Quantitative evaluation of beech and locust dowels in contemporary window systems

The increasing sizes of windows and their related glass panes including 3-pane insulating glass create higher static and dynamic stresses in the window corners. This trend emphasizes the use of durable wood species in the dowelled window corner joints. Locust (Robinia pseudoacacia) dowels with its pronounced technical performance in numerous applications and high natural durability has therefore become a competitor to the more traditionally used dowels made of European beech (Fagus sylvatica). However, so far, there have not been tests executed confirming the technical superiority of locust dowels applied in window corners. This study was conducted as a collaborative effort between the University of Applied Sciences Bern (BFH, Biel) and Eicher wood supplies (Eicher Holzwaren AG, Schwarzenegg), as a manufacturer of dowels. Eicher produces dowels made of European beech from local forests. The goals of the study were to determine the differences in performance of dowels made of locust and beech in respect to their performance in wooden window corners. The main criteria in window construction are strength, durability, process ability and reliability. The study did subject triple-dowelled and glued window corners to standard and cyclic conditioning followed by the determination of their mechanical strength using commonly accepted standards. The analysis and interpretation of the test results was done using statistics and comparisons with the relevant literature. Results from the tension and shear tests show that window corners with dowels made of beech did not perform significantly better in any of the five tests executed compared to the window corners with locust dowels. Nevertheless, when cyclic conditioning is employed, the strength properties of the window corners with beech dowels clearly exceed the results of the dowels made with locust. This finding is important, as cyclic climate conditions are present in all applications.