Application of local post-tensioning to new and existing structures

While the technology of post-tensioning has been widely used and well developed in concrete structures, it has also been applied to other structures, such as timber frame construction. In addition, post-tensioning, along with other strengthening techniques such as retrofitting and carbon fibre reinforced polymers (CFRP), has been used to strengthen and upgrade existing steel structures using similar materials and setup as in post-tensioned concrete beams. However the technology required for prestressed members can be very expensive and not widely available in many developing countries, or even in remote areas of developed countries such as Australia, and, if specified, may prove to be uneconomical if for example personnel and equipment have to be imported or relocated. The objective of this thesis is therefore to introduce and investigate local post-tensioning (LPT) as a simple and low-cost alternative to modern high-tech post-tensioning techniques. LPT uses conventional reinforcing bars and a manual screw jack (in the case of concrete or steel members) or bracing straps and bracing tensioners (in the case of timber members), instead of tendons and hydraulic jacks, to apply post-tensioning to the new or existing structure. This research investigated LPT reinforced concrete, timber and steel beams, resulting in the following findings: ‚Äö Four large-scale concrete beams were locally post-tensioned and tested. Theoretical predictions were made based on AS 3600-2009 and a new theoretical approach was proposed to determine the cracking moment of tested beams, since the current design guide was not applicable to this post-tensioning method. ‚Äö LPT timber panels were investigated as an application to new structures. Six identicalpanels made of butt jointed and nailed timber beams were post-tensioned and tested under four-point bending. These were compared with equivalent panels made from continuous unjointed beams. In addition, the dynamic behaviour of these panels was experimentally investigated. Results showed that the post-tensioning created hogging deflections and decreased the overall serviceability deflections by almost half of their values; moreover, LPT affected the dynamic behaviour of these panels, in particular the damping ratio, which significantly decreased but remained within the acceptable limits. ‚Äö The effectiveness of LPT to enhance the repair of damaged steel beams was studied. Six repaired beams with different bar diameters and levels of pre-stress were tested under three-points bending. The level of post-tensioning controlled the beams' stiffness, while restoration of their ultimate load-carrying capacity was governed by the bar size. Significantly higher repaired capacities were achieved by this method than by other published methods used for the strengthening of steel beams. ‚Äö Upgrading of intact steel beams using LPT was also investigated. In total, seven beams were upgraded and tested under three-point bending, using different configurations of reinforcing steel. The level of post-tensioning, the type of LPT (internal or external) and the diameter of reinforcing bars used significantly affected the beams' stiffness and their ultimate load-carrying capacity. The results obtained in this study compared favourably with those of other methods used for upgrading steel beams.