# Flexural Capacity of a New Composite Beam with Concrete-Infilled Tubular Lower Flange
## 1 Introduction  
It is well known that steel-concrete composite members are structural system that can optimize the mechanical properties and material characteristics of steel and concrete. In addition to the conventional composite members utilizing wide flange shapes and shear studs, various types of steel-concrete composite members have recently been proposed, and their usage has increased on construction sites.  
One of the main goals of the recently developed composite systems is the reduction of floor height. 'Thor-beam' and 'Delta-beam' developed in Scandinavia [1] and the'slimflor beam' by the Steel Construction Institute [2] are representative examples with this feature. These slim floor beams are generally used in conjunction with deep decks and hollow core slabs for 5 m to 9 m span [1; 2; 3]. [PERSON] et al. [4; 5] suggested a methodology that can determine the optimal locations of shear connectors by performing a test on several slim floor beam specimens. [PERSON] et al. [6] proposed a new slim floor beam system without shear connectors that can guarantee a sufficient level of composite action between the two materials and derived its design strength equations. [PERSON] et al. [7; 8] developed a composite slim floor beam (CoSFB) that retains an enhanced flexural capacity by utilizing dowel bars passing through the web of the beam section. The typical span of slim floor beams is approximately 8 m, but the maximum span of the CoSFB system can be extended to 14 m while limiting the overall floor depth within 400 mm.  
The reduction of the amount of steel material and improved constructability are other goals of recently developed composite systems. The 'Versa :T:' beam by [PERSON] is one such example; its main concept is that the steel cross section plays the role of a form for concrete and functions as a structural component. [PERSON] et al. [9; 10] developed a similar composite beam utilizing high-strengthsteel for the improvement of its flexural and seismic capacity. [PERSON] et al. [11; 12] developed a new composite beam that includes bolted cold-formed steel components and steel rebars for improved bending strength.  
This study proposes a new steel-concrete composite beam system with tubular lower flange illustrated in Figure 1. The composition of its cross-sectional components is shown in Figure 2. This new composite member has openings in the web for perfect integration of the concrete and steel materials, and steel rebars are installed inside the tubular lower flange to enhance its flexural strength. It allows the use of deep decks and piping members passing through the web for floor depth reduction.  
In this paper, the bending capacity of the new composite beam is investigated by performing a two-point loading test on seven specimens, which can be categorized mainly into two types, non-composite and fully composite specimens. The load versus displacement curves are plotted for all the specimens and their ultimate failure modes are identified. Theoretical equations to estimate the flexural strengths of the new composite beam members are proposed, and their accuracy is examined by comparing the predictions of the equations with the test results.