AN ALTERNATIVE PROCEDURE FOR ACCIDENTAL ECCENTRICITY IN DYNAMIC MODAL ANALYSES OF BUILDINGS

SUMMARY Most building codes enforce the inclusion of accidental torsion in the equivalent static load and dynamic analyses of asymmetric buildings. The computations of accidental torsional moments in the equivalent static load procedure are straightforward and already implemented in most of building analysis tools. However, the application of accidental torsion in dynamic analysis can be performed in one of the following two basic approaches: 1) Shifting the centre of mass by the required amount of eccentricity in either direction. This will lead to change in global stiffness matrix of the system, therefore natural frequencies and modal parameters are needed to be computed for each eccentricity cases. 2) Run static analyses considering torques at each storey level to approximate accidental torsion for each eccentricity cases then combine the results with the dynamic load results. In this study an alternative procedure for the application of accidental torsion in dynamic modal superposition technique by modification of global force vectors to include the effect of accidental torsion for each modal shape separately. The proposed method has been applied to three sample multi-storey buildings and the results are compared with those obtained by using conventional approaches.