Torsional Response of Unsymmetric Buildings to Incoherent Ground Motions

The behavior of unsymmetric buildings excited by incoherent seismic ground motions is studied, and the assessments and information reported make it possible to gain valuable insight into the resulting dynamic response. The effects of the ground motion incoherence and of the structural eccentricity are first considered independently of one another, and attempts are made to identify their relative importance. The torsional effects induced by the incoherence actions are interpreted in terms of normalized accidental eccentricities; and comparisons are made of these eccentricities with the value of 0.05 specified by design codes. Such comparisons form a frame of reference for the assessment of the 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 represented by proper combinations of the corresponding independent effects, under conditions that are of practical importance. This approximation leads to the definition of effective modified eccentricities, which can be useful for the appreciation of the torsional effects induced in building structures by earthquakes.