On the fire performance of unbraced composite frames

Abstract

Purpose The purpose of this study is to gain a deeper understanding of the structural behaviour of fire-exposed unbraced composite frames. Designers to date paid little attention to unbraced one-bay composite frames as structural system. There are two main reasons for this. First, codes lack simplified methods for the fire design of these frames due to their sway and the linked P-Δ effects when subjected to fire, which complicates the design. Second, it is demanding to construct external composite joints for the regarded one-bay frames. Thus, external joints in composite constructions are mostly constructed as steel joints. Nevertheless, these frames offer advantages. These include increased usable space and flexibility in the building’s use, large spans, fast construction times and inherent fire resistance. Design/methodology/approach To profit from these benefits, two different external semi-rigid composite joint were developed for the considered one-bay composite frames. The first solution based on concrete-filled steel tube columns and the second on concrete-filled double skin tube columns. Furthermore, a numerical model was established to study the fire performance of unbraced composite frames. The model was validated against four fire tests on isolated composite joints and two large-scale fire tests on unbraced composite frames. Findings Overall, the predictions of the numerical model were in good agreement with the test results. Thus, the numerical model is appropriate for further investigations on the fire performance of unbraced composite frames. Originality/value The sequence of construction results in significant stresses in the steel section, which creates difficulties in numerical modelling and may account for the relatively few studies carried out at room temperature. For the fire design, there was, to the best knowledge of the author, to date no numerical model available that was capable of considering the sequence of construction.

How bracing and heating regimes influence the fire performance of composite frames

“…The penetrating shear panel coupled the tension force T and the compression force C. The cleat also introduced the shear force V in the inner tube. Further details about the assembling of this new joint are given in (Bahr, 2011; Schaumann et al , 2010).…”

“…On this basis, it was possible to consider shear failure. In addition, the model took the interaction of internal forces in the joints into account (Bahr, 2018).…”

“…Related to the span L of the frames, this corresponded to L/26, L/17 and L/33. As discussed in (Bahr, 2011), deflections larger than L/30 put the vertical compartmentation at risk as cracks will occur in the concrete slab. Thus, the vertical deflection L/30 of the beam represented a further failure criterion.…”

“…To gain insight into the fire performance of these frames, the author conducted further numerical investigations using the finite-element software “Safir” (Franssen, 2005). The established numerical model is described in the accompanying paper (Bahr, 2018). Its main feature is that it includes the interaction of local forces and integrity failure of the vertical compartmentation, where deformations of the beam were limited to L/30 according to (Breunese et al , 2003), with L being the span of the slab.…”

“…Its main feature is that it includes the interaction of local forces and integrity failure of the vertical compartmentation, where deformations of the beam were limited to L/30 according to (Breunese et al , 2003), with L being the span of the slab. The established numerical model for the fire design of these frames was validated in (Bahr, 2018) with good agreement against four fire tests on isolated composite joints and two fire large-scale fire tests on unbraced composite frames.…”