WITHDRAWAL CAPACITY OF SCREWS IN EUROPEAN ASH (FRAXINUS EXCELSIOR L.)

摘要

The accrescence of hardwoods in Austria, Germany and Switzerland exceeds the current demand by about one third. The medium quality log classes B and C of European hardwoods according to EN 1316 (1997) can be used for the production of glue laminated timber (GLT). The European ash (Fraxinus excelsior L.) is the third frequent deciduous trees in the aforementioned countries and impresses by very good mechanical properties, the relatively low shrinkage and the good gluing behaviour. Wood screws are commonly used and powerful connectors in load bearing timber structures. The calculation model for the withdrawal capacity according to ON EN 1995-1-1 (2009) is based on withdrawal tests with spruce (Picea abies (L.) Karst.). The wood species influences the withdrawal strength by means of charakteristic density. Due to the considerably higher mean density of European ash compared to spruce it seems reasonably to verify the given calculation model for hardwoods. 2571 withdrawal test results will be presented and analysed regarding the influence of screw diameter d (4 to 20 mm), nominal penetration length l_(mom) and effective penetration length 4/(3 s to 8 d), the angle of the screw axis to the grain a (0°, 15°, ..., 90°) and the density p on the withdrawal capacity R_m. A modified calculation model for the withdrawal capacity of European ash will be suggested.