Development of steel-timber composite system for large scale construction

Abstract

Connecting timber panels to steel girders using mechanical fasteners (e.g. screws and bolts) is an attractive and novel method for developing a fully prefabricated and sustainable hybrid steel-timber composite (STC) floor that can also facilitate future dismantling and recycling of the structural components. This study concerns the short-term behaviour of innovative STC floors comprising of cross-laminated timber (CLT) and/or laminated veneer lumber (LVL) panels connected to steel girders by various mechanical fasteners and/or glue. The mechanical characteristics of STC connections play an essential role in the safe and economical design of hybrid STC structures and floor systems. Accordingly, at the first stage of this research project, short-term behaviour of STC connections has been investigated. Push-out tests on different types of STC lap joints with coach screws (with and without a reinforcing nail plate), high-strength bolts and a combination of glued and screwed connectors are reported, and the load-slip behaviour, stiffness, strength and failure modes of the STC connections are characterised. At the second stage, short term behaviour of STC system was investigated through four-point bending tests. The results of four-point bending tests performed on full-scale STC beams are reported and the structural behaviour (i.e. the load-deflection response, short-term stiffness, and peak load capacity and failure modes) of the proposed STC system are studied. At the last stage of the project, extensive non-linear 1D and 2D finite element (FE) analyses of STC beams are carried out and the numerical results are verified against the test results. It is shown that 1D and 2D FE models can adequately capture the structural response of the STC beams. Using the FE and experimental results, the composite efficiency of STC beams that can significantly influence their structural behaviour beams is determined. In addition, the mechanical behaviour of lap STC connections was investigated through using a nonlinear 3D continuum-based FE model. In the FE models developed, the non-linear behaviour and failure of the timber is captured by a stress-based failure criterion formulated in the framework of continuum-damage mechanics. Yielding of the steel plates and fasteners (i.e. coach screws and bolts) is captured by an elastic-hardening plastic constitutive law. These FE models are validated against experimental (push-out) tests conducted on LVL and Steel-CLT composite lap connections. It is shown that the FE models developed can adequately predict the load-slip response and failure mode of the hybrid steel-timber composite connections tested. The validated FE model is used to undertake a parametric study that elucidates the influence of the yield strength and the length of the fasteners and the post-tensioning force in the bolts on the stiffness, load carrying capacity and load-slip behaviour of hybrid steel-timber composite connections.