The critical role of floor diaphragms in the seismic retrofit of existing buildings

Floor diaphragms are key components of the Lateral Force Resisting System (LFRS) of existing buildings, both in the as-built and retrofitted configurations. Nevertheless, their effectiveness is rarely verified in the seismic vulnerability assessments, nor in the conceptual design for the retrofit of existing buildings. Floor diaphragms are essential for responding to horizontal loads because their primary functions are: ensuring the integrity of the structure by connecting the vertical elements of the LFRS and enabling their activation; transferring the inertial loads of the floor to the vertical elements, providing load paths for lateral forces; accommodating transfer forces resulting from incompatible deformation patterns of the existing structure and the retrofit vertical system, or from discontinuities or irregularities in infill distribution; providing lateral support to the vertical elements against buckling and second-order effects; collecting the out-of-plane loads of the perimeter infills