beam bridge

beam bridge, simple bridge in which a horizontal beam is supported at each end. The beam bridge is the most common and oldest bridge form. The earliest examples were simple wooden beams placed across a stream or other declivity to serve as a crossing. Later, beam bridges were supported by wooden or stone piers, enabling the construction of larger and stronger structures. Modern beam bridges, made of steel or reinforced concrete, are often used in highway overpasses and railways. When a bridge is made up of beams spanning between only two supports, it is called a simply supported beam bridge. If two or more beams are joined rigidly together over supports, the bridge becomes continuous.

Mechanics

In architecture, a horizontal beam carries vertical loads by bending. As the beam bridge bends, it undergoes horizontal compression on the top. At the same time, the bottom of the beam is subjected to horizontal tension. The supports carry the loads from the beam by compression vertically to the foundations. Prestressed concrete is a particularly effective material for beam bridges because the concrete is able to resist the forces of compression, while the embedded steel withstands the forces of tension. Concrete is also the most cost-effective material. Given the bending inherent in beam bridges, both simply supported and continuous beam bridges are limited to relatively short spans between supports. In general, the distance between supports is not more than 76 metres (250 feet) or the bridge stability is compromised.

Britannica Quiz

Bridge Quiz

Construction

In the case of a typical overpass beam bridge with one support in the middle, construction begins with the casting of concrete footings for the pier and abutments. Where the soil is especially weak, wooden or steel piles are driven to support the footings. After the concrete piers and abutments have hardened sufficiently, the erection of a concrete or steel superstructure begins. Steel beams are generally made in a factory, shipped to the site, and set in place by cranes. For short spans, steel beams are usually formed as a single unit. At the site, they are placed parallel to each other, with temporary forms between them so that a concrete deck can be cast on top. The beams usually have metal pieces welded on their top flanges, around which the concrete is poured. These pieces provide a connection between beam and slab, thus producing a composite structure.

For longer spans, steel beams are made in the form of plate girders. A plate girder is an I beam consisting of separate top and bottom flanges welded or bolted to a vertical web. While beams for short spans are usually of a constant depth, beams for longer spans are often haunched—that is, deeper at the supports and shallower at mid-span. Haunching stiffens the beam at the supports, thereby reducing bending at mid-span.

Examples

The Lake Pontchartrain Causeway in southeastern Louisiana is among the longest overwater bridges in the world. Its twin spans are formed of two continuous beam bridges that run parallel to each other for nearly 39 km (24 miles) across Lake Pontchartrain. Completed in 1956 and 1969, respectively, both the southbound and northbound structures are made of thousands of individual beam bridge spans. The Manchac Swamp Bridge, also in Louisiana, is another major beam bridge. Supported by concrete trestles, it extends 36,690 metres (almost 23 miles).

Opened in 2005, China’s Donghai Bridge is one of the longest oversea bridges in the world. Extending more than 32,500 metres (20 miles), the continuous beam bridge serves to connect Shanghai with a port in Zhejiang. The Tianjin Grand Bridge in northern China extends 113.7 km (70.6 miles) between Langfang and Qingxian and serves as a railway viaduct for the Beijing-Shanghai High-Speed Railway. The bridge opened in 2011 and is the third longest bridge of any type in the world. One of the tallest beam bridges is the Labajin Bridge in Sichuan, China. Completed in 2012 with a unique design to withstand seismic activity, its tallest pier rises 183 metres (600 feet) above the bottom of the rugged gorge below.