Establishing R w and C d Factors for Confined Masonry Buildings

Abstract

Seismic design provisions, including the Chilean seismic code, prescribe seismic design forces by reducing a linear elastic response spectra using a response modification factor. The codes also estimate expected maximum inelastic displacement that may occur during a severe earthquake, by amplifying the elastic displacement computed from the design seismic forces by a displacement amplification factor. In this paper, the seismic force reduction factor R w and displacement amplification factor C d are computed for typical confined masonry buildings, comparing base shear and first-story drift that develop in the structure when linear or nonlinear behavior is adopted. The computations are performed after analyzing several three and four story buildings. These buildings were subjected to the action of a number of earthquakes records obtained in Chile on March 3, 1985 and in Mexico on Sept. 19, 1985. The R w factor, evaluated for this type of building, depends both on the structure's wall density and on the source mechanism of the earthquake; in contrast to most code recommendations, the ratio of R w/C d obtained is less than 1.