Bridge Types

Introduction/Motivation

What impacts do bridges have on our communities and cities? Bridges provide essential links between places, providing us with access to resources, other places and other people. Bridges enable roadways to pass through varying terrain, over waterways and through mountains with minimal deviation, saving time in transport or commute or even connecting areas that would otherwise be inaccessible. Who designs these bridges? Civil engineers do. Think about bridges as a way that engineers help us bring worlds together. (Show a map of Vancouver, BC, Canada, or another city with many bridges.) For example, the jutting features of Vancouver would be difficult to access if it were not for the bridges that tie this region together.

Three basic types of bridges used in transportation are: beam and truss bridges, arch bridges and suspension bridges. To understand how bridges work, we must understand the forces that act on every bridge. Two major forces act on a bridge at any given time: compression and tension. Compression, or compressive force, is a force that acts to compress or shorten the thing it is acting on. Tension, or tensile force, is a force that acts to expand or lengthen the thing it is acting on. As a simple example, think of a spring. If we push both ends of the spring towards each other, we are compressing the spring. Thus, a force of compression is acting on it to shorten the spring. If we pull both ends of the spring away from each other, we are stretching the spring. Thus, a force of tension is acting on it to lengthen the spring. It is the purpose of the bridge design to handle these forces without breaking or failing in some manner.

Beam and Truss Bridges

Beam bridges are the simplest and least expensive type of bridge to build. The most simple beam bridges consist of a horizontal beam that is supported on each end by columns or piers. The weight of the beam and any additional load on the bridge is transferred directly to the piers. However, the beam itself must be able to support its own weight and loads between the piers. When a load pushes down on the beam, the top portion of the beam is pushed together by a compressive force while a tensile force stretches the lower portion. The farther apart the supports or piers, the weaker a beam bridge becomes. For larger beam bridges designed for heavy car and railroad traffic, the beams are substituted by simple trusses, or triangular units, which are more economical than solid beams. Engineers have used many different truss patterns in bridges. Therefore, most beam bridges rarely span more than 200 feet (61m), however, old truss bridges crossing major rivers are often as long as 500-600 feet (152-183m), not including end supports such as piers.

Arch Bridges

Arch bridges are the easiest type of bridge to recognize. They are one of the oldest types of bridges and have extraordinary natural strength. Instead of pushing straight down as beam bridges do, the weight of the arch bridge and any additional load on the bridge is carried outward along the curve of the arch to the supports at each end. These supports are called abutments. Abutments distribute the load from the bridge and keep the ends of the bridge from spreading out. The Romans were masters of the arch bridge. Many of their arch bridges used little or no mortar, or "glue," to hold the stones together. The goal of an arch bridge is to carry all loads in compression, without any tensile loads present. The stones in the structures stay together by the sheer force of their own weight and the compression transferred between them. The size of the arch, or the amount of curvature, has a major effect on the effectiveness of this type of bridge. Sometimes, in very large arch bridges, the arch is often reduced in size or flattened down, which results in significant tensile forces that must be factored into the design. Most modern arch bridges span between 100-1,500 feet (30-457m).

Suspension Bridges

Two categories of suspension bridges are: modern suspension bridges and cable-stayed bridges. Modern suspension bridges are characterized by an M-shaped cable pattern. Cables are strung over two towers and then anchored on both ends. The roadway is suspended from the cables by thinner cables or rods. The roadway's weight and any additional load are transferred to the cables, creating a tension force in the cables. The cables then transfer their force to the towers and anchors. Typical modern suspension bridges span distances from 2,000 to 7,000 feet (610-2,134m). Cable-stayed bridges are characterized by an A-shaped cable pattern. Cables are anchored directly to the towers and eliminate the need for an anchorage system. The same tensile and compressive forces are seen in a cable-stayed bridge as they are in a modern suspension bridge. Typical cable-stayed bridges span distances from 500 to 3,000 feet (152-914m), fast becoming the bridge of choice for medium length spans. Cable-stayed bridges also look cool!

Today, we are going to create simple models of each type of bridge that we just discussed to help us learn more about how the forces of tension and compression act on each one. We are also going to think about the situations when an engineer might decide to use each type of bridge when designing roadways.

Vocabulary/Definitions

abutment: A mass, as of masonry, receiving the arch, beam, truss, etc., at each end of a bridge.

anchor: Any device for securing a suspension bridge at either end.

arch bridge: A bridge that forms the shape of an arch.

beam: A long, rigid, horizontal support member of a structure.

beam bridge: A bridge that consists of beams supported by columns (piers, towers).

cable: A very strong rope made of strands of metal wire, as used to support cable cars or suspension bridges.

cable-stayed bridge: A bridge that consists of one or more towers (or columns) with cables supporting the bridge deck. Characterized by A-shaped cable patterns.

compression: A pushing force that tends to shorten objects.

deck: The "top" of the bridge on which we drive or walk.

engineer: A person who applies her/his understanding of science and math to creating things for the benefit of humanity and our world.

suspension bridge: A bridge in which the deck is hung from cables.

tension: A pulling or stretching force that tends to lengthen objects.

Three main types of bridges are beam (or truss), arch and suspension bridges. Usually, the obstacle to be overcome — another roadway, a river, a valley, a canyon, railroad tracks — is the main factor in determining which bridge type is best to use. The obstacles that require bridges are many and varied. One unusual bridge might be a catwalk high above and across a theater stage, to provide access for lighting and props. Besides how these bridges look, the main difference in the three types of bridges is the distances each type can safely span. Typical span lengths are: beam = up to 200 feet (61 m), arch = 130-500 feet (40-152 m), and suspension = 2,000-7,000 feet (610-2,134 m). The primary reason for the differences in span lengths is how each bridge type handles tension and compression forces.

At any given time, two main forces act upon a bridge: compression and tension. Compression is a force that acts to compress or shorten the thing it is acting on. Tension is a force that acts to expand or lengthen the thing it is acting on. As an example, think of a spring (see Figure 5). If we push both ends of the spring towards each other we are compressing the spring. Thus, a force of compression is acting to shorten the spring. If we pull both ends of the spring away from each other we are stretching the spring. Thus, a force of tension is acting to lengthen the spring.

Compressive and tensile forces are present in all bridges, and it is the job of engineers to design bridges capable of withstanding these forces without buckling or snapping. Buckling occurs when compressive forces overcome an object's ability to handle compression, and snapping occurs when the tensile forces overcome an object's ability to handle tension. The best way to deal with these forces is to either dissipate them or transfer them. To dissipate force is to spread it out over a greater area, so that no one spot has to bear the brunt of the concentrated force. To transfer force is to move it from an area of weakness to an area of strength, an area designed to handle the force. An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference. Figures 6 and 7 illustrate tension and compression forces acting on three bridge types.

Can you think of any bridges in your community? On roadways? On bike paths? On walking paths? Describe what they look like. What type of bridges do you think they are? Countless types of bridges exist in the world. Natural bridges are made of trees or logs positioned to cross over rivers and ravines. Bridges have also been made of wood boards, rope, wire, metal, concrete and anything that might hold a weight to allow a person or load of goods to pass.

What are some reasons that engineers design bridges? People might want a bridge for the access it provides to resources, for expansion, for trade or industry, for the purpose of being connected to another community, city or region. Bridges can also bring money to a city through trade or tourism.

What are some of the things that engineers must consider when designing bridges? Civil and structural engineers think about the type of bridge that fits the situation, the available materials (What materials are needed to build the bridge?), the site conditions (What type of soil and rock are they working on?), the geologic and environmental factors (Are native animals and plants in the area? What is the weather like?), the budget (How much money do they have?) and their audience (Who will use the bridge and for what purposes?).