motion suites considered:IB, OB, NGA IB, NGA OB and UHS. Incremental dynamic analysis involves scaling eachground motion in a suite until it causes collapse of the structure [19]. Each horizontal componentof ground motion is individually applied to the 2D frame model. The ground motions areincreasingly scaled until sidesway collapse occurs. This occurs when the maximum storyinterstory drift ratio (IDR) increases without bounds (IDA curve becomes flat) as seen in Figure4. To approximately account for 3D ground motion effects (i.e., the maximum ground motioncomponent), the lower collapse capacity from each pair of motions is recorded as the buildingcollapse capacity [17]. This process produces a set of values for a given intensity measure, suchas the average spectral acceleration (SAavg), associated with the onset of collapse for each groundmotion, as illustrated in Figures 4a through 4e. Fragility function parameters can then beestimated from this data as outlined in [20]. The resulting collapse fragilities for each groundmotion suite are summarized in Figure 4f.Figure 4: Incremental dynamic analysis results for all ground motion suites (a though e) andassociated collapse fragilities (f). 0.02 0.04 0.06 0.08 0.1SAavg(g)Maximum IDR (-)IBµ=0.12; β=0.10(a) 0.02 0.04 0.06 0.08 0.1SAavg(g)Maximum IDR (-)OBµ=0.13; β=0.32(b) 0.02 0.04 0.06 0.08 0.1SAavg(g)Maximum IDR (-)NGA IBµ=0.12; β=0.17(c) 0.02 0.04 0.06 0.08 0.1SAavg(g)Maximum IDR (-)NGA OBµ=0.13; β=0.26(d) 0.02 0.04 0.06 0.08 0.1SAT1(g)Maximum IDR (-)UHSµ=0.18; β=0.16(e) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5ProbabilityofCollapseSAavg (g)IBOBNGA IBNGA OBUHS(f)μPrevious studies suggest that SAavg provides more stable collapse risk estimates than themore traditional spectral acceleration at the fundamental period (SAT1) [18]. Additionally, tallbuildings generally have responses that are sensitive to excitation at a range of periods, bothshorter (higher mode effects contributions to the response) and longer (lengthened periods due tononlinear behaviour). SAavg is selected as the intensity measure of choice as it enablesconsideration of spectral acceleration (SA) demands at a range of periods. Consideration ofhigher mode effects is particularly important for the 50-story 1970s archetype steel moment-frame considered because its design was strictly based on a first mode translational response [5].Nonlinear dynamic analysis results illustrate a non-uniform distribution of IDR up the buildingheight, with large concentrations of deformation often observed in middle and upper stories, dueto contributions from higher modes. In this study SAavg is defined as the geometric mean of 5%damped SA values at the following periods: T1, T2, T3 and Tmax. Previous studies recommend aperiod range of 0.2T1 and 3T1 [18]. The selected range meets the lower bound SA. However, anupper bound of 10 seconds is used for practical reasons, primarily due to a general lack ofground motion prediction equations that predict spectral values at periods longer than 10 s [21].Discussion and ConclusionsThe incremental dynamic analysis results and the associated collapse fragilities for all groundmotion suites vary due to the characteristics of the recordings in each set. The ratio of collapsecapacities between two ground motion sets is known as the relative collapse capacity. In thisstudy, this ratio enables an evaluation of the effects of (i) deep basins, (ii) duration, and (iii) UHSon the structural collapse of the 50-story 1970s tall steel moment-frame archetype building. Theresulting relative collapse capacities are summarized in Table 1.Table 1: Relative Collapse Capacities.Basin Effects Duration Effects UHS EffectsμOB / μIB = 1.08 μNGA IB / μIB = 1.0 μUHS / μIB = 1.50μNGA OB / μNGA IB = 1.08 μNGA OB / μOB = 1.0 μUHS / μOB = 1.38The basin amplification phenomenon is evaluated by the relative collapse capacity of theoutside (OB and NGA OB) to the inside basin (IB and NGA IB) ground motion suites. Theresults indicate that the inside of basin ground motions are more likely to cause collapse than theoutside of basin motions. The relative collapse capacity of 1.08 coincides with the results of [16],which estimated a mean relative collapse capacity of 1.18 for outside-to-inside basin motions inthe Konsen basin. The study results in [16] considered 30 archetype buildings with afundamental period range from 0.42 to 2.36 seconds. The results of this study suggest thatsimilar trends may apply at longer periods.The effects duration are evaluated by comparing the relative collapse capacity of theinside and outside basin ground motion suites to their corresponding spectrally equivalent NGAsuites. The results indicate that duration effects have a limited impact on the collapse fragility ofthe 50-story steel moment-frame archetype building considered in this study. These observationsare in agreement with the results of [22], which suggest that duration effects have greater impactin the collapse fragilities of modern ductile buildings than older buildings designed per outdatedcodes, such as the 1970s archetype considered in this study.UHS, derived from probabilistic seismic hazard curves, accounts for the contributions ofall seismic sources that may affect a site, but are usually not representative of any oneearthquake. Actual time history records show significant variations in spectral ordinates fromthat of the UHS. Subduction earthquakes are particularly important in the Pacific Northwest asthey dominate the seismic hazard, particularly for long period structures. To quantify thedifferences in ground motion characteristics that affect structural collapse, ground motionswhose spectral shape is consistent with the 2475 year return period UHS (contributions of allseismic sources) are compared against the inside and outside of basin ground motions (large-magnitude subduction earthquakes).The relative collapse capacities of the UHS suite versus theinside and outside of basin suites are 1.50 and 1.38 respectively. These results are consistent withthe results of [16], where the 30 archetype buildings previously discussed, have a mean relativecollapse capacity (UHS versus inside basin) of 1.41.In conclusion, for the archetype 50-story 1970s steel moment-frame building, (i) insidebasin motions are more likely to cause collapse than outside basin motions, (ii) duration effectshave negligible impact on the relative collapse capacity, and (iii) UHS motions are the leastlikely to cause collapse. While the trends identified are consistent with the results of previousstudies, future work should focus on validating these observations particularly related to basineffects, for instance by carrying out similar evaluations using ground motions recorded in basinsother than Konsen, and by carrying out a comparative study of a modern code-conformingmoment frame to evaluate whether these trends hold in an equivalent system with greaterductility.References1. 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