Experimental study on the longitudinal shear transfer mechanisms and resistance in CSTC composite beams

Abstract

This contribution presents an experimental study on composite steel truss in concrete (CSTC) beams consisting of prefabricated structural steel latticed girders embedded in concrete. Unlike typical composite beams with downstand steel profiles using headed stud shear connectors, CSTC beams are composed of straight and sinusoidally bent bars with a circular cross-section, in which the transfer of the longitudinal force is ensured via a mechanical interlock between the steel diagonals and the surrounding concrete slab. Because of the lack of specific tests, it is necessary to comprehend the local behaviour (stiffness, resistance and slip capacity) through new experimental investigations which support the development of a reliable design model able to predict the respective longitudinal shear resistance.This paper presents and discusses the setup and results of a wide experimental campaign of more than 30 push-out tests on CSTC beams. Different geometrical and mechanical parameters were varied in accordance with their typical range of application. Based on the experimental results, it was found that the shear transfer mechanism and resistance is governed by localized concrete failure combined with plastic bending deformation of the diagonal bars. The load-slip behaviour was found to be qualitatively comparable with headed stud shear connectors and the significant slip capacity, in most cases beyond 20 mm, indicates that the sinusoidal web bars lead to a shear transfer mechanism that may be considered to be ductile according to EN 1994-1-1. In addition, to isolate the local contribution to the resistance provided by the bars near the weld seams, the diagonal bars of some specimens were intentionally sawed off in some of the specimens, resulting in a “steel tooth”. The load carried by this isolated component was found to be about 58 % of the full longitudinal shear resistance. The experimental values of the resistance were finally used to calibrate statistically an analytical equation for predicting the design of longitudinal shear resistance in compliance with the reliability levels given in EN 1990. Additional considerations concerning the potential interaction with the vertical shear load capacity given by the diagonal bars were included in the final design proposal.