5 th International Conference on Advances in Steel Structures Experimental Investigation of the Robustness of Fin Plate Connections in Fire Hongxia Yu; Ian Burgess; Buick Davison Department of Civil and Structural Engineering, The University of Sheffield, Sheffield S1 3JD, UK Roger Plank School of Architecture, The University of Sheffield, Sheffield S10 2TN, UK Abstract Fire hazards and full-scale structural tests have indicated that steel connections could be subjected to large deformations and fracture in fire. This is not currently considered in design approaches because the connections are assumed to heat up more slowly than the structural frame members, and therefore retain more relative strength. A project at the Universities of Sheffield and Manchester is currently investigating the robustness of common types of steel connections when subjected to fire. This paper reports on a part of the test results on fin plate connections. The test results illustrate that bolts have their strength reduced faster than hot- rolled steel with increase of temperature, and failure of fin plate joints is quite often controlled by bolt failure in shear. As a result of bolt shear fracture, fin plate connections have unexpectedly low resistance and ductility when subject to elevated temperatures and large rotations. 1. Introduction Current design codes generally consider that steel connections will be heated more slowly than beams or columns in fire situations, and are therefore less likely to be the critical components in fire safety design. However, evidence from the collapse of the WTC buildings (FEMA, 2002) and full-scale fire tests at Cardington (Newman et al., 2004) indicates that connections may often be the weakest link in a structural frame in fire conditions. This is because, at ambient temperature, connections are designed to transfer shear and/or moment, whereas in fire they can be subjected to additional compressive or tensile forces due to restraint to thermal expansion or to catenary action arising from large deflections. At very high temperatures, beams lose most of their bending capacity, and develop axial tensile forces which, in combination with large deflections, may support the lateral loads by second-order effects. In consequence, the connections may eventually be subjected to large rotations and significant tensile forces. Under such conditions there is a clear possibility of connection fracture, which may lead either to fire spread to upper floors, or to progressive collapse of the building. In the past, connections have been extensively investigated to determine moment- rotation behaviour. However, the importance of connections in providing tying resistance to hold the whole structure together should not be overlooked (BSI, 2001). Previous researches (Yu and Liew, 2005) have shown that in a fire situation connections can be subjected to significant tying force when the beams are heated and deformed to high deflection so that they perform like cables to resist upper floor loads. 722