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.
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.
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.
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.
The work presented here has been carried out with a financial grant from the Research Fund for Coal and Steel of the European Community. All fire tests were conducted by the partner CTICM under the supervision of Dr Bin Zhao. The author conducted the numerical studies during his time at the Institute of Steel Construction under the supervision of Professor Peter Schaumann. The author gratefully acknowledges the financial support and appreciates the intensive cooperation among the project partners.
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