Enhancing structural robustness of existing buildings through roof-level retrofit beams

The increasing occurrence of extreme events has underscored the importance of ensuring that existing buildings can withstand localised damage without triggering progressive collapse. Various retrofitting strategies have been proposed to enhance structural robustness, such as local strengthening of connections, bracing systems, or reinforcement of slabs and beams. However, these solutions are often invasive, difficult to implement in existing buildings, or limited in scope, as they primarily address local failure mechanisms without engaging the structure as a whole. As an alternative, roof-level retrofit beams have emerged as a promising global strategy to enhance robustness. By stiffening the roof-level beam, this solution aims to activate efficient alternative load paths in the event of a column failure. The retrofit beam assumes most of the load of the failed column that would otherwise be carried by slabs and beams above the failed column, possibly enabling their failure. The roof-level beam redistributes then the load among the connected columns, thus engaging the entire structure in a more global load-resisting mechanism. This article presents an in-depth analysis of the functioning of this retrofitting strategy through nonlinear dynamic simulations carried out on two representative case studies: a reinforced concrete and a steel building. The analysis explores the load redistribution mechanisms, the structural demands in key elements before and after retrofitting, and the system’s main limitations. Different combinations of damage scenarios and retrofit beam configurations were also investigated. The article provides a foundational understanding of the structural behaviour of roof-level retrofit beams and highlights key considerations to guide their application in real structures. • Roof-level retrofit beams enhance the structural robustness of existing buildings • Roof-level retrofit beams “hang” the floors in the event of a column failure • Load is redistributed among columns after a load-bearing element fails • Demands on beams and floors, which usually trigger collapse, are reduced • Retrofit layouts beyond perimetral can address interior column failures