TORSIONAL RESPONSE OF UNSYMMETRIC BUILDINGS TO INCOHERENT GROUND MOTIONS

The behavior of unsymmetric buildings excited by incoherent seismic gr ound motions is studied, and the assessments and information reported make it possible to gain valuable insight into the resulting dynamic r esponse. The effects of the ground motion incoherence and of the struc tural eccentricity are first considered independently of one another, and attempts are made to identify their relative importance. The torsi onal effects induced by the incoherence actions are interpreted in ter ms of normalized accidental eccentricities; and comparisons are made o f these eccentricities with the value of 0.05 specified by design code s. Such comparisons form a frame of reference for the assessment of th e importance of the torsional effects resulting from the ground motion incoherence. The coupled effects of incoherence and eccentricity are then considered, and it is reported that they can be approximately rep resented by proper combinations of the corresponding independent effec ts, under conditions that are of practical importance. This approximat ion leads to the definition of effective modified eccentricities, whic h can be useful for the appreciation of the torsional effects induced in building structures by earthquakes.