Behaviour of Grade 8.8 bolts under natural fire conditions—Tests and model

Recent full-scale experimental tests performed on steel and composite structures have demonstrated that the presence of tensile forces in axially-restrained beams during the cooling phase of a fire could lead to bolt failures. In order to understand this observation and design structures that are not prone to such a failure mode, it is essential to have a deep knowledge of the material behaviour of all the components, including bolts, during both the heating and cooling phase.In the present article, the test set-ups and the results of the tensile and shear tests performed at the Centro Sviluppo Materiali (Italy) on Grade 8.8 bolts under heating–cooling cycles are described.Then, material laws are defined for characterising the mechanical behaviour of Grade 8.8 bolts under heating and cooling phases. These laws account for the non-reversibility of the mechanical properties of Grade 8.8 bolts.

► The behaviour of Grade 8.8 bolts is not reversible after heating up to 500∘C.
► The loss of bolts resistance after a complete heating–cooling cycle can be 40%.
► The ductility of bolts is clearly increased when the peak temperature of the cycle is 800∘C.