Long Term Behaviour of Laminated Veneer Lumber Members Prestressed with Unbonded Tendons

Abstract

This paper briefly presents and discusses the results of experimental tests undertaken to resolve an issue regarding the viability of multi-storey prestressed timber structures, specifically the reduction in prestress load over time. The test programme included full-scale specimens, reduced scale specimens and small blocks of laminated veneer lumber (LVL). Based on the experimental data, the creep functions of LVL in bending and compression parallel and perpendicular to the grain were evaluated. The creep perpendicular to the grain was found to be significantly larger (about four times) than the creep parallel to the grain. The prestress losses were found to be relatively low (around 10%) when timber is loaded parallel to grain only, such as a beam. However, the losses markedly increased and reached a 34% value when 11% of the frame’s length was loaded perpendicular to grain. An attempt has been made to separate the contributions made to prestress losses by key factors, namely creep and mechano-sorption parallel and perpendicular to the grain of the LVL. Lastly, an analytical solution is proposed, developed and compared with the experimental values.