What is a bridge in engineering?

In engineering, a bridge is a structure that spans horizontally between supports and carries vertical loads. The supports must be strong enough to hold the structure, and the span between them must be strong enough to carry loads. There are six main types of bridges: beam, truss, arch, suspension, cantilever, and cable-stay. The beam bridge is the most common type, carrying vertical loads through bending, resulting in horizontal compression on the top and tension on the bottom. Wood , stone , iron, and concrete are the primary materials used for bridges. Modern bridges are mainly built with steel, reinforced concrete, and prestressed concrete.

What are the main parts of a bridge?

Bridges consist of several main parts that work together to support loads and ensure structural integrity. The basic bridge forms include the beam, truss, arch, suspension, cantilever, and cable-stay. Key components include: * Deck: The surface that carries traffic. * Supports: These transfer loads from the bridge to the foundations. Common supports include piers and abutments . * Foundations: These prevent vertical settling and horizontal sliding. * Cables: Curved cables in tension that carry vertical loads. * Towers: These carry loads by vertical compression to the ground.

Why do engineers build different types of bridges, like beam and arch bridges?

Engineers choose different bridge types like beam and arch bridges based on efficiency, economy, and aesthetics. The beam bridge is the most common type, where a horizontal beam is supported at each end and carries vertical loads by bending. They are cost-effective for shorter spans because they use less material. Arch bridges carry loads primarily through compression, requiring strong foundations to handle vertical and horizontal forces. They can span wider openings compared to beam bridges because they can be constructed from smaller blocks of material. Ultimately, the selection depends on balancing these factors while meeting the specific requirements of the site.

How does a suspension bridge support its weight and traffic?

A suspension bridge supports vertical loads using curved cables under tension. These loads are then transferred to the towers, which send the weight as vertical compression to the ground. The loads are also transferred to anchorages, which resist the inward and sometimes vertical pull of the cables. Bridges are designed to handle their own permanent weight, known as dead load, as well as the weight of traffic, referred to as live loads. Suspension bridge design incorporates deflection theory to ensure the horizontal deck and curved cables work together when bearing weight.

What materials are commonly used to construct bridges and why?

The primary materials for bridges are wood, stone, iron, and concrete; iron had the greatest effect on modern bridges. Today, steel, reinforced concrete, and prestressed concrete are almost exclusively used. Wood is inexpensive and available, but it is weak in compression and tension. It is suitable for small bridges with light loads. Stone is strong in compression but weak in tension, so it is mainly used in arches, piers, and abutments. Concrete is strong in compression but weak in tension. It is a mix of water, sand, gravel, and a binder like cement. Reinforced concrete has steel bars to handle tension, which allows for less concrete usage and protects steel from corrosion. Prestressed concrete reduces the amount of steel and concrete needed, resulting in lighter and less expensive designs. Steel is stronger than iron in both tension and compression.

How do engineers make sure a bridge is safe and can last for a long time?

To ensure a bridge's safety and longevity, engineers consider multiple factors during design and construction. They calculate the bridge's dead load (the bridge's weight) and the expected live loads (traffic). They also account for environmental factors such as wind and seismic activity. Modern bridges mainly use steel, reinforced concrete, and prestressed concrete. These materials provide strength and durability. Bridges are designed to handle static wind load (horizontal pressure) and dynamic wind load (vertical motion). Expansion joints and bearings accommodate material expansion and contraction due to temperature changes, reducing stress. Aerodynamic design also minimizes wind impact.

bridge

Introduction
The elements of bridge design
- Basic forms
  - Beam
  - Truss
  - Arch
  - Suspension
  - Cantilever
  - Cable-stay
- Materials
  - Wood and stone
  - Iron and steel
  - Concrete
- Construction
  - Beam bridges
  - Arch bridges
  - Suspension bridges
  - Cantilever bridges
  - Cable-stayed bridges
- Performance in service
  - Live load and dead load
  - Forces of nature
The history of bridge design
- Early wood and stone bridges
  - The ancient world
    - Beam bridges
    - Roman arch bridges
    - Asian cantilever and arch bridges
  - The Middle Ages
  - The Renaissance and after
    - Stone arch bridges
    - Timber truss bridges
- Iron and steel bridges, 1779–1929
  - Iron
    - Early designs
    - Railway bridges
    - Suspension bridges
  - Steel
    - Railway bridges
    - Suspension bridges
- Concrete bridges
  - Reinforced concrete
    - Early bridges
    - Maillart’s innovations
  - Prestressed concrete
    - Eugène Freyssinet
    - Ulrich Finsterwalder
    - Christian Menn
- Steel bridges after 1931
  - Long-span suspension bridges
    - Tacoma Narrows
    - Lessons of the disaster
  - Truss bridges
- Cable-stayed bridges
  - European designs
  - U.S. designs
- Asian long-span bridges and viaducts
  - China
    - Danyang-Kunshan Grand Bridge
    - Tianjin Grand Bridge
    - Hong Kong–Zhuhai–Macau Bridge
  - Japan
    - Ōsaka Harbor
    - Island bridges
  - Taiwan

References & Edit History Related Topics

Images & Videos

The vidoe thumbnail image shows a two-sided bridge lifting and separating at the center. A blue circle highlights the open section.

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.

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.

Diagram of a single-span truss bridge over water, with red lines showing areas under tension and green lines showing areas under compression in the bridge structure.

Illustration of an arch bridge spanning two cliffs, with a green line along the curve of the arch indicating the path of compression forces through the structure.

Diagram of a suspension bridge spanning over water, with red lines illustrating tension forces in the cables and green lines showing compression forces in the vertical towers.

Diagram of a cantilever bridge spanning water, with red lines marking areas under tension and green lines marking areas under compression in the bridge structure.

Diagram of a cable-stayed bridge spanning water, with red lines showing tension forces in the cables and green lines showing compression forces in the bridge structure.

At a Glance

bridge summary

bridge

engineering

Written by

Philip N. BillingtonAll

Fact-checked by

Britannica Editors

Last updated

Oct. 30, 2026 •History

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Cable thieves strike new 1,281m Sejingkat Bridge in Sarawak • Oct. 23, 2026, 2:37 AM ET (Toronto Star)

bridge, structure that spans horizontally between supports, whose function is to carry vertical loads. The prototypical bridge is quite simple—two supports holding up a beam—yet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must be strong enough to hold the structure up, and the span between supports must be strong enough to carry the loads. Spans are generally made as short as possible; long spans are justified where good foundations are limited—for example, over estuaries with deep water.

All major bridges are built with the public’s money. Therefore, bridge design that best serves the public interest has a threefold goal: to be as efficient, as economical, and as elegant as is safely possible. Efficiency is a scientific principle that puts a value on reducing materials while increasing performance. Economy is a social principle that puts value on reducing the costs of construction and maintenance while retaining efficiency. Finally, elegance is a symbolic or visual principle that puts value on the personal expression of the designer without compromising performance or economy. There is little disagreement over what constitutes efficiency and economy, but the definition of elegance has always been controversial.

Modern designers have written about elegance or aesthetics since the early 19th century, beginning with the Scottish engineer Thomas Telford. Bridges ultimately belong to the general public, which is the final arbiter of this issue, but in general there are three positions taken by professionals. The first principle holds that the structure of a bridge is the province of the engineer and that beauty is fully achieved only by the addition of architecture. The second idea, arguing from the standpoint of pure engineering, insists that bridges making the most efficient possible use of materials are by definition beautiful. The third case holds that architecture is not needed but that engineers must think about how to make the structure beautiful. This last principle recognizes the fact that engineers have many possible choices of roughly equal efficiency and economy and can therefore express their own aesthetic ideas without adding significantly to materials or cost.

Generally speaking, bridges can be divided into two categories: standard overpass bridges or unique-design bridges over rivers, chasms, or estuaries. This article describes features common to both types, but it concentrates on the unique bridges because of their greater technical, economic, and aesthetic interest. See also list of bridges.

The elements of bridge design

Basic forms

There are six basic bridge forms: the beam, the truss, the arch, the suspension, the cantilever, and the cable-stay.

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

Beam

The beam bridge is the most common bridge form. A 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.

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.

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The elements of bridge design

Basic forms

Beam