Report - PEER - University of California, Berkeley

with some thoughts on the challenges still facing engineers when dealing withhorizontally irregular structures in seismic zones.2. RECENT DEVELOPMENTSWith the availability of more powerful computers, an increasing interest in the postyieldbehaviour of asymmetric buildings was taking place. While the computation toobtain the seismic responses is relatively straight forward, the ability to translate theobservations to design procedures is more problematic. Even in single-storey models(Fig.1), the number of parameters is very larger. In addition to the usual ones such asdamping ratio, peak displacement or drift, type of hysteresis model, target ductilitydemand, there are the strength distribution among the resisting elements.Figure 1. Typical 1-storey model studied by investigators.While disagreement among researchers due to different definitions of parameterswere noted already in the linear range (e.g., the 3 possible and legitimate definitionsof the torsional-to-lateral frequency ratio), conflicting conclusions from parametricstudies on nonlinear structures have been much more common. Hence it is notsurprising that, considering also the complexity of the problem, practitioners were notquick to adopt the emerging recommendations. In the mid 1990s De la Llera andChopra (1995) brought some order into model characterization by suggesting the useof the base-shear base-torque interaction diagram for this purpose.One-storey models have been used to study the effect of bidirectional seismicinput on asymmetric systems, usually monosymmetric ones. The picture emergingfrom the studies on the subject is that bidirectional excitation has some effect on theresponse, but it does not appear substantial (for summary see Rutenberg 2002). Itappears that with the advent of efficient, reliable and user-friendly 3-D linear andnonlinear programs the need to extrapolate from unidirectional to bidirectionalresponse will lose its practical importance.370