Determining the Flexural Capacity of Long-Span Post-Tensioned LVL Timber Beams

Abstract

Post-tensioning can be used to introduce a precamber in timber beams, similar to concrete applications, resulting in decreased deflections and, hence, optimizing material usage. However, the amount of tendon post-tensioning or eccentricity can be significantly higher than in concrete applications because of the higher tensile strength of timber. Therefore, the secondary forces (generated by the tendon elongation) can increase the ultimate capacity of the post-tensioned member. To investigate these potential benefits, experimental testing to failure was carried out on four full-scale laminated veneer lumber (LVL) beam specimens, three of which were post-tensioned with unbonded tendons. A three-dimensional finite-element-model approach was proposed to simulate the behavior of the specimens. An analytical model was also developed to calculate the static response of the beams for a preliminary design by hand. Both modeling approaches, i.e., numerical and analytical, provided good results compared with the experimental data. Results indicated that post-tensioning can increase the load-carrying capacity of timber beams at the ultimate limit state up to 56%, especially if draped tendon profiles are used and the top flange of the beam is properly dimensioned to resist combined compression stresses from post-tensioning and bending moments.