# Numerical Study on the Bending Resistance of Lightweight Built-Up Steel-Concrete Composite Beams
## 1 Introduction  
Cold-formed steel (CFS) elements are widely used and investigated due to their high stiffness and strength, ease of prefabrication and installation, and low transportation cost and handling [1]. On the other hand, the combination of CFS with concrete slabs can provide effective and lightweight composite floor systems. Apart from the low self-weight and the possibility of building larger spans, the use of CFS profiles also makes it possible to form various shapes of built-up open and closed cross-sections (e.g., from C and Z sections). The thickness of CFS profiles ranges from 1.2 mm to 6.4 mm [2] (the minimum thickness is 0.378 mm, according to [1]). The resistance of built-up sections and members can be increased by arranging profiles in different positions and arrangements. Different built-up CFS profiles with closed sections, as well as open profiles and single profiles with holes, were presented in paper [3]. Back-to-back CFS profiles have also been investigated; for example, two bolted C profiles were studied in [4]. There are also innovative types of CFS profiles, such as the rectangular hollow flange beams where hollow flanges increase their buckling resistance [5]. To improve the buckling performance of the back-to-back steel system, [PERSON] et al. [6] placed reinforced plates inside the hollow flanges and studied cases with different spacings between the connections and different configurations of web beads. The results show that the profiled web reinforcement (beads) increases the ultimate load by 10%, whereas the spacings between the connections (150 mm and 300 mm) reduces the load-bearing capacity by 30%. Some other cross-section shapes from CFS C-sections are investigated in [7]. Regarding the applications of CFS in composite structures, Ref. [8] summarizes the strength of state-of-the-art composite beams made of CFS concrete. The results of the investigation show that the strength of the composite beam is maintained when a thin-walled CFS profile replaces the standard reinforcement with the same cross-sectional area. Furthermore, the development of the empirical equation, which aims to evaluate the ultimate shear bond capacity under bending conditions, and the interaction between shear connections and bending, is discussed. Ref. [2] predicts the moment resistance of the composite beam; the CFS box section was composed of two channel sections embedded in the concrete, in accordance with Eurocode 4 [9]. A CFS with hollow flanges was investigated in [10], wherein the hollow flanges of an I section were filled with normal and high-strength lightweight concrete. The results show that a lightweight infill, whether it is lightweight, normal, or lightweight high-strength concrete, positively affects the shear capacity because it delays shear buckling. Bolts were used for the connections between back-to-back C-profiles in the composite members studied by [PERSON] et al. [11]. They considered three types of shear connections, under bending and shear resistance conditions, and they compared the experimental and theoretical results. Single channels and back-to-back channels in steel-concrete composite beams were also investigated in Ref. [12]. Considering that the degree and type of shear connection significantly influence the composite systems, different types and arrangements of shear connectors are studied in the existing literature. Among the other types of shear connectors [13], there are shear connectors which are permanently connected to steel beams [14] or that are demonutable [15; 16]. Guided by the idea of forming a composite system with built-up CFS profiles connected by spot welds and demountable shear connections, the LWT-FLOOR system is proposed [13]. The system consists of four cold-formed channel sections that are spot-welded to the corrugated web (CW) and reinforced by shear plates near the supports, whereas the concrete slab is connected to the steel beam with demountable shear connectors.