Out-of-plane mechanism in the seismic risk of masonry façades

A framework for evaluating the risk of the out-of-plane mechanism of masonry façades subjected to earthquake excitations is presented. The effect of seismic events on masonry façades is investigated and results are presented in terms of fragility curves and seismic risk for several masonry supporting restrictions. The restrictions considered are (1) masonry façade unrestricted to rock on a firm base, (2) masonry façade restricted to rock in one direction, and (3) restrained rocking masonry façade in one-sided motion. Because the available empirical information is insufficient to obtain fragility functions for masonry façades, the study is based on numerical analysis and recent studies on seismic risk. The conceptual framework is illustrated with information from masonry façades characteristic of Mexican religious structures and within the seismic hazard areas of Mexico. Since the frequency characteristics of ground motions strongly depend on the distance from the epicenter to the site, which is of cardinal importance to the vulnerability of these structures, the impact of this distance for hard-soil sites is thoroughly investigated. The probability of failure for the three types of masonry façades was calculated. It was found that the probability of failure decreases with an increased width-to-height relation; conversely, the probability of failure (reported in terms of the reliability index) increases with increased block size dimensions. Results in this study show that restraining conditions of the façades inconsequentially affect the probability of failure. Although the reliability of freestanding masonry façades are marginally greater, this finding holds true regardless of whether or not one considers far-field ground motions. The results in this study can be useful for evaluation and reinforcement tasks of masonry façades located in areas of high seismic hazard.