Seismic Design of Cast-in-Place Concrete Diaphragms, Chords, and Collectors

Building structures generally comprise a three-dimensional framework of structural elements configured to support gravity and lateral loads. Although the complete three-dimensional system acts integrally to resist loads, we commonly conceive of the seismic force-resisting system as being composed of vertical elements, horizontal elements, and the foundation (Figure 1-1). The vertical elements extend between the foundation and the elevated levels, providing a continuous load path to transmit gravity and seismic forces from the upper levels to the foundation. The horizontal elements typically consist of diaphragms, including collectors. Diaphragms transmit inertial forces from the floor system to the vertical elements of the seismic force-resisting system. They also tie the vertical elements together and thereby stabilize and transmit forces among these elements as may be required during earthquake shaking. Diaphragms are thus an essential part of the seismic force-resisting system and require design attention by the structural engineer to ensure the structural system performs adequately during earthquake shaking.

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). Although horizontal elements can consist of truss elements or horizontal diagonal bracing, in most cases diaphragms are constructed as essentially solid, planar elements made of wood, steel, concrete, or combinations of these. Concrete diaphragms can be conventionally reinforced or prestressed, and can be cast-in-place concrete, topping slabs on metal deck or precast concrete, or interconnected precast concrete without topping, though the last system is seldom used in structures assigned to Seismic Design Categories D, E, or F. The scope of this Guide is restricted to cast-in-place concrete diaphragms, either conventionally reinforced or prestressed. However, many of the concepts that are presented here apply equally to other diaphragm types.