The Varied Uses of Cast-in-Place Reinforced Concrete
Many units of bridge construction — foundations, columns, girders, beams, slabs, and walls — are akin to those of building construction and serve similar purposes. The bridge designer combines these units to meet the demands of design, construction, use, and setting.
Regardless of type or design, cast-in-place reinforced concrete has increasingly become the material of choice for nearly all bridge construction, because reinforced concrete bridges are typically more economical, durable, and aesthetically pleasing than bridges built of other materials.
Slab Bridges
Slab bridges may be composed of single or multiple slabs. Because of their thin profile, they are extremely useful where headroom is limited. This rigid frame bridge type consists generally of footings, walls, and a deck slab designed so that the walls and deck form a single element. The weight of the bridge deck and any loads upon it then help the legs to resist earth pressure.
- Simple, easy to construct.
- Well-suited for spans up to 50 feet.
- For longer spans, continuity with abutments and piers can immobilize frame action.
I-Girder Bridges
A simple beam bridge is commonly used for relatively short spans. When a single span is used, the end supports are called abutments or end bents. When two or more spans are used, the intermediate supports are called piers or intermediate bents. Girders can be designed as an integral part of each span or they can be cast separately. Cross-beams also can be placed between girders to provide extra support.
- Most popular bridge type.
- Well-suited for spans up to 160 feet.
- Common widths: 28 feet, 54 feet, 63 feet, and 72 feet.
Box-Girder Bridges
A box girder bridge is supported by abutments and piers in the same way as a simple- or continuous-beam bridge. The span consists of a deck slab, a series of walls, and a bottom slab, which creates voids or "boxes," provides for a saving in concrete weight, and permits longer spans. In longer spans, the depth is usually accomplished by varying the height of the walls to provide a curved under-surface.
- Second most-popular bridge type.
- Well-suited for spans from 60 feet to 105 feet.
- Common widths from 27 feet to 42 feet.
- Use of side-by-side boxes with no wearing course provides for faster construction.
Arch Bridges
Concrete arch bridges may consist of either a single arch or multiple arches supported by abutments and intermediate piers. Typical shapes include the high, round, semicircular arch, and the low, flat, elliptical arch. Concrete arch bridges are well suited for short and long spans where the foundations are solid, and may be constructed with either open or earth-filled spandrels. Arch bridges typically are used in situations, such as an entry to a city or park, where design and aesthetics are particularly important.
- Most efficient shape for supporting gravity loads.
- First segmented precast arch bridge in the U.S.: Natchez Trace Parkway; Franklin, Tennessee.
- Longest existing concrete arch bridge: Wanxian Bridge, China; 1,378 feet.
Cable-Stayed
Bridges
A more recent type of bridge currently gaining popularity as a "signature" structure, the cable-stayed bridge with a cast-in-place reinforced concrete deck, is designed so that straight cables connect the bridge's towers or pylons to its deck. Unlike suspension bridges, whose towers are generally symmetrical, cable-stayed bridges offer the design advantage of having one or multiple towers of varying dimensions.
- Economical and aesthetically pleasing.
- Structurally efficient use of materials.
- Longest concrete cable-stayed bridge in the U.S.: Dames Point, Jacksonville, Florida; 1,300 feet.
