Cable-suspended bridges that rely on very high strength steel cables as major structural elements may be classified as suspension bridges or cable-stayed bridges. The fundamental difference between these two classes is the manner in which the bridge deck is supported by the cables. In suspension bridges, the deck is supported at relatively short intervals by
vertical suspenders, which, in turn, are supported from a main cable (Fig. 15.6a). The main cables are relatively flexible and thus take a profile shape that is a function of the magnitude and position of loading. Inclined cables of the cable-stayed bridge (Fig. 15.6b), support the bridge deck directly with relatively taut cables, which, compared to the classical suspension bridge, provide relatively inflexible supports at several points along the span. The nearly linear geometry of the cables produces a bridge with greater stiffness than the corresponding suspension bridge.
Cable-suspended bridges are generally characterized by economy, lightness, and clarity of structural action. These types of structures illustrate the concept of form following function and present graceful and esthetically pleasing appearance. Each of these types of cablesuspended bridges may be further subclassified; those subclassifications are presented in articles that follow.
Many early cable-suspended bridges were a combination of the suspension and cablestayed systems (Art. 15.1). Such combinations can offer even greater resistance to dynamic loadings and may be more efficient for very long spans than either type alone. The only contemporary bridge of this type is Steinmans design for the Salazar Bridge across the Tagus River in Portugal. The present structure, a conventional suspension bridge, is indicated in Fig. 15.7a In the future, cable stays are to be installed to accommodate additional rail traffic (Fig. 15.7b).