title: Structural Behaviour and Vibration Response of Continuous Cold-formed Steel-Concrete Composite Beams
Creators: name: Oluwadahunsi, Silas Eniola
Publisher: name: University of Liverpool Repository
Year: 2029

description: The use of cold-formed steel (CFS) sections as a replacement for the conventional hot-rolled I-sections in composite floor systems has been gaining significant interest among structural engineers and researchers alike. This steel type possesses a number of advantages over hot-rolled steel sections such as higher strength-to-weight ratios, sustainability, and cost effectiveness. However, CFS has a major drawback, which is its susceptibility to buckling. Closed CFS sections offer the best resistance to buckling phenomena. However, very limited research has been done to examine their suitability for composite beams. Thus, this research investigated the application of a rectangular hollow section (RHS) in continuous composite beams with the aim of developing efficient beam sections for short-span applications, which meet ultimate and serviceability requirements. Two slab types were considered – solid and composite slabs. The study is divided into three main experimental parts, with each complemented with numerical modelling (NM). The parts are push-out, beam bending and vibration response tests. Extensive parametric studies were conducted with the validated numerical models. 
For the push-out tests, the connector types investigated were headed studs, stiffened angles, and Z-tab connectors. The highest shear capacity was recorded with stiffened angle connectors (SAC). Increasing concrete compressive strength was shown to have a positive impact on connector capacity but a negative impact on relative slip. The connector failure modes observed were concrete crushing, connector facture and yield. The NM results corroborated these outcomes. Furthermore, the parametric studies also examined the effect of other parameters such as slab and connector geometry and connector spacing on connector capacity; Concrete compressive strength demonstrated the highest impact on connector capacity.
Five-point bending tests were carried out on two-span continuous composite beams to examine their load-deflection behaviour, end slip, ductility, the location of the neutral axis, and failure. The beams were 5.7 m and 1.2 m in length and width, respectively. Beams with solid slabs outperformed the ones with composite slabs on most metrics. On average, they showed higher load capacity and superior ductility. For light floor sections such as the CFS-concrete ones investigated, vibration performance is critical. Thus, vibration response tests were carried out through walking and drop (sandbag) tests to determine the fundamental frequency of each beam section and their acceleration response. All beams tested surpassed the 3 Hz minimum fundamental frequency requirement. Moreover, the floor sections met the one-second RMS acceleration requirements for most building use cases. The NM parametric studies showed that increasing RHS thickness had a significant and positive impact on the fundamental frequency of the sections, while increasing slab thickness had little impact.
A design procedure based on conventional structural design principles and Eurocodes 3 and 4 guidelines was proposed. The design method provided conservative values for moment capacity in the sagging region, while it yielded values with good accuracy for negative bending moment capacity. Moreover, the results indicate that the design is more appropriate for solid slab beams, as they are able to achieve full shear connection at the interior supports.
The findings of the research, as well as the proposed design, will provide structural engineers the basis for designing CFS-concrete floors for practical use.