Peak floor accelerations in unreinforced masonry buildings with flexible diaphragms

Abstract

The non-structural components in buildings are often subject to greater earthquake accelerations than that of the ground. This acceleration amplification is due to the building’s main lateral resisting system being subject to additional ‘relative’ accelerations as they deform when subjected to ground motion. Floor accelerations can be conservatively estimated using linear dynamic analysis, however even relatively simple modal analysis for buildings with flexible floor(s)/roof is often impractical. A simple modal analysis is proposed that is based upon the assumption of two vibration modes for the buildings, one being the first building mode if the diaphragm vibrations are excluded and the other being a diaphragm-related mode. These modal properties are simple to calculate and the results from the proposed method has practical value. For the evaluation of the method, parametric nonlinear time-history analyses were conducted on four building typologies with diaphragm flexibility being taken as a variable. Results show that significant increase in floor accelerations can occur due to the diaphragm vibrations, and that most of this increase can be captured by the proposed method.