Other Bridge Types
A cable-stayed bridge is a segmental bridge in which the superstructure is supported by inclined cables extending from a pylon above the deck. This type of structure provides economic and aesthetically pleasing solutions for bridge spans from 300 to 2500 ft. The bridges can be built using either precast or cast-in-place concrete segments and are built in a similar manner to the balance cantilever method for box girder bridges.
The cables are generally arranged in one of three configurations:
- Harp in which the inclined cables are parallel.
- Fan in which the inclined cables emerge from a point near the top of the pylon.
- Semi-fan in which the inclined cables emerge over a vertical length near the top of the pylon.
Each of these configurations may be combined with a central cable plane or twin edge cable planes. The planes, as viewed along the bridge, may be vertical or inclined. The cross section of the superstructure may be rigid as in a box section or flexible as in an edge girder system. Usually, the edge girder system is used with twin cable planes, whereas a single cable plane requires the torsionally rigid cross section of the box. These factors influence both the visual appearance of the bridge and its structural design.
The pylon configuration is influenced by the number of cable planes. A single pylon or an A-frame is generally used with a single plane of cables, whereas a portal or H-frame is used with two planes of cables.
Extradosed bridges are similar in appearance to cable-stayed bridges but have shorter pylons and a flatter stay-cable inclination. As such, the deck system becomes the primary load carrying member for dead and live loads. This type of bridge has been described as a hybrid of a cable-stayed bridge and a post-tensioned box girder bridge. The bridges are constructed in a similar manner as cable-stayed bridges. They are particularly useful where there are restrictions on the height of the pylon.
Concrete arches may be constructed segmentally using either precast concrete segments or cast-in-place segments. In both methods, the arch is erected by cantilevering out from the base of the arch. Temporary inclined cables are used to support the arch until the two halves meet at midspan. At that point, the arch is self supporting and the cables removed. This type of construction allows the arch to be built without any interference on the ground below. As such, segmental arches may be used to span environmentally sensitive areas and deep gorges.
Segmental methods may also be used for the construction of concrete columns for bridges. The method is similar to that for precast segmental superstructures using the short-line method of casting except the match casting is done vertically and not horizontally. During erection, the segments are stacked vertically on top of each other and incrementally post-tensioned. The primary advantage of using segmental columns is speed of erection.