Precast concrete diaphragm design by ASCE 7-16

Abstract

Floor and roof diaphragms are an important aspect of a building’s structural lateral system. The diaphragm resists in-plane seismic loads and transfers them to the vertical lateral force resisting system. If they are not properly designed, the failure could be devastating. After the 1994 Northridge earthquake the failures of multiple precast structures were investigated, and it was determined that inadequate diaphragm design was the cause of the failures. It was found that previous code provisions underestimated the diaphragm design forces. Additionally, diaphragms were assumed to behave elastically during seismic events, but it was observed that a large earthquake could cause inelastic behavior of diaphragms. Code provisions prior to the ASCE 7-16 determined diaphragm design forces based solely on the fundamental modal response of structures. However, further research proved that higher modes also influenced diaphragm forces. Therefore, a change was warranted to diaphragm design provisions in the ASCE 7-16. The diaphragm design provisions in the ASCE 7-16 ensure adequate ductility and strength of diaphragms and provides different options for elastic or inelastic diaphragm design. Not all design options are applicable and permitted for every scenario. The selection of the design option will depend on the seismic design category, the geometry of the diaphragm, and the number of stories of the building. The new diaphragm design provisions in the ASCE 7-16 are required for precast diaphragm design. This report provides a detailed design procedure for precast diaphragms and briefly discusses the research done on precast diaphragms that led to the provisions in the ASCE 7-16. In addition, a parametric study was conducted to illustrate how the design options change the diaphragm design forces and required reinforcement.