Ultimate tendon stress of strengthened and non-strengthened unbonded post-tensioned I-beams

There is a lack of sufficient information on the assessment of stress in unbonded tendons at ultimate in continuous post-tensioned members. Although many investigations have been conducted on simply supported members, few studies have focused on continuous members, particularly strengthened ones. An investigation into the stress in unbonded post-tensioned tendons at ultimate in continuous I-beams externally strengthened with carbon-fibre-reinforced polymer sheets is described, with special emphasis on the effects of non-prestressed steel and the implementation of ultrahigh-strength self-compacting concrete. Six two-span 9 m I-beams were constructed, monitored and tested until failure. Finite-element analysis (FEA) was carried out to analyse the ultimate stress in the tendons (fps) and generate the responses of non-strengthened and strengthened beams. The test results were compared with prediction equations proposed in design standards and other previous studies. The tendon stress approached the point of yielding at the onset of failure for the two groups of beams. This is an important finding, resulting in higher moment redistribution and ductility demands for such members. The FEA accurately predicted fps and was applicable to both non-strengthened and strengthened beams. The code equations were found to provide fps values for practical design on the safe side.