Report - PEER - University of California, Berkeley

Dutta and Das (2002, 2002b and refs. therein) also used one-storey models tostudy the effect of strength deterioration on the bidirectional response of codedesignedasymmetric structures. They concluded that the displacement and ductilitydemands on the flexible edge as well as on the rigid edge were much larger than thoseof their symmetric counterparts and of similar models but without strengthdeterioration. They also observed that the unidirectional input might grosslyunderestimate the response. These issues deserve further study.Stathopoulos and Anagnostopoulos (2003) examined critically the use of 1-storeymodels as proxies for multistorey asymmetric frame structures. These are due to theassumption that plastic hinges form in columns, rather than in beams. For examplethey found that the simple 1-mass shear-spring (or shear-beam) models commonlyused by researchers to assess seismic provisions may not be appropriate, since suchmodels over-predict the flexible edge displacements, as noted earlier by Ghersi et al.(1999).The easier accessibility of nonlinear 3-D computer programs in the 1990s freedresearchers from the need to extrapolate from the 1-storey models. Yet, single masstorsional behaviour continues to attract many researchers, mainly because it is able toprovide qualitative information on the global behaviour at low computational effort,and even to reveal hitherto unknown phenomena. Indeed, the second part of this paperpresents a design procedure based on single mass response.Most of the interest focused on multistorey frame structures, while severalstudies on wall-frames were also reported, and will be referred to subsequently.Studies by Duan and Chandler (1993) on multistorey structures modelled as shearbeams showed that there could be problems with uncritical extrapolation of 1-storeyresults. Nassar and Krawinkler (1991) observed that such modelling is likely to beconservative because realistic cases, in which plastic hinges form at beam-ends,usually show smaller ductility demands. Moghadam and Tso (1996) observed thatshear beam modelling does not lead to reliable estimates of important designparameters. They also concluded that the seismic provisions could not adequatelyprotect torsionally flexible buildings.De Stefano et al. (2002) studied the response of a code-designed unidirectionallyexcited 6-storey frame building. They attributed the excessive ductility demands inunexpected locations to overstrength, and concluded that code-designs, which arecalibrated to 1-storey models, may not achieve their goal of bringing the ductilitydemands in asymmetric structures in line with their symmetric counterparts. Thismatter should be further explored.Reports on multistorey asymmetric structures under bi-directional excitation alsobegan to emerge in the late 1990s, along with studies on unidirectionally excitedstructures. Fajfar and coworkers (e.g., Marusic and Fajfar 1999) compared theresponse of mass eccentric perimeter frame 5-storey models with their torsionallyflexible counterparts, i.e., those with lateral load resisting internal frames. Theydemonstrated that, as in 1-storey frames, increasing the ground motion intensitylowers twist amplification of the torsionally flexible structures. They also suggested371