Statics: Prestress

An eccentrally post-tensioned beam

Fig. 11-9: An eccentrically post-tensioned beam

In order to reduce the maximum sagging bending moment of a beam and its maximum deflection when loaded, prestressing can be used to produce initial hogging bending moments and upward deflections, which will offset parts of the bending moments and deflections induced by the subsequent downward vertical loading.

To produce a hogging bending moment and upward deflection of a beam, the post-tensioned steel bar needs to be placed below the neutral axis of the cross-section of the beam. The position of the tendon should be as low as possible to produce larger hogging moments and deflections or to allow smaller post-tension forces.

Fig. 11-9 shows a composite beam that has a span of over 10 m [11.7]. The cross-section of the beam is T shaped and the post-tensioning tendon is placed at a position much lower than the neutral axis of the section.