Diaphragms typically are horizontal or nearly horizontal planar elements that serve to transfer lateral forces to vertical elements of the lateral-force-resisting system (Fig. R12.1.1). Diaphragms also tie the building elements together into a complete three-dimensional system and provide lateral support to those elements by connecting them to the lateral-force-resisting system. Typically, diaphragms also serve as floor and roof slabs, or as parking structure ramps and, therefore, support gravity loads. A diaphragm may include chords and collectors

Diaphragms are required to be designed as part of the seismic force-resisting system of every new building assigned to Seismic Design Category B, C, D, E, or F of the International Building Code (IBC 2009, referred to here as the IBC)

Failure of some connections between diaphragms and walls in the 1994 Northridge earthquake triggered code changes for collectors. According to ASCE 7 § 12.10.2, collectors must be capable of transferring the seismic forces originating in other portions of the structure to the element providing the resistance to those forces