Seismic response of low-rise shear wall structures with flexible diaphragms

In the 1994 Northridge earthquake, parking garages constructed using precast concrete diaphragms appeared to suffer more damage than other buildings. The presence of ramps and openings combined with long floor spans led to complex load paths and interdependencies in the flow of lateral forces through the floor diaphragms to shear walls.; Analyses motivated by damage and collapse of parking structures in the 1994 Northridge earthquake have shown that flexible diaphragms can experience larger horizontal accelerations and interstory drifts than those considered in current codes of practice. This leads to lateral displacements that can be in excess of the drift capacity of gravity load resisting systems as well as the potential for damage to the diaphragms.; In the present study, the amplification of forces and displacements in flexible diaphragms was investigated by inelastic dynamic response time history analyses of lowrise shear wall structures. Various degrees of diaphragm flexibility, shear wall flexural overstrength, and numbers of stories were considered in these analyses. The results confirmed recent findings that current code provisions (e.g. contained in UBC 97, IBC 2000 and IBC 2003) consistently underestimate diaphragm forces at the upper and lower floors under common conditions. Proposed herein are methods for the strength design of diaphragms that consider both an elastic and inelastic diaphragm response.; Current seismic codes allow buildings to be designed using an equivalent lateral force procedure if the interstory drifts calculated on the basis of the design lateral forces are less than the specified allowable story drifts. While this approach assures that structures have some minimum lateral stiffness, the calculated interstory drifts may significantly underestimate the actual peak interstory drifts determined on the basis of nonlinear dynamic analyses particularly for structures with flexible diaphragms. Consequently, the gravity framing systems in such structures may be exposed to interstory drift demands several times greater than would be expected on the basis of design calculations, thereby jeopardizing their integrity.; In this study, simple methods to more accurately estimate peak interstory drifts that account for higher mode effects are described for low-rise perimeter shear wall structures. The proposed methods are based on the principal modes obtained from a principal components analysis (PCA) of computed inelastic dynamic responses. The method can consider variations in the design response spectrum and gives reasonably good estimates for use for preliminary design of the structure. (Abstract shortened by UMI.)...