ICO-SHEAR | Investigations on the longitudinal shear transfer in Composite Steel Truss in Concrete (CSTC) beams

Project description

Self-bearing composite steel truss and concrete (CSTC) beams provide high structural efficiency within compact construction depths and allow extensive prefabrication. Unlike conventional steel-concrete composite beams, the longitudinal shear transfer in CSTC beams results from the mechanical interlock between the sinusoidal steel diagonals and the surrounding concrete, rather than through discrete connectors, such as headed studs. Whilst this aspect is crucial for ensuring the composite action and a safe design of the composite beam, it has remained largely unexplored.

The research project ICO-SHEAR, conducted at ETH Zurich in collaboration with Tecnostrutture s.r.l., was initiated to address this question and to develop strong scientific evidence and a reliable design framework for such construction solutions. The study focused on the NPS® Basic beam, a semi-prefabricated slim-floor system made of a welded steel truss embedded in in-situ concrete. To investigate the underlying mechanisms, an extensive experimental campaign of more than forty push-out tests was conducted. These tests explored different geometric and material configurations, including variations in diagonal diameter, number of sinusoids, and concrete strength. The experimental work was complemented by high-resolution fibre-optic sensors integrated within the steel truss, enabling detailed monitoring of strain and deformation during load transfer. The results revealed that the longitudinal shear resistance is primarily governed by local concrete crushing and plastic bending of the steel diagonals, while maintaining remarkably ductile behaviour with large slip capacities.

To gain further insight into the local stress distribution and load paths, detailed numerical simulations were performed using advanced three-dimensional finite element models. These analyses reproduced the experimental results with high accuracy and allowed the decomposition of the total shear resistance into its fundamental components. Most of the longitudinal shear force was found to be transferred through bearing at the lower weld zones, while the remaining part resulted from the combined effects of friction, anchorage, and deformation of the upper truss chords.

Based on the combined experimental and numerical findings, a design equation was developed to predict the longitudinal shear resistance of CSTC beams. The proposed formulation extends the principles of Eurocode 4 by introducing correction factors that account for the specific truss geometry and the number of sinusoidal bars. The analytical model was statistically calibrated in accordance with the reliability principles of EN 1990, yielding a robust and practical tool for engineering design.