Behaviour of Beams with External Tendons

G.B. Guimarães and C.M.O. Campos

Dept. of Civil Engineering, Pontifical Catholic University of Rio de Janeiro, Brazil

C.J. Burgoyne

Dept. of Engineering, University of Cambridge, Cambridge, UK

Presented at FIP Symposium on Post-tensioned Concrete Structures, 1996

Beams prestressed with external tendons show significant differences in behaviour from beams with conventional tendons, especially after the ultimate load has been reached. The behaviour is often characterised by large deflections caused by rotations at single crack locations, and since the tendon can move relative to the concrete, there can be a reduction in the lever arm of the tendon. This leads to a sudden reduction in load carrying capacity, which is undesirable. The compression flange can also reach its limiting capacity as the beam rotates, again leading to catastrophic loss of strength.

Beams prestressed with external prestressing tendons have a number of attractions for engineers. They allow a reduction in weight, since concrete is not provided merely to act as cover to the tendon or duct; they allow the tendons to be inspected for signs of corrosion (although that corrosion is rendered more likely in steel tendons because of the absence of an alkaline environment) and the tendons can be replaced or retensioned if necessary.

External prestressing is also an ideal application of tendons made of new materials, such as aramids; since the tendons are brittle, it is in any event necessary to avoid the strain concentrations that occur at crack locations with bonded tendons. Since aramid fibres are non-corrodable, there is no problem about the lack of an alkaline environment.

Tests on Externally Prestressed Beams

A number of tests have been carried out by the authors and their colleagues on prestressed with external tendons (Burgoyne et al, Guimaraes and Branco). Although these were carried out to demonstrate the viability of prestressing wiih aramid fibre ropes, the beam behaviour was not dependent on the type of tendon and a number of common features were observed. These features will also be applicable in beams prestressed with tendons made from steel.

References

Hillerborg, A., 1991, "Size dependency of the stress-strain curve in compression", Procs. Int. Rilem Workshop on analysis of concrete structures by fracture mechanics, 171.

Tam, A. and Pannel, F.N., 1976, "Ultimate moment resistance of unbonded partially prestressed reinforced concrete beams", Magazine of Concrete Research 28, 203-208.

Pannel, F.N., 1969, “Ultimate moment of resistance of unbonded prestressed concrete beams", Magazine of Concrete Research 21, 43.

Shah, S.P., Stroeven, P., Dalhuisen, D. and van Stekelenburg, P., 1978. “Complete stress-strain curve for steel fibre reinforced concrete in uniaxial tension and compression", Chap 7.3 in Procs Rilem Symp. Testing and test methods of fibre cement composites, ed. R.N., Swamy, The Construction Press.

Burgoyne, C.J., Guimaraes, G.B. and Chambers, J.J., 1991, "Tests on beams prestressed with unbonded polyaramid tendons", Technical Report CUED/D - Struct/TR 132, Department of Engineering, University of Cambridge.

Campos, C.M.O., in preparation, "Analysis of the flexural behaviour of concrete beams prestressed with external parallel-lay ropes", PhD Thesis, PUC Rio de Janeiro.

Guimaraes, G.B. and Branco, M.M.C., 1996, "An experimental investigation of the flexural behaviour of concrete beams prestressed with aramid tendons", to be presented at Advanced Composite Materials in Bridges and Structures II, Montreal.

CEB-FIP Model Code 1990, Comite Euro-International Du Beton, Lausanne.