Inelastic Torsion for Preliminary Seismic Design

Because of torsion, the demanded strength and interstory drift at certain parts of the structure increase above those required by just translational deformation. These effects differ for elastic and inelastic behavior, tending to be larger when inelastic behavior involves a torsional mechanism of the structure. A clear contradiction exists in the present consideration of torsion effects during the design process because torsion effects are elastically considered, while most of the seismic code design methodology is based on the safety level earthquake (EQ). This paper shows that for a special class of buildings, the reduction in the building strength resulting from inelastic torsion can be obtained using the classical theorems of plastic analysis. By analyzing an auxiliary structure, a simplified formula for the reduction in strength due to inelastic torsion is obtained. Using this simplified formula, guidelines to control inelastic torsion during preliminary seismic design are offered.