ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, D.C. Charmpis, N.D. Lagaros, Y. Tsompanakis (eds.) Rethymno, Crete, Greece, 13–16 June 2007 “NUMERICAL ANALYSIS OF COMPOSITE FLOOR DIAPHRAGMS BEHAVIOR UNDER LATERAL LOAD” LEON SARKISSIAN ¹ SEYED MEHDI ZAHRAI ² 1- M.Sc of Earthquake Engrg., University of Tehran, Tehran, Iran levik_504@Yahoo.com 2- Assistant Professor, Dept. of Civil Engrg., University of Tehran, Tehran, Iran mzahrai@ut.ac.ir Keywords: composite floor system, nonlinear FEM analysis, diaphragm, seismic load, cyclic loading, crack pattern, ultimate strength, in-plane stiffness, experimental data. Abstract: In current design procedure of structures, structures' diaphragms are usually assumed to be rigid and elastic; however in some cases, depending on the type of the floor system, the floor systems may represent inelastic behavior or act as a flexible diaphragm. 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 is inevitable. Since in recent years use of the composite floor systems is increasing, in this study response of the composite diaphragms in steel structures, subjected to lateral seismic loads is analyzed. As a part of a comprehensive study of effective parameters in the diaphragm behavior of the composite diaphragms, two half- scale single story building models with the parameter of the joists direction, which was set parallel and perpendicular to the lateral load direction, were studied. The scale-model structures were steel buildings with composite roof system consisted of four square panels supported by side frames with X bracing. First the diaphragms behavior under the lateral load specified in the seismic code of Iran was studied, then by increasing the lateral load amplitude, the inelastic behavior of the diaphragms was studied, using FEM and nonlinear structural analysis by ANSYS. The in-plane characteristics of the composite diaphragms such as the in-plane stiffness, ultimate strength, stiffness degradation and the crack pattern of the floor slab were studied analytically, and the results were compared with the results obtained from experiments. The results showed that the composite diaphragms present good performance under lateral loads and they were relatively rigid subjected to the seismic load specified in the seismic code; however under lateral loads with higher amplitudes, by developing the cracks in the floor slabs, nonlinear behavior and stiffness degradation of the diaphragms were observed. Also the joists direction was found an effective factor in the diaphragm behavior of composite floor systems which affects the diaphragms in-plane characteristics significantly.