L. Sarkissian - Earthquake Engineering Department, Tehran University
K. Khalili Jahromi - Building and Housing Research Center (BHRC)
S.M. Zahrai - Department of Civil Engineering, Tehran University

The influence of the in-plane flexibility of floor systems on the seismic response of the structures may become significant, particularly when considerable floor slab cracking and yielding is expected. Since in recent years the composite floor systems (steel beams with upper concrete slab) have been used widely in the steel buildings, the investigation of the diaphragm behavior of composite floor systems subjected to lateral loads, is inevitable. As a part of a comprehensive study of the effective parameters in the diaphragm behavior and in-plane characteristics of the composite floor systems, two half-scale single story building models with the parameter of the direction of the joists were tested. The scale-model structures were steel buildings with composite floor system consisting of four square panels supported by side frames with X bracing. The structures were designed under the seismic load specified in the seismic code of Iran, but to ascertain the inelastic behavior of the diaphragm, the stiffness of the lateral load resisting system of the structures were increased by doubling the number of bracings, then the cyclic lateral load was applied to the structures up to failure. The paper reports on the results of the test program performed at the structural engineering laboratory of Building and Housing Research Center of Iran (BHRC). The results show that the composite diaphragms present good performance under lateral loads; however the diaphragms' in-plane characteristics such as  stiffness, ultimate strength, stiffness degradation and the crack pattern of the floor slabs are affected significantly by the relative direction of the floor joists to the lateral load (perpendicular or parallel).

Composite Floor System, Direction of the Joists, Diaphragm, Seismic Load, Cyclic loading, experimental data, In-plane Stiffness