Fire safety of bonded structural timber elements

Bonded connections in timber structures have been used for more than 100 years with the first patent for glued-laminated timber beams dating back to 1906 received by Otto Hetzer. In the beginning, casein adhesives were mostly used for gluing of engineered wood products and in the early 1940s replaced by phenolic-formaldehyde adhesives. Later on, to achieve lower hardening temperatures and to improve the bond strength, resorcinol was added as an additional component of the adhesive. Those (phenolic)-resorcinol-formaldehyde adhesives (RF/PRF) performed successfully on the market given their excellent structural performance, long-term performance and resistance to elevated temperatures, which also guaranteed excellent structural performance in fire. Since the 1970s, new adhesives have entered the market, such as melamine-(urea)formaldehyde (MF/MUF) adhesives, which have lower costs and shorter hardening times. The development of polyurethane (PUR) adhesives for engineered wood products started in Switzerland in 1985. Those adhesives satisfied the need for formaldehyde free adhesives, which is mainly attributed to health and environmental reasons. However, due to new requirements concerning the high temperature resistance of adhesives, especially in North America, newly developed adhesives are basically banned from the market, and adhesive manufacturers face a new barrier to approve their new adhesive technologies on the market. The work presented in this thesis clarifies the influence of adhesives on the fire design of glued-laminated timber beams. Additionally, clear scientifically based requirements are identified, which should be met by adhesives used in glued-laminated timber beams in case of fire. Nowadays, fire design models for timber structures, such as the “Reduced cross-section method” given in EN 1995-1-2 (2004), usually consider both the loss in cross-section attributed to charring and the temperature-dependent reduction of strength and stiffness of the uncharred residual cross-section. For glued structural timber elements such as glued-laminated timber beams, it is assumed that the adhesive does not significantly influence the fire resistance. In this thesis, twelve different adhesives for both structural and non-structural applications were tested in large-scale fire tests on finger-jointed timber lamellas. Those fire tests indicated that structural adhesives certified according to current European testing standards exhibit sufficient strength in fire for the use in glued-laminated timber beams. Taking into account the crack pattern observed in the fire tests, no significant influence on the fire resistance was found between the studied structural adhesives. Therefore, it is not necessary to consider the influence of adhesives in the design of glued-laminated timber beams, given that the adhesive is approved according to current European testing standards. For the investigation of the performance of the adhesive used in glued-laminated timber members, it is most favourable to use a simple small-scale test method that the adhesive manufacturers can use in their own laboratories. Therefore, establishing a link between small-scale tests at elevated temperatures and large-scale fire tests is of high importance, in order to replace expensive fire tests with easy-to-perform small-scale tests. In this thesis, twelve different adhesives were tested in large-scale fire tests on finger-jointed timber lamellas and in small-scale