Experimental and computational study of the vertial shear behaviour of partially encased perforated steel beams

A comprehensive study has been undertaken by the authors to conduct advanced analysis and enable design tools for innovative Ultra Shallow Floor Beams (USFB) in buildings. In the USFB, the concrete slab lies within the steel flanges and is connected to the slab through the web opening, providing enhanced longitudinal and vertical shear resistance. There are additional benefits in providing increased fire and buckling resistance to the steel beam. In this study four specimens of symmetric steel-concrete composite beams with large circular web openings in the steel section and low concrete grade were tested under static monotonic loading. One of the specimens was from a lower quality of concrete and was tested in order to further investigate the failure mechanism and the actual behaviour of the concrete confinement. The load carrying capacity of the perforated bare steel beam is also presented for direct comparison. For the computational approach to the problem, a three-dimensional (3D) Finite Element (FE) model was created, employing solid elements with material, geometrical and interfacial non-linearity. Two-dimensional (2D) FE contact models using shell elements were established to examine the steel-concrete interface condition. The results show that the FE models are able to satisfactorily predict the load carrying capacities and the crack patterns of these new composite beams against the Vierendeel failure mechanism. A sensitivity study of material models and contact strengths using various constitutive models from the literature and the dominant parameters which affect the structural behaviour of the USFBs, are presented and discussed. Furthermore, the FE models provide detailed information on the structural behaviour of the confined concrete between the flanges and the section of concrete that passes through the web openings, as this is of paramount importance for the load carrying capacity and the failure mode of the USFBs. The comparison between the experimental and computational results leads to useful conclusions. The results for the composite beams show a significant increase in vertical shear resistance, even though mechanical shear connectors were not used. A previous design method is presented and modified to be able to be used for the load carrying capacity prediction of this new composite structural system. The results compare very well and the shear enhancement demonstrated in this study is now used in design practice.