Nonlinear seismic response analysis of steel-concrete composite frames
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Frame finite-element models permit obtaining, at moderate computational cost, significant information on the dynamic response behavior of steel–concrete composite (SCC) frame structures. As an extension of conventional monolithic beam models, composite beams with deformable shear connection were specifically introduced and adopted for the analysis of SCC beams, in which the flexible shear connection allows development of partial composite action influencing structural deformation and distribution of stresses. The use of beams with deformable shear connection in the analysis of frame structures raises very specific modeling issues, such as the characterization of the cyclic behavior of the deformable shear connection and the assembly of composite beam elements with conventional beam–column elements. In addition, the effects on the dynamic response of SCC frame structures of various factors, such as the shear connection boundary conditions and the mass distribution between the two components of the composite beam, are still not clear and deserve more investigation. The object of this paper is to provide deeper insight into the natural vibration properties and nonlinear seismic response behavior of SCC frame structures and how they are influenced by various modeling assumptions. For this purpose, a materially nonlinear-only finite-element formulation is used for static and dynamic response analyses of steel–concrete frame structures using composite beam elements with deformable shear connection. Realistic uniaxial cyclic constitutive laws are adopted for the steel and concrete materials of the beams and columns and for the shear connection. The resulting finite-element model for a benchmark problem is validated using experimental test results from the literature for quasi-static cyclic tests. The paper then focuses on the numerical simulation, based on various modeling assumptions, of the eigenproperties and seismic response of a realistic two-dimensional five-story two-bay moment resisting frame made of steel columns and SCC beams and designed according to the Eurocode. It is found that the inclusion of the deformability of the shear connection in the finite-element model has a significant effect on the global dynamic response of SCC frame structures. In modeling this type of structures by using frame elements with deformable shear connection, a proper representation of the shear connection boundary conditions for all composite beams is crucial for accurate response simulation.