Design for Lateral Loads/Systems
343359 - New Studies to Assess and Improve Seismic Torsion Design Provisions
Friday, April 20
11:00 AM - 12:30 PM
This study evaluates ASCE/SEI 7 design requirements for seismic-induced torsion in buildings, using collapse resistance as the primary metric. Two of the main objectives of the study are to: (1) determine the effectiveness of ASCE/SEI 7 torsion design requirements for preventing seismic-induced collapse of torsionally irregular buildings, (2) propose modifications to ASCE/SEI 7 torsion design requirements to eliminate deficiencies and/or unnecessary sources conservatism.
The study finds that ASCE/SEI 7 torsion design provisions are generally conservative for most building configurations—exceptions are some buildings that rely heavily on lines of lateral resistance orthogonal to the design earthquake force to resist torsion moments. It is also observed that the ASCE/SEI 7 requirements that drift and stability limits are satisfied at the building edge are important for ensuring that torsionally sensitive buildings have adequate collapse resistance.
Based on preliminary findings, the following modifications to ASCE/SEI 7 would provide better consistency in collapse resistance over a large range of building configurations and degrees of torsional irregularity: (1) altering the triggers for requiring a redundancy factor of 1.3 so that fewer buildings are affected, (2) considering the effects of the orthogonal ground motion component for computing torsion moments in buildings classified as extremely torsionally irregular, (3) reducing the accidental torsion amplification factor.
Furthermore, the study demonstrates that buildings classified as extremely torsionally irregular need not be banned from Seismic Design Categories E and F, as long as the lateral system is proportioned properly; neither should the equivalent lateral force procedure be prohibited on the basis of torsional irregularity.
This study is of national significance, as it is anticipated to directly affect future editions of ASCE/SEI 7. The content of this study is of interest to building code developers, engineers who use ASCE/SEI 7 or codes that reference it, and earthquake engineering researchers. The audience will take away the following: (1) a better understanding of how torsion design provisions in ASCE/SEI 7-16 affect building collapse resistance, including when they are conservative vs. unconservative, (2) design strategies for obtaining more consistent performance among torsionally regular and irregular buildings, (3) insight into potential revisions of future editions of ASCE/SEI 7 and their technical basis.
The study is part of a project managed by the Applied Technology Council (ATC) under FEMA contract HSFE60-12-D-0242 task order HSFE60-16-J-0223, entitled “ATC-123: Improving Seismic Design of Buildings with Configuration Irregularities.” Any opinions are those of the authors and not necessarily ATC or FEMA.