What is a beam bridge?

A beam bridge is a simple type of bridge characterized by a horizontal beam supported at each end. It is the oldest and most common bridge form, with early versions consisting of wooden beams placed across a stream. Modern beam bridges are often made of steel or reinforced concrete and are used for highway overpasses and railways. Beam bridges carry vertical loads through bending, resulting in horizontal compression on the top and tension on the bottom of the beam. The supports then carry the loads from the beam vertically to the foundations. Beam bridges can be either simply supported, with beams spanning between two supports, or continuous, with multiple beams joined over supports.

How does a beam bridge support weight?

A beam bridge supports weight through horizontal beams that carry vertical loads, causing the beam to bend. As it bends, the top undergoes horizontal compression, while the bottom experiences horizontal tension. The supports then transfer the load from the beam through vertical compression to the foundations. Beam bridges are suited for shorter spans because of the bending they undergo. Modern beam bridges use steel or reinforced concrete and are often seen in highway overpasses and railways.

What materials are commonly used to build beam bridges?

Beam bridges are commonly built using steel , reinforced concrete , and prestressed concrete . Construction typically starts with concrete footings for the pier and abutments; if the soil is weak, wooden or steel piles are used for support. For shorter spans, steel beams are made as a single unit. For longer spans, steel beams are often made as plate girders, which consist of top and bottom flanges connected to a vertical web. Prestressed concrete is effective because the concrete resists compression, while the embedded steel withstands tension.

What are the main parts of a beam bridge?

A beam bridge is a simple type of bridge that consists of a horizontal beam supported at each end. The main parts of a beam bridge include: * Beams Horizontal structural members that carry vertical loads through bending. They can be made of steel, wood, or reinforced concrete. * Supports The supports carry the loads from the beam by compression vertically to the foundations. Beam bridges often have piers or abutments as vertical load-bearing members. * Deck A concrete surface cast on top of the beams.

How do engineers decide when to use a beam bridge instead of a different type of bridge?

Engineers consider several factors when deciding to use a beam bridge instead of other types. The beam bridge is the simplest and most common type, consisting of a horizontal beam supported at each end. Beam bridges are suitable for relatively short spans because of bending inherent in the design. The distance between supports is generally not more than 76 meters (250 feet), or the bridge's stability may be compromised. For longer spans, engineers may consider cantilever or suspension bridges. When choosing a bridge design, engineers aim for efficiency, economy, and aesthetics. They also consider the materials, load, and shape of the bridge.

beam bridge

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Images

Long view of the East Sea Bridge, also known as Donghai Bridge, curving across the water toward the horizon in the East China Sea near Shanghai, with distant support towers visible.

Diagram of a beam bridge spanning over water, with red lines along the top showing areas under tension and green lines along the supports and lower sections indicating compression forces.

A simple diagram of a beam bridge showing an orange horizontal beam supported at both ends by two vertical supports.

Aerial view of the Lake Pontchartrain Causeway near New Orleans, Louisiana, showing the long twin bridges stretching straight across blue water toward the horizon.

Aerial view of the Tianjin Grand Bridge, a long railway viaduct crossing over water and green landscape, with the city skyline visible in the distance.

Illustrations of six bridge types show their structural differences: a simple beam bridge, a truss bridge with triangular supports, a cantilever bridge with projecting sections, an arch bridge, a suspension bridge with cables hanging between towers, and a cable-stayed bridge with cables radiating from towers.

Quick Facts

Also called:: girder bridge

Related Topics:: cantilever bridge; pontoon bridge; continuous beam bridge; clapper bridge; simply supported beam bridge

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beam bridge

civil engineering

Also known as: girder bridge

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Top Questions

What is a beam bridge?
A beam bridge is a simple type of bridge characterized by a horizontal beam supported at each end. It is the oldest and most common bridge form, with early versions consisting of wooden beams placed across a stream. Modern beam bridges are often made of steel or reinforced concrete and are used for highway overpasses and railways.

Beam bridges carry vertical loads through bending, resulting in horizontal compression on the top and tension on the bottom of the beam. The supports then carry the loads from the beam vertically to the foundations. Beam bridges can be either simply supported, with beams spanning between two supports, or continuous, with multiple beams joined over supports.
2 Britannica Sources
1. beam bridge
2. bridge: Cantilever
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How does a beam bridge support weight?
A beam bridge supports weight through horizontal beams that carry vertical loads, causing the beam to bend. As it bends, the top undergoes horizontal compression, while the bottom experiences horizontal tension. The supports then transfer the load from the beam through vertical compression to the foundations.

Beam bridges are suited for shorter spans because of the bending they undergo. Modern beam bridges use steel or reinforced concrete and are often seen in highway overpasses and railways.
2 Britannica Sources
1. beam bridge: Mechanics
2. bridge: Cantilever
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What materials are commonly used to build beam bridges?
Beam bridges are commonly built using steel, reinforced concrete, and prestressed concrete. Construction typically starts with concrete footings for the pier and abutments; if the soil is weak, wooden or steel piles are used for support.

For shorter spans, steel beams are made as a single unit. For longer spans, steel beams are often made as plate girders, which consist of top and bottom flanges connected to a vertical web. Prestressed concrete is effective because the concrete resists compression, while the embedded steel withstands tension.
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What are the main parts of a beam bridge?
A beam bridge is a simple type of bridge that consists of a horizontal beam supported at each end. The main parts of a beam bridge include:
* Beams Horizontal structural members that carry vertical loads through bending. They can be made of steel, wood, or reinforced concrete.
* Supports The supports carry the loads from the beam by compression vertically to the foundations. Beam bridges often have piers or abutments as vertical load-bearing members.
* Deck A concrete surface cast on top of the beams.
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2. pier
3. beam bridge
4. bridge: Beam
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What are the advantages and disadvantages of beam bridges?
Beam bridges are simple structures that are supported at each end and can be made of wood, steel, or reinforced concrete.

Advantages:
* They are the most common and oldest bridge form.
* Concrete is a cost-effective material for beam bridges.

Disadvantages:
* Beam bridges are limited to relatively short spans between supports. The distance between supports is generally no more than 76 meters (250 feet), or the bridge stability is compromised.
* As the beam bridge bends, it undergoes horizontal compression on the top, while the bottom is subjected to horizontal tension.
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2. bridge: Beam bridges
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How do engineers decide when to use a beam bridge instead of a different type of bridge?
Engineers consider several factors when deciding to use a beam bridge instead of other types. The beam bridge is the simplest and most common type, consisting of a horizontal beam supported at each end.

Beam bridges are suitable for relatively short spans because of bending inherent in the design. The distance between supports is generally not more than 76 meters (250 feet), or the bridge's stability may be compromised. For longer spans, engineers may consider cantilever or suspension bridges.

When choosing a bridge design, engineers aim for efficiency, economy, and aesthetics. They also consider the materials, load, and shape of the bridge.
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1. beam bridge
2. bridge: Beam
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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.

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.

The northern terminus of the Akashi Strait Bridge in Terumi ward, southern Kobe, Hyogo prefecture, west-central Japan. The bridge spans the Akashi Strait and links Awaji Island to Honshu.

Britannica Quiz

Bridge Quiz

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.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Melissa Petruzzello.

beam bridge

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