EUROSTEEL 2008, 3-5 September 2008, Graz, Austria EXPERIMENTAL STUDY ON FLEXIBLE END PLATE CONNECTIONS IN FIRE Ying Hu a , Buick Davison a , Ian Burgess a , Roger Plank b a Department of Civil and Structural Engineering, The University of Sheffield, Sheffield S1 3JD, UK b School of Architecture, The University of Sheffield, Sheffield S10 2TN, UK INTRODUCTION Over the last decade research has shown that steel and steel-composite structures can have a significantly greater fire resistance than is suggested by conventional tests on isolated components. Burgess [1] explains that this is largely due to the interaction between the beams and floor slabs in the fire compartment, and the restraint afforded by the surrounding structure. In the design of real projects, it is implicitly assumed that the connections have sufficient fire resistance because they are heated more slowly than the connecting members in fire situations. However, the evidence from the collapse of the WTC buildings suggests that the progressive collapse may have been triggered by the failure of steel connections [2], [3]. From full-scale fire tests, it has been found that steel connections may be the weakest components in fire conditions [4]. In research on the performance of large substructures in fire, non-linear three- dimensional analysis shows that the axial forces generated in beams are seen to reach very high values [5]. Typically theses forces can vary from compression in the early stages of a fire, when thermal expansion is restrained by the surrounding structure, to tension in the later stages due to the heated members hanging essentially in catenary. Consequently, the connections at the ends of these members are subjected in turn to these axial forces whilst also undergoing large rotations. In 1987 a joint SCI/BCSA connection group was established to produce a series of publications which would standardise detailed design methods for commonly used steel connections. Owens and Moore [6] carried out a series of tests to investigate the ability of simple steel connections to resist tying forces, as specified in UK design codes to ensure a minimum level of robustness and prevent progressive collapse. The test programme comprised 11 tests for web cleat steel connections and 10 tests for flexible endplate beam-to-column connections. It was found that conventional steel connections have inherent robustness and may provide tensile ties to resist progressive collapse. It should be noted that Owens and Moore tested tying forces applied to simple steel connections as horizontal tie effects, as indicated by UK codes. But in a fire situation the tying force is most likely to be inclined due to the connections experiencing large rotations/deformations. Hence, the connection will be subjected to both horizontal and vertical force components. Furthermore, the strength of steel connections exposed to a fire will result in a reduction of resistance, thus an understanding of the tying resistance of simple steel connections in fire is essential in the investigation of robustness of steel structures. The research group at the University of Sheffield developed a series of tests for steel connections, including three commonly used simple connections and one moment connection. This paper reports on the experiments conducted on flexible end plate connections and details of the other connections may be found in Yu et al. [7]. 1 TEST PROGRAM In this connection programme, twelve tests were separated into three classes according to initial loading angles (α): 35 o , 45 o and 55 o , which implicitly defined the horizontal and vertical components of tying forces applied to these connections. As a result of the loading mechanism, the angle of inclination of the applied tensile force varied during the test. The magnitude and angle of the force were monitored and recorded throughout the experiments. Flexible end plate (Fep)