Influence of Elevated Temperatures on the Composite Joint of a Composite Beam in Fire

The use of partial connection theory for composite beams at ambient temperatures is state of the art according to Eurocode 4 (EN 1994-1-1 2009), hereby the number of shear connectors can be reduced up to a minimum degree of shear connection. When designing for fire resistance, the partial shear connection theory is, however, not directly applicable in accordance to Eurocode 4—Part 1-2 (EN 1994-1-2 2008). Due to the different behaviour of steel and concrete components at elevated temperatures, high residual stresses are to be expected in the composite beam. In certain situations, the effect of these stresses on the composite joint may lead to a slip of the composite joint higher than the minimum ductility of the headed stud shear connectors given in the Eurocode 4 (EN 1994-1-1 2009). This paper shows how the usage of partial connection theory affects the load carrying capacity of a composite beam in the event of fire. A series of four large-scale fire tests was conducted to investigate various effects of a reduced degree of shear connection on the deflection behaviour and the composite joint. The experimental setup will be described, as well as the 9.0 m composite beams and test sequence. Furthermore, the deflection and temperature measurements as well as the given boundary conditions will be explained. The presented results of one fire test provide an initial conclusion about the deflection behaviour of the composite girder with a reduced degree of shear connectors as well as the behaviour of the composite joint at elevated temperatures. Due to the transient thermal conditions in the composite section and the following variation of the reduction of the yield stresses in the components of a composite section, a rising degree of shear connection has to be expected with rising temperature. Furthermore, three major effects on the slip caused by rising temperature on the composite joint will be presented.