Thermal response of hybrid steel-timber floor cross-sections exposed to standard fire: experimental and numerical investigations

Hybrid systems that combine steel beams with cross-laminated timber (CLT) floor slabs can be vulnerable to fire, given the combustible nature of timber. Specifically, when unprotected, heat from a fire can conduct through steel beams to the CLT panels, which in turn may experience loss of mechanical properties, and possible charring (and combustion) in the connection zone between the CLT panel and the steel beam. Accordingly, this paper aims to establish thermal profiles in hybrid steel-timber floor cross-sections exposed to fire through experimental and numerical investigations. Results from fire tests and numerical validation studies on hybrid cross-sections exposed to a standard fire are presented; a total of six experiments with unprotected, partially protected, and fully protected steel sections were conducted following an ISO 834-1 standard fire exposure. Furthermore, a two-dimensional numerical heat transfer model was developed using SAFIR software, to predict the evolution of temperatures in the hybrid cross-section. The results confirm that passive fire protection of the steelwork using intumescent coatings plays a key role in determining the extent of charring in the connection region between the CLT panel and the steel section. In addition, temperature predictions from the developed numerical model show reasonable agreement with the experimental measurements.