Effect of prior in-plane damage on the out-of-plane behavior of masonry infill walls

Out-of-plane collapse is one of the most important failure modes for masonry infill walls in frame structures during earthquakes and may result in significant economic loss, blocked evacuation pathways and even injuries and fatalities. To investigate the effect of prior in-plane damage on the out-of-plane behavior of masonry infill walls, a series of full-scale masonry infilled RC frame specimens that conform to the Chinese practice were subjected to sequential in-plane and out-of-plane quasi-static loading. The results show that the out-of-plane strength of the masonry infill walls was reduced by 22% and 47% at 1/500 and 1/200 prior in-plane drift ratios, respectively, whereas the out-of-plane stiffness was not significantly affected by the prior in-plane damage. Based on the test results of the present experiment and the collected data available in the literature, an empirical equation was derived to relate the out-of-plane strength reduction factor with both the prior in-plane drift ratio and the slenderness of the wall panel. According to the equation, the out-of-plane strength starts to degrade when the in-plane drift ratio reaches approximately 0.1% to 0.3% depending on the wall slenderness. Both the rate and extent of the strength degradation are greater for slenderer wall panels. Through a numerical example, we demonstrate that neglecting such an in-plane-out-of-plane interaction may lead to substantial underestimation of the loss by seismic damage of masonry infill walls in buildings.