Rotation Capacity of Beams Prestressed with Synthetic External Tendons

Giuseppe Guimarães

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

Claudia Campos

Dept. of Civil Engineering, Military Institute of Engineering, Rio de Janeiro, Brazil

Chris Burgoyne

Department of Engineering, University of Cambridge, Cambridge, UK

External prestressing techniques have been used both in new structures and in rehabilitation of existing ones. Some advantages of this technique are the possibility of controlling and adjusting the tendon forces, the ability to inspect, replace and add tendons, and lower weight. To overcome problems due to corrosion of steel, the non-corrodable nature of parallel-lay ropes, made of aramid fibres, makes them suitable for external prestressing applications. Experimental works (Forgamini 1999; Araujo 1997; Branco, 1993) and the recent construction of a bridge (Kherkof, 1998) have demonstrated the viability of aramid ropes as external prestressing tendons.

Aramid tendons exhibit a linear behaviour up to failure with no plasticity, and a lower modulus of elasticity than steel. Despite the difference in the prestressing material, the overall behaviour of beams prestressed with external tendons is similar, with the final failure given by concrete crushing in the compression zone at a particular section, leading to a complete collapse of the structure (Burgoyne et.al., 1991). In the loading regime before concrete cracking, the behaviour of beams shows no difference between those prestressed with aramid tendons and those prestressed with steel. After concrete cracking, however, the beams show a sudden increase in their deflections; the tendon modulus influences the flexibility, ultimate load and final rotation capacity of the beams, because of the different development of tendon force in the post-cracking range.

This paper focuses on a numerical parametrical study of the flexural resistance of concrete beams prestressed with external paralley-lay aramid ropes, with particular emphasis on the rotation capacity of a critical section. A relationship between the rotation capacity and the relative position of the neutral axis is proposed. The results highlight the effects of different modulus of elasticity of tendons on the behaviour of the beams. The comparison between numerical predictions and experimental results shows good agreement. It is also concluded that the proposed relationship can be applied to a rigid-plastic model for predicting the ultimate force in unbonded tendons.