Partial interaction behaviour of composite steel-concrete members at elevated temperatures accounting for geometric nonlinearities

This paper presents a numerical model for the analysis of composite steel-concrete beams at elevated temperatures accounting for both longitudinal and transverse interaction within the framework of nonlinear finite deformation theory. A reduced formulation, useful for solving structural problems, is then derived on the basis of the theory of small strain and moderate rotations. The numerical solution proposed relies on the use of the finite element method. As the scope of this study is restricted to moderately elevated temperatures, elastic material properties are assumed for all materials while still accounting for their degradation with temperature. A bilinear constitutive model is adopted for the transverse interface connection to reflect the more realistic case in which an extremely high connection stiffness exists for the bearing condition of the concrete slab against the steel joist, and a relatively weaker stiffness is manifested when the joist and slab are separating. A brief numerical example is then presented for a beam configuration subjected to a thermal distribution typical of real fire scenarios and pinned at its ends.