Wood as Fire Protection of Steel in Hybrid Structural Elements

Abstract

Various solutions are used for steel-timber hybrid elements that associate two timber beams on both sides of a steel plate or a steel profile. Steel is used as mechanical reinforcement to obtain hybrid elements with high performance regarding the strength and the stiffness. Wood is a combustible material but it burns in a controlled way with low thermal conductivity. Steel is non-combustible and shows high performances in normal conditions. However, its mechanical properties decrease dramatically at elevated temperatures. Previous work on wood-steel connections highlighted the wood protection effect enhanced by the migration of water in the wood element and the absorption of some of the fire energy by vaporization of the water contained in wood. The present study focuses on the thermal performances of steel-timber cross-sections to obtain the evolution of temperature on steel element during fire exposure. The aim is to evaluate the wood material as passive fire protection for steel elements in comparison with other common solutions. Experimental program is performed using different types of wood as insulation to protect steel profiles. The thermal gradient is measured in the protected steel section using different pieces of wood in different configurations: partial or full protection with three or four exposure sides of the hybrid cross section. The tests were performed on a furnace prototype developed in the laboratory. The temperatures are measured using type K thermocouples. Results are compared with those given by a finite element model and the analytical expressions of EN1993-1-2.