How Collapse Potential is Affected by the Method of Considering Accidental Torsion

Structural Engineers have long observed that torsional building response is an indicator of earthquake collapse risk. The Building Code’s explicit treatment of torsion dates back at least to the 1961 Uniform Building Code (UBC), which introduced the requirement of adding 5% eccentricity to any inherent torsion when distributing lateral earthquake forces to the vertical seismic force-resisting elements. Although today’s code includes additional penalties for torsionally irregular structures, the treatment of “accidental torsion” remains much the same. This often-maligned but critically important provision prohibits the design of cruciform-type structures without any torsional strength. It also offers increased collapse protection by indirectly accounting for the non-uniform degradation of the vertical seismic force-resisting elements that occur in the true non-linear response of structures. …