Fire resistance of axially restrained and partially unprotected Ultra Shallow Floor Beams (USFB®) and DELTABEAM® composite beams

Ultra-shallow floor types such as the USFB® and DELTABEAM® beam ‘plug’ composite flooring systems are recently developed and have seen many applications in contemporary construction. They involve partially encased steel beams in concrete, with the lower flange remaining exposed. Besides the satisfactory behavior of the system at ambient conditions, understanding their response to elevated temperatures is critical in evaluating their overall performance. Previous numerical studies of the authors have investigated their fire resistance when simply supported. The computational analyses demonstrated that such flooring systems are experiencing severe thermal gradients and bowing. When such beams are axially restrained, the compression due to the restraining may produce second order effects on the bowed beams. On the other hand, the effect of axial restraints is difficult to be estimated because of the temperature’s non-uniformity across the cross-sections. For this reason, comprehensive finite element analyses (FEA) were implemented in this paper to simulate the response of such restrained beams subjected to fire. Material properties were modelled according to Eurocodes. The coupled thermal-structural parametric analyses involved different variations of the “shortest” cross-sections. From the FE analyses, useful conclusions are drawn.