Flexural behavior of post-tensoned steel trusses with high performance conrete omposite decks

Abstract

The hypothesis of using post-tensioning techniques in strengthening in situ steel trusses built compositely with high performance concrete decks is examined and evaluated experimentally and theoretically. In realizing the proposed technique, unbonded high-strength tendons are prestressed externally to provide axial compression to counteract tensile stresses caused by external loading. The effects of post-tensioning on composite truss systems are investigated as related to flexural strength and behavior. The analytical portion of the research utilizes a technique which incorporates the concepts of transformed section, principle of virtual work, and principle of superposition. In the experimental portion of the study, two full size truss specimens were simply supported at their ends and subjected to third-point monotonically increasing loads. Experimental parameters included the level of post-tensioning as applied to a complete composite system and a nominally identical truss only, without a concrete slab. Test results indicated that for the extra partial prestressing ratio (PPR) of 6.12%, the post-tensioned composite truss resisted a maximum total load 20.4% higher than that in the case of a composite truss without prestressing. The failure mode of the posttensioned composite truss occurred by yielding of the bottom chord while the tendon was still within the elastic range. This behavior resulted in a ductile failure mode displaying detectable warning deformation and is therefore preferred to sudden failure occurring without any noticeable ductile deformations. Good agreement between experimental and theoretical results was attained. The successfully developed and verified mathematical model is capable of evaluating the strength and stability of in situ structures similar to the type studied herein. The model can also be used to establish the maximum amount of post-tensioning required for structural rehabilitation as well as to evaluate the associated serviceability, and ductility factors. Such information can then be used for upgrading existing composite spans such as may be found in bridges or buildings and thereby to upgrade the flexural capacity of such linear structures by taking advantage of the high strength tendons.