The cable-stayed bridge has come into wide use since the 1950s for medium- and long-span bridges, because of its economy, stiffness, esthetic qualities, and ease of erection without falsework. Cable-stayed bridges utilize taut cables connecting pylons to a span to provide intermediate support for the span. This principle has been understood by bridge engineers for at least two centuries, as indicated in Art. 15.1.
Cable-stayed bridges are economical for bridge spans intermediate between those suited for deck girders (usually up to 600 to 800 ft but requiring extreme depths, up to 33 ft) and the longer-span suspension bridges (over 1,000 ft). The cable-stayed bridge, thus, finds application in the general range of 600- to 1,600-ft spans, but spans as long as 2,600 ft may be economically feasible.
A cable-stayed bridge has the advantage of greater stiffness over a suspension bridge.
Cable-stayed single or multiple box girders possess large torsional and lateral rigidity. These factors make the structure stable against wind and aerodynamic effects.
Structural Characteristics of Cable-Stayed Bridges
The true action of a cable-stayed bridge is considerably different from that of a suspension bridge. As contrasted with the relatively flexible main cables of the latter, the inclined, taut cables of the cable-stayed structure furnish relatively stable point supports in the main span.
Deflections are thus reduced. The structure, in effect, becomes a continuous girder over the piers, with additional intermediate, elastic (yet relatively stiff ) supports in the span. As a result, the stayed girder may be shallow. Depths usually range from 1â„60 to 1â„80 of the main span, sometimes even as small as 1â„100 of the span.
Cable forces are usually balanced between the main and flanking spans, and the structure is internally anchored; that is, it requires no massive masonry anchorages. Second-order effects of the type requiring analysis by a deflection theory are of relatively minor importance for the common, self-anchored type of cable-stayed bridge, characterized by compression in the main bridge girders.
Types of Cable-Stayed Bridges
Cable-stayed bridges may be classified by the type of material they are constructed of, by the number of spans stay-supported, by transverse arrangement of cable-stay planes, and by the longitudinal stay geometry.
A concrete cable-stayed bridge has both the superstructure girder and the pylons constructed of concrete. Generally, the pylons are cast-in-place, although in some cases, the pylons may be precast-concrete segments above the deck level to facilitate the erection sequence. The girder may consist of either precast or cast-in-place concrete segments. Examples are the Talmadge Bridge in Georgia and the Sunshine Skyway Bridge in Florida.
All-steel cable-stayed bridges consist of structural steel pylons and one or more stayed steel box girders with an orthotropic deck (Fig. 15.15). Examples are the Luling Bridge in Louisiana and the Meridian Bridge in California (also constructed as a swing span).
Other so-called steel cable-stayed bridges are, in reality, composite structures with concrete pylons, structural-steel edge girders and floorbeams (and possibly stringers), and a composite cast-in-place or precast plank deck. The precast deck concept is illustrated in Fig. 15.16.
In general, span arrangements are single span; two spans, symmetrical or asymmetrical;
three spans; or multiple spans. Single-span cable-stayed bridges are a rarity, usually dictated by unusual site conditions. An example is the Ebro River Bridge at Navarra, Spain (Fig. 15.17). Generally, back stays are anchored to deadman anchorage blocks, analogous to the simple-span suspension bridge (Fig. 15.9a).
Span Arrangements in Cable-Stayed Bridges
A few examples of two-span cable-stayed bridges are illustrated in Fig. 15.18. In two-span, asymmetrical, cable-stayed bridges, the major spans are generally in the range of 60 to 70% of the total length of stayed spans. Exceptions are the Batman Bridge (Fig. 15.18g) and Bratislava Bridge (Fig. 15.18h), where the major spans are 80% of the total length of stayed spans. The reason for the longer major span is that these bridges have a single back stay anchored to the abutment rather than several back stays distributed along the side span.
Three-span cable-stayed bridges (Fig. 15.19) generally have a center span with a length about 55% of the total length of stayed spans. The remainder is usually equally divided between the two anchor spans.
Multiple-span cable-stayed bridges (Fig. 15.20) normally have equal length spans with the exception of the two end spans, which are adjusted to connect with approach spans or the abutment. The cable-stay arrangement is symmetrical on each side of the pylons. For convenience of fabrication and erection, the girder has drop-in sections at the center of the span between the two leading stays. The ratio of drop-in span length to length between pylons varies from 20%, when a single stay emanates from each side of the pylon, to 8% when multiple stays emanate from each side of the pylon.
Transverse to the longitudinal axis of the bridge, the cable stays may be arranged in a single or double plane with respect to the longitudinal centerline of the bridge and may be positioned in vertical or inclined planes (Fig. 15.21). Single-plane systems, located along the longitudinal centerline of the structure (Fig. 15.21a) generally require a torsionally stiff stayed box girder to resist the torsional forces developed by unbalanced loading. The laterally displaced vertical system (Fig. 15.21b) has been used for a pedestrian bridge. The V-shaped arrangement (Fig. 15.21e), has been used for cable-stayed bridges supporting pipelines. This variety of transverse-stay geometry leads to numerous choices of pylon arrangements (Fig. 15.22).
There are four basic stay configurations in elevation (Fig. 15.23): radiating, harp, fan, and star. In the radiating system, all stays converge at the top of the pylon. In the harp system, the stays are parallel to each other and distributed over the height of the pylon. The fan configuration is a hybrid of the radiating and the harp system. The star system was used for the Norderelbe Bridge in Germany primarily for its esthetic appearance. The variety of configurations in elevation leads to a wide variation of geometric arrangements, as indicated by Fig. 15.23.
The number of stays used for support of the deck ranges from a single stay on each side of the pylon to a multistay arrangement, as illustrated in Figs. 15.18 to 15.20. Use of a few stays leads to large spacing between attachment points along the girder. This necessitates a relatively deep stayed girder and large concentrations of stay force to the girder, with attendant complicated connection details. A large number of stays has the advantage of reduction in girder depth, smaller diameter stays, simpler connection details, and relative ease of erection by the cantilever method. However, the number of terminal stay anchorages is increased and there are more stays to install.
A list of major modern cable-stayed bridges is provided in Table 15.2.