Fire tests on steel-timber composite beams

The fire resistance and the thermomechanical behaviour of Steel-Timber Composite (STC) beams are studied through 4 fire tests. 35 non-loaded reduced specimens and 2 mechanically loaded real-scale beams are tested considering standard fire conditions (ISO 834 temperature-time curve). They consist in various steel profiles associated with timber elements in such a way that steel is fully or partially protected from fire. Timber is used as a fire protective material since it has a low conductivity and a low charring rate. Fire tests on non-loaded reduced specimen allow to investigate a wide variety of configurations, and to identify key parameters. It is found that full timber protection is very efficient as steel remains below 250°C during 35 or 70 minutes when timber protection is respectively 30 or 50mm thick. Moreover, timber moisture is found to have a beneficial impact on steel temperature, while hollow sections favour timber combustion and steel heating during the cooling phase. Full-scale mechanically loaded fire tests highlight the importance of assembly joints, because deflexion can open it, which accelerate the heating of steel. Finally, an 81 min fire resistance is measured for a STC beam with 45mm timber protection. These finding contribute to better understand the behaviour of steel-timber structural elements in fire situation. It appears that judicious mixing of timber and steel can lead to improved performances in both normal and fire situations. The presented results can be used to improve the design of STC beams.