Fire Safety Science Digital Archive

Design models for timber structures in fire usually take into account the loss in cross-section due to charring as well as the temperature dependent reduction in strength of timber. For bonded timber elements like glued laminated timber beams it is assumed that the adhesive used does not influence the resistance of structural timber beams significantly. To investigate the fire resistance of bonded timber elements a comprehensive research project is currently ongoing at the Institute of Structural Engineering of ETH Zurich, Switzerland. The aim of this research project is the development of a simplified design model for the fire resistance of bonded structural timber elements taking into account the behavior at elevated temperature of the adhesive used. The paper presents results of first numerical and experimental analyses on the influence of the adhesive on the load-carrying capacity of glued laminated timber members in fire. It is shown that the temperature dependent material properties of the adhesive used in the bondline of glued laminated timber beams have small influence on the shear resistance. For the investigated timber beams failure occurs mainly due to exceeding the bending moment resistance. Because of the steep temperature gradient in timber beams during fire exposure the temperature in the inner region of the cross-section is rather low; hence the shear capacity in the bondline between the different lamellas is still sufficient. However, in case of finger jointed lamellas of glued laminated timber beams the influence of adhesive used in the finger joints is more significant and has to be considered. Tensile tests at elevated temperatures on finger jointed specimens confirmed the significant influence of the adhesive used.