The role of masonry infill in progressive collapse mitigation of multi-storey buildings
Creators: name: Brás Xavier, Helder Francisco
Publisher: name: Imperial College London
Year: 2015

description: The research presented in this thesis addresses the influence of non-structural masonry infill on the resistance of multi-storey buildings to progressive collapse under sudden column loss scenarios. In particular, the mechanical response of infilled frames in peripheral bays is investigated within the scope of a design-oriented robustness assessment framework. A ductility-centred progressive collapse assessment methodology recently developed at Imperial College is employed as a rational procedure to quantify structural robustness for sudden column loss. This allows due consideration of structural redundancy, ductility, strength, dynamic effects and energy absorption capabilities in a unified manner. In this way, robustness quantification is shifted from typical code recommendations into a sound performance-based mechanical assessment. The realistic contribution of masonry panels towards collapse arrest is examined considering the results from full-scale laboratory tests and accurate numerical simulations. Novel real-scale tests were performed on different two-bay frames with brick-masonry infill subjected to incremental pushdown deformation, capturing the dominant deformation mode actually found following removal of an edge column. In these physical tests, it was observed that the failure mechanisms and damage patterns displayed by the infilled frames under pushdown deformation are similar to those activated by lateral pushover loading. Chiefly, clear evidence of diagonal cracking and shear sliding, eventually culminating in crushing of the compressed corners were recorded during the tests. Different infill configurations were tested, including central openings and initial gaps between masonry and frame elements. Overall, a global stable response was observed even at the expense of severe damage in the masonry panels. Importantly, a monotonic supply of energy absorption was noticed with increasing vertical deformation, which translates into considerable robustness reserve associated with the confined infill walls. Secondly, advanced mesoscale finite element simulations were employed in order to capture the complex frame-infill interaction in the early stages of pushdown response, where it has been observed in a realistic case study that progressive collapse is effectively arrested at small dynamic deformations, with minimal damage to the masonry panels and surrounding structural elements. Finally, application of the robustness assessment framework allowed a critical comparison between the collapse resistance arising from secondary mechanisms typically considered in this context (such as floor membrane and beam catenary effects) and that related to the presence of masonry infill. While the former are quite effective at relatively large deformations, the latter is shown to add substantial contribution at small displacements. The conclusions in this thesis are 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. On the other hand, due account for masonry infill subject to proper quality control during the construction process is recommended for rational robustness design of new buildings.