Relaxation behaviour of prestressed wood assemblies. Part 1: Experimental study

Abstract

Transverse prestress has been found to increase significantly the strength and stiffness of laminated timber bridge decks and timber joints using timber connectors. A fundamental requirement to the successful use of prestressing with timber is the ability to predict the reduction of prestress with time for transversely prestressed timber applications. The loss of prestress with time for transversely prestressed assemblies was studied analytically and experimentally. In the experimental study, Douglas fir specimens were subjected to two different relative humidity levels and to various stressing histories. The relaxation behaviour and the moisture content of the specimens were continuously monitored for 9 months. Results show that the relaxation behaviour of transversely prestressed wood is affected by the relative humidity level in the surrounding environment and by the stressing frequency. The magnitude of the observed prestress losses and the graphs showing prestress levels and moisture content of specimens as a function of time under various externally imposed conditions are presented. Key words: connections, wood, prestress, Douglas fir, rheology.