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Inelastic response of torsionally unbalanced multistorey shearwall buildings designed using elastic static and dynamic analyses Sewell, Cheryl Dale

Abstract

Damage to buildings during recent earthquakes caused by increased torsional response supports the need to improve upon the existing building code design guidelines through developing a better understanding of the response of asymmetric buildings with the intent to restrict the construction of torsionally precarious structures. The effects of torsion on building response is a complex problem for even single storey structures because so many parameters are involved in the description of linear and nonlinear torsional response. Discrepancies exist between the results of many previous studies due to the number of factors governing torsional response. Researchers also have varying opinions as to how to effectively incorporate torsional effects into analytical models for building design. These controversies contribute to the fact that there are wide variations between the torsional provisions of major world design codes. Current building codes torsional provisions are only applicable to buildings which are essentially uniform vertically with relatively symmetric floor plans. Most studies examining torsional response of multistorey buildings focus on shear frame structures. This study investigates the adequacy of elastic design methods to predict and control the increased displacement and ductility demand on edge-elements of vertically uniform, multistorey, shear core buildings, designed to yield in flexure, with varying degrees of asymmetric stiffness distribution. A comparison is made between the elastic and nonlinear time history response of models designed using three elastic methods of determining element strength; the NBCC static torsional provisions (NBC), revised static torsional provisions proposed by Humar and Kumar (H/K), and a dynamic analysis with a statically applied torsional moment of 0.1b (Dyn+Tl) where b is the length of the building perpendicular to the direction of earthquake motion. The elastic static methods grossly overestimate nonlinear displacements of elements on both the stiff- and flexible-edges for torsionally flexible structures. The elastic response spectrum analysis (RSA) with shifted centre of mass (CM) best estimates inelastic displacements for all elements. Inelastic displacements of stiff- and flexible-edge elements generally increase with increasing torsional flexibility for structures with a torsional to lateral frequency ratio, Q < 1. Deformation demand increases with the magnitude of static stiffness eccentricity for the flexible-edge elements. The inelastic displacements of stiff-edge elements of torsionally stiff structures (for Q = 1.25) increase for the Dyn+Tl and sometimes for the H/K design method, leading to large ductility demands for these elements. The NBC design method best controls the displacements and, therefore, ductility demand of stiff wall elements at Ω = 1.25. The displacement response of structures with a lateral period > 2 seconds is relatively insensitive to the design method used for determining element strength distribution. The ductility demand of the flexible wall elements is below the design target for all methods of design. Dynamic magnification of base shear and storey shear forces, found by the nonlinear analyses, due to the contribution from higher modes can be more than double those predicted by elastic analysis, regardless of the elastic method employed in determining wall strengths. Also, the inelastic moment demand from the nonlinear dynamic time history analyses varies substantially from that predicted by the elastic analyses. Higher mode effects are evident in the moment and shear envelopes of the stiff and flexible walls and are more pronounced for the buildings with a lateral period > 2 seconds.

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