Compatibility Forces in Floor Diaphragms of Steel Braced Multi-Story Buildings

Abstract

Floors have a key role in the seismic behaviour of structures, especially in multi-story buildings. The in-plane behaviour of a floor system influences the seismic response of the structure significantly and affects the distribution of lateral forces between seismic resisting systems and over the height of the structure. In buildings where the seismic resisting systems are in the same location in plan on each floor over the height of the building, inertial and displacement compatibility shear forces are the principal shear forces generated at the interface between the floor system and the seismic-resisting system. These two are called interface diaphragm forces. These interface forces must be transferred into the appropriate lateral load resisting system and the interface must be well designed and detailed. Determination of the magnitude of the interface loads on concrete diaphragms are not well understood and still a matter of debate. There is no consensus of a design procedure for determining the diaphragm actions and distribution into the seismic resisting systems. In this paper, interface forces generated in floor diaphragms by asymmetrical actions of the braced framing system on each side of the building in the direction of analysis have been investigated. A numerical study using Numerical Integration Time History Analysis (NITH), has been undertaken to evaluate the interface forces of concrete floor diaphragms in a 12-story braced steel building. The results of nonlinear time history analyses using ground motion records from three different earthquakes are presented.