Pushdown tests on masonry infilled frames for assessment of building robustness

The research presented in this paper addresses the influence of nonstructural masonry infill on the resistance of multistory buildings to progressive collapse under sudden column loss scenarios. In particular, the structural response of infilled frames in peripheral bays is investigated within the scope of a design-oriented robustness assessment framework previously developed at Imperial College London. This allows due consideration of structural redundancy, ductility, strength, dynamic effects, and energy absorption capabilities in a unified manner. The realistic contribution of masonry panels toward collapse arrest is examined considering the results from full-scale laboratory tests performed on different two-bay frames with brick-masonry infill subjected to incremental pushdown deformation, capturing the dominant deformation mode found following removal of an edge column. In these physical tests, it is observed that the failure mechanisms and damage patterns displayed by the infill panels under pushdown deformation are similar to those activated by lateral pushover loading. Clear evidence of diagonal cracking and shear sliding, eventually culminating in crushing of the compressed corners, is noted. Different infill configurations are tested, including central openings and an initial gap between masonry and frame elements. Overall, a global stable response is observed even in the presence of severe damage in the masonry panels, delivering a monotonic supply of energy absorption with increasing downward displacement. The outcome from this experimental research provides mechanically sound and quantifiable evidence that nonstructural masonry infill panels in peripheral frames offer a reliable and efficient source of enhanced robustness under column loss events. Because of the widespread application of masonry infill panels, this is believed to be particularly relevant within the context of retrofitting operations for robustness enhancement of existing structures, as a result of the growing demand for upgraded resilience of urban infrastructure. Similarly, due account for masonry infill subject to proper quality control during the construction process is recommended for rational robustness design of new buildings.