Experimental observations on the in-plane behaviour of masonry wall infilled RC frames; focusing on deformation limits and backbone curve

Abstract

The ratio of frame strength and stiffness to masonry infill are major parameters that influence the seismic capacity of masonry infill. The influence of such parameters to seismic capacity in terms of strength, stiffness and deformation has significant variations between different design codes and past literature on the topic. This study focused on the in-plane behaviour of unreinforced masonry infill walls installed in reinforced concrete (RC) frames with different strengths. In the first part of this study, two ½ scale specimens with different RC frames and identical masonry infill walls were tested using a static cyclic loading protocol. The main objective was investigating the influence of changing frame strength to seismic capacity in terms of: strength, stiffness and deformation. Results of the presented experiment showed that as the ratio of frame shear strength to masonry shear strength increased, there was great improvement of the masonry infill walls in terms of strength and avoidance of sudden brittle behaviour of the masonry infill. However, varying frame strength did not significantly influence the initial stiffness and story drift at maximum strength. In the second part of this study, an investigation was conducted on the deformation limits of masonry infilled RC frames and the influence of various parameters, based on data collected from many recent experimental tests. Based on these experimental results, the deformation limits of masonry infill were found to be directly proportional to both the compressive prism strength of the masonry infill and the ratio of shear strength of frame to that of the masonry infill. The influence of aspect ratio showed large variation and it is difficult to conclude its level of influence on deformation. A simplified procedure based on experimental data was proposed that can estimate the backbone curve and gave good estimation of the post-peak lateral strength degradation slope based on the ratio of frame strength with simple hand calculation. Such a method is useful in the preliminary design process to help practicing engineers understand the general behaviour expected by infilled RC frames.