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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.
Item Metadata
Title |
Inelastic response of torsionally unbalanced multistorey shearwall buildings designed using elastic static and dynamic analyses
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2000
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Description |
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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Extent |
7356533 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-07-20
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0063507
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2000-11
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.