Experimental investigation of steel–CLT hybrid beams under serviceability demands

Mass timber products have gained popularity due to notable advantages over traditional construction materials, such as their light weight and low embodied carbon. However, these products have disadvantages when it comes to longer spans due to serviceability demands. The development of hybrid floor systems combining cross laminated timber (CLT) slabs with steel beams and girders provides a potential solution to these challenges. While research on steel-timber composite (STC) floors has produced valuable results and data, there is a lack of data and research on the behavior of STC floors under serviceability demands. This research summarizes a series of beam-scale tests, on 6.1 m long beams, that explored the natural frequency of STC beams and girders as well as composite flexural stiffness and ultimate flexural capacity. Three different composite connectors were tested: through bolts, screws inserted from above the CLT deck through the CLT and top flange of the steel girder, and screws inserted from below the bottom flange into the cross laminated timber. The results showed that the natural frequency of the STC beam and girders tested within this study is controlled by the CLT due to the large stiffness differential between CLT and steel. In addition, the results demonstrated that unlike shear studs used in steel-concrete composite floors, the connectors used in STC floors have large yield slips and therefore, composite action is load-dependent. This load-dependent composite behavior was approximated using various methods from the literature and compared to experimental results.