Experimental analysis on small-scale finger-jointed specimens at elevated temperatures
Finger joints have to meet strength performance requirements for the use in structural engineered wood products such as glued-laminated timber beams (glulam). Among these, the thermal stability of the adhesive used in the finger joint is an important criterion to determine the suitability of the connection. The influence of adhesives on the load-carrying capacity of glulam beams and finger-jointed members can be assessed by large-scale fire tests; however, there are obvious benefits in using small-scale specimens tested at elevated temperatures as an alternative. In Europe, there is currently no small-scale test available to test the fire performance of adhesives in structural timber members. The work presented in this paper addresses this issue and was supported by members of a European standardisation committee to develop such a small-scale test. This paper presents, as a first step, the results of a series of tensile tests on small-scale finger-jointed specimens tested at elevated temperatures. The tests were performed with different types of adhesives which are currently also tested in large-scale fire tests. In the small-scale tests here, structural adhesives showed nearly no weaknesses with increasing temperature up to 140 °C. Further, MUF and PRF adhesives showed mostly wood failure even up to temperatures of 220 °C. In the next step of this investigation, the results obtained from the small-scale tests at elevated temperature will be compared to large-scale fire test results in order to find a possible link between both testing methods.
Mechanical behaviour of finger joints at elevated temperatures
Finger joints are commonly used to produce engineered wood products like glued laminated timber beams. Although comprehensive research has been conducted on the structural behaviour of finger joints at ambient temperature, there is very little information about the structural behaviour at elevated temperature. A comprehensive research project on the fire resistance of bonded timber elements is currently ongoing at the ETH Zurich. The aim of the research project is the development of simplified design models for the fire resistance of bonded structural timber elements taking into account the behaviour of the adhesive used at elevated temperature. The paper presents the results of a first series of tensile and bending tests on specimens with finger joints pre-heated in an oven. The tests were carried out with different adhesives that fulfil current approval criteria for the use in load-bearing timber components. The results showed substantial differences in temperature dependant strength reduction and failure between the different adhesives tested. Thus, the structural behaviour of finger joints at elevated temperature is strongly influenced by the behaviour of the adhesive used for bonding and may govern the fire design of engineered wood products like glued laminated timber beams.
Effect of glue type on flexural and tensile properties of finger-jointed tropical African hardwoods
A study was undertaken to evaluate the flexural and tensile properties of finger-joints produced from Obeche (Triplochiton scleroxylon), Makore (Tieghemella heckelii), and Moabi (Baillonella toxisperma) using resorcinol formaldehyde (RF), melamine formaldehyde (MF), and isocyanate (ISO) adhesives. The results of the study have indicated that, generally, glue type has a statistically significant effect on the flexural and tensile strengths of finger-joints from the three tropical African hardwoods. No significant effect was, however, observed on modulus of elasticity in both bending and tension. All three glues proved to be nearly equivalent in finger-joint bending strength for Obeche, but in tension, ISO glue was significantly superior. MF glue was significantly superior in both bending and tensile strengths for Makore and Moabi. On the basis of joint efficiency and percentage wood failure, RF and ISO glues proved unsatisfactory as a bonding medium in the medium-density Makore and the high-density Moabi. The performance of the three different glues appeared to be related to wood density, exhibiting the highest joint efficiencies in the low-density Obeche, moderate efficiencies in the medium-density Makore, and the lowest efficiencies in the high-density Moabi. MF, however, appeared to perform better in the medium-density Makore than in Obeche. The high-density Moabi resulted in the significantly lowest joint efficiency and percentage wood failure, even with MF glue. MF adhesive is recommended for finger-jointing in the medium-density Makore and the high-density Moabi, and ISO adhesive for the low-density Obeche.