Report - PEER - University of California, Berkeley

They tested the procedure on three asymmetric variants of the Los Angeles 9-storeySAC building. The results show similar accuracy as for symmetric buildings. Someproblems were noted when strong modal coupling occurs.Pushover analysis was also applied to asymmetric wall-frame structures withsome success by De Stefano and Rutenberg (1998) and Moghadam and Tso (2000).This, however, remains an area in need of further study, since the response is verysensitive to the relative stiffnesses of the walls and frames, and hence extrapolationsfrom either walls or frames are problematic.Experimental work on the nonlinear response of asymmetric structures, whichis needed to provide support for the standard analytical tools, was quite limited in thepast. More recently several shaking-table test results have been published by Fardis etal. (1998) and De Stefano et al. (2002), already briefly summarized (Rutenberg 2002).Mola et al. (2004) concluded that codified approaches for assessing the response ofhorizontally irregular structures, including pushover analyses, “failed to give correctand safe-side estimation of the important features of structural response”, includingthe failure mechanism of the structure. Hence, their findings (also Negro et al. 2004),together with the criticism launched against 1-storey approximations, should beexamined very carefully since they all cast very serious doubts about the viability ofstandard time-honoured procedures.Passive control is continuing to attract researchers. Several studies have beenpublished during the last two years. Tena-Colunga and Gomez-Soberon (2002)studied the response mass eccentric structure with symmetric base-isolation. Lin andChopra (2003a, 2003b) presented a design procedure for 1-storey asymmetricstructures with nonlinear viscous or viscoelastic dampers. De La Llera and Almazan(2003a, 2003b) studied, the former experimentally, the response of multistoreyeccentric structures provided with friction pendulum dampers. Yoshida et al. (2003)presented an experimental verification for design using magnetorheological dampers.Space limitations preclude description of these interesting studies.3. RECENT STUDIES BY AUTHORSThe nonlinear response of asymmetric multistorey buildings laterally supported byflexural walls has not attracted much research interest because it has been accepted bymany that design based on linear analysis would give adequate protection. Results ofa recent study on symmetric structural systems comprising walls of different lengths(e.g., Rutenberg et al. 2004) and work in progress on torsionally flexible wall systemsshow that predicting the peak base shear on the walls can be problematic. This shouldbe of interest since the standard capacity spectrum approach cannot capture thisbehaviour. Summary of preliminary results is given in section 3.1.In most studies on asymmetric structures it is assumes that the eccentricity is given,and procedures have been developed to design the structures so that the responsewould not be larger than that of the corresponding symmetric structure. This was aconvenient approach because it assumed that element yield displacements vary with373