Fire design of composite structures exposed to natural fires

In this contribution a new approach is introduced for structural fire design of load bearing members. Detailed knowledge of natural fires and their relevant parameters as well as the development in numerical methods for structural fire safety permit an approach that leaves the traditional procedure based on standard fire tests. The basic idea is simple. As in cold design the relevant effects result from mechanical loading, the relevant effects result from the fire load density in the fire situation. Consequentially a limit fire load according to a particular structural member may be defined analogical to the ultimate load bearing capacity in cold design. In real fires stability of the structural member is maintained, if the fire load density is below this limit value. On the basis of numerous numerical simulations it could be demonstrated that composite members, which are classified to the fire resistance class R 60 following the traditional design method, have limit fire load densities higher than 1000 MJ/m2 and do not fail under natural fires in general buildings. For practical design purposes a correlation is given between the traditionally fire resistance classes and design fire scenarios, so that the fire design can be carried out realistically and economically on the bases of a natural fire safety concept.