ASCE 7-10 allows for two methods of considering accidental torsion when conducting a modal response spectrum analysis. The first method is to apply a static accidental torsion moment to the modal results calculated from eigenvalue analysis where the mass is modeled at the center of mass. The second method is to physically offset the center of mass and conduct the eigenvalue analysis. Research has shown that the second method is potentially very unconservative in extremely torsionally irregular structures - the modal response can become uncoupled from the translational response and not directly considered in the design. Consequently, the second method can produce designs that are much weaker and have much higher collapse rates than the first method. There is a supplement for ASCE 7-16 planned to address this issue. However this article is intended to present the problem, and to encourage practicing engineers to use the first method in their designs.
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