Flexural behaviour of post-tensioned timber beams
An experimental program was carried out to investigate the effect of post-tensioning on timber beams and the performance of timber bridge decks with post-tensioned stringers. The experimental program contains three phases. As part of the first phase, twenty shear tests were performed to determine the ultimate capacity of lag screws that could be used in the post-tensioning process. Results showed that six lag screws with a diameter of 12.7 mm were adequate for the anchorage system. The second phase included testing 22 full scale timber beams. Six beams were control without any reinforcement. The remaining sixteen beams were post-tensioned with either draped or straight carbon fiber reinforced polymer (CFRP) tendons (Groups CD and C) or steel cables (Groups SD with draped cables and S with straight cables).The results revealed that the strength and the stiffness for Group CD were increased on average by 70% and 10%, respectively, while the strength for Group C was increased on average by 56%. In contrast, the strength and the stiffness for Group SD were increased on average by 45% and 4%, respectively, while the strength for Group S was increased on average by 49%. The bending strength of the post-tensioned timber beams was predicted using analytical model. The third phase of the research included the construction of four timber bridge decks: control deck (BD-CO), deck with stringers post-tensioned with draped CFRP tendons (BD-DC), deck with stringers post-tensioned with draped steel tendons (BD-DS), and a deck with stringers strengthened with near-surface mounted glass fibre-reinforced polymer bars (NSM-GFRP) referred to as BD-G. Post-tensioning the stringers of BD-DS and the stringers of BD-DC increased the apparent stiffness on average by 2% and 3%, respectively. In contrast, the apparent stiffness for the stringers of BD-G increased on average by 29%. SECAN program was used to analyze the four timber bridge decks. It was found that SECAN can predict the behaviour of the timber bridge deck based on the experimental stiffness of the individual tests. Cost analysis was done for the three strengthening techniques. It was found that post-tensioning stringers with draped steel cables can be cost effective.
timber, post-tensioned, beams, CFRP