Progressive collapse capacity of a gravity-load designed RC building partially collapsed during structural retrofitting

Abstract The problem of progressive collapse has been a major topic in structural forensic engineering since its inception, but it has recently received a growing interest. In the last two decades, the occurrence of extreme events and their catastrophic impact on the built environment and people has promoted several research programmes and guidelines for structural design and assessment, spotlighting the importance of back-analysis and numerical prediction of progressive collapse phenomena. This paper presents a numerical study aimed at investigating the ability of existing reinforced concrete structures to prevent progressive collapse during structural retrofitting. The progressive collapse capacity of a real, reinforced concrete framed building constructed in the 1950s and partially collapsed in 2001 is discussed in relation to a couple of structural retrofitting operations that were found to be the primary causes of the accident. Existing information from past forensic investigations was integrated with a simulated design procedure to develop the capacity model of the structure, according to design codes and practice rules used at the time of construction. Nonlinear pushdown analysis with displacement control was carried out both in intact conditions and during retrofitting operations. In addition to global capacity models of the structure, the use of partial capacity models representing the building corner under retrofitting was evaluated, highlighting a very high computational efficiency of those simplified models. Analysis results also indicate that the load contribution from infill walls can significantly influence the progressive collapse resistance, particularly in relation to retrofitting works involving the corner of the building plan as actually happened. Retrofitting interventions on the real building included the concrete cover removal from ten ground-floor columns and soil excavation around column bases. The removal of concrete cover from single and multiple ground-floor columns together with the loss of supports due to soil excavation around column bases was found to significantly affect the collapse capacity of the structure. The major impact of retrofitting operations in terms of location and extent provides a simulation-based proof of the progressive collapse suffered by the case-study building, evidencing that soil excavation around more than three columns was able to produce the accident.