Statics: Smaller Internal Forces

A cable-stayed bridge

Fig. 9-14: A cable-stayed bridge in Lisbon

There are many long-span cable-suspended and cable-stayed bridges in the world. Cables are used for the bridges not only because they are light and have high tensile strength, but also because they can create self-balanced systems in the structures and because the cables provide elastic supports to reduce the effective span of the decks. The latter reason is more significant.

Fig. 9-14 shows a cable-stayed bridge in Lisbon. The stayed cables act as elastic supports to the bridge decks, which effectively reduce the internal bending moments in the decks allowing the large clear spans. As the internal forces become smaller, the bridge deck becomes stiffer and can span greater clear distance, producing a more economical design.

This behaviour of the bridge can also be considered using the concept of direct force paths. Due to the use of cables, the loads acting on the bridge decks are not transmitted to their supports primarily through bending actions. Rather, loads are transmitted mainly through the tensile forces in cables to the support tower (Fig. 9-14), with the horizontal components of the cable forces induced by the self-weight of the bridge being self-balanced due to the symmetry of the structure. Vertical components of the cable forces pass directly through the tower to the foundation.

As a partially self-balanced system is created, forces are transmitted in a relatively straightforward manner from the deck to the supports, producing a stiffer and more economical design.