This paper presents a study on the failure modes of steel building in fire, with the aim of identify basic collapse mechanisms and design characteristics that play a role in the development and propagation of failures through the structural system. In particular, the effect of deformations and eigen-stresses induced by a restrained thermal expansion are not considered by current design methods and regulations, but are known to have driven the collapse of several steel and composite structures. In this study, the effect of restrained thermal expansions of steel beams exposed to fire is investigated with respect to two different structural typologies, i.e. single- and multi-story frames. In single-story buildings, such as car parks or industrial halls, the presence of stiff beams, typically required by large spans and higher service loads due to the different occupancy of the premises, may favour an outward collapse of the structures in case of fire. The consideration of failure modes of single story buildings is not required by current codes. However, an outward structural collapse represents a serious hazard, as it endanger people and premises on the outside. Such an occurrence should therefore be avoided even in case a timely evacuation of the premises is ensured by a proper fire design. In multi-story buildings instead, stiff continuous columns are typically required in order to resist horizontal actions and sustain the weight of the tall structure. As a consequence, the thermal expansion of horizontal members exposed to fire is highly restrained and may lead to a buckling failure of the beams. This failure may be detrimental in case the loss of horizontal restrain determines the failure of the columns and triggers a vertical propagation of the collapse. This hazard is especially significant in case the fire has spread through several floors, occurrence which is not contemplated by current fire design requirements, but that has been observed in most high-rise building fires. The results of the studies show that unfavourable collapse of single-story buildings and progressive collapse of multi-story buildings can be avoided by proper consideration of the relative axial and flexural stiffness of the horizontal and vertical elements of the structural system. In addition, a methodology is proposed for the development of design tables that relate the profiles of the elements needed for a safe fire design to the required soliciting loads on the beams.
Proceedings of the 24th Italian Steel Conference, 2013