Effect of out-of-plane displacements on the in-plane capacity of lightly precompressed rocking unreinforced masonry piers

Abstract

Unreinforced masonry (URM) buildings subject to bidirectional seismic excitation are under the effects of simultaneous in-plane and out-of-plane actions. Seismic assessment carried out without considering the combined effects due to these actions could result in non-conservative capacity estimates and inadequate strengthening requirements. Bidirectional effects are not explicitly considered in the seismic assessment of existing URM buildings, owing to the limitations in existing protocols and analytical models and the simplicity achieved in performing the assessment. The current work attempts to quantify the effect of out-of-plane displacements on the in-plane capacity of masonry walls undergoing a flexural rocking mechanism of failure, thereby improving the existing understanding of bidirectional interaction in masonry structures. Response of lightly precompressed masonry piers with different aspect ratios in the presence of out-of-plane displacements, were studied experimentally and replicated using numerical and analytical studies. Results indicate that the interaction problem in URM structures should not be considered only from a force capacity reduction perspective. The force capacity reduction predicted using the interaction equations employing an exceedance of net tensile stress-based approach shall be applicable only for the flexural cracking strength and not the ultimate flexural capacity. The in-plane flexural cracking capacity reduces in proportion to the applied out-of-plane displacements when the applied out-of-plane displacements are in the elastic range. In the inelastic range of out-of-plane displacements, the ultimate flexural capacity corresponding to toe-crushing and also the initial stiffness of the pier was observed to decrease due to the reduction in net area available for compression. Also, even small range of out-of-displacements could result in inelasticity in a URM pier leading to a reduced toe-crushing capacity. Hence, it may not be conservative to define the in-plane restoring shear as the failure capacity for rocking URM piers under bidirectional loading. Accordingly, modifications are proposed for the response of masonry walls undergoing rocking mechanism.