Comparative assessment of retrofit strategies for progressive collapse mitigation in steel-framed structures

Abstract

A large proportion of the existing building stock has been designed without consideration for progressive collapse and may therefore need retrofitting. To this end, this study compares the performance of three typical retrofit strategies for existing multi-story steel-framed structures subjected to column loss scenarios. The strategies include global measures, such as horizontal bracings placed above the removal zone and a truss system added at the rooftop level, and a local measure based on strengthening the 1st story columns through concrete encasement. Five moment resisting frames, prone to different progressive collapse mechanisms, are considered for case study purposes. Detailed numerical models validated against experimental results are developed to investigate the robustness of the frames before and after retrofitting. Parametric non-linear static analyses are also conducted to optimize the design of the retrofit measures. Additionally, the study assesses the impact of retrofitting on the dynamic response through non-linear dynamic analyses. The results show that the effectiveness of retrofitting depends on several factors, including, most significantly, their ability to redistribute loads, the number of stories, and the type of collapse mechanism (e.g., failure of beams or columns). Moreover, it is shown that global measures have the most favorable influence on the dynamic behavior. The study reveals that no single strategy is effective across all configurations and that case-specific decisions are typically necessary, based on the vulnerabilities in each structure. The paper offers fundamental insights and practical considerations for designing and implementing the various retrofit measures, enabling more holistic and informed selection approaches.