How does tension relate to bridges

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.

Broadway Bridge, Boulder, CO. Used with permission. 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 feet 61m , however, old truss bridges crossing major rivers are often as long as feet m , not including end supports such as piers. 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. A difference is that they use compression and tension differently. Bridges are able to hold so much weight because of the way they are built and the tension that holds them together. Compresssion,tension,torsion,shear ,gravity. Compression, Tension, Torsion, and the other is either bending or shear. Tension and compression are the two forces that act upon a bridge.

The strengths of Truss bridges are that Truss bridges can support and resist lateral loads. Another is that unlike the Arch and Beam bridges, the Truss bridge prevents twisting and swaying during earthquakes and high winds.

Truss bridges also resist the forces of compression and tension. Suspension bridges use steel ropes because steel is very strong and can hold under tension. Suspension bridges are built to make use of tension, whereas most other types of bridges make use of compression to bear their load.

Suspension bridges are usually designed with the deck suspended below a series of towers by cables. Other types of bridges are generally either designed with the deck being supported from below by pillars, or made up of an arch.

Concrete bridges have steel reinforcement embedded within the concrete, because concrete is weak in tension. Steel is strong in tension and compression, which makes suspension bridges practical. Every nut and bolt, steel member size and thickness, and every other component on a bridge has been located and sized by using mathematics to calculate all the forces applied to each component of the bridge, which must be resisted by the materials used.

Forces Acting on Truss BridgesThere are two major forces that act on bridges: compression and tension. The compression force bears down on an object to shorten or compress it, while tension is the directly opposing force that lengthens and stretches the object.

A spring is a good example of a simple mechanism that works with both forces. Compression pushes the coils together, thus shortening the spring and tension pulls the coils further apart, lengthening the spring. The surface tension of water is 72 mN. Adding detergent lowers the surface tension to less than this number. Some common ones will lower it to 25mN. The deck is in tension. The trusses handle both tension and comprehension, with the diagonal ones in tension and the vertical ones in compression.

A cantilever bridge is one of the simpler forms to understand. Basically, it addresses the forces of tension pulling above the bridge deck and those of compression pushing below. This sculptural structure is a type of bridge commonly referred to as a curling bridge.

A system of hydraulic pistons is used to roll it into its closed, octagonal shape. It uses an advanced hydraulic system to lift it out of the way when boats pass. While this seems simple enough, this bridge must deal with unique tension and compression issues. It leverages features of suspension and cable-stayed designs that are pushed and stretched to extreme limits when the bridge is in motion.

This structure adds a new dimension to standard bridge engineering. Bridge design is simple and complex at the same time. A bridge is constantly balancing compressive forces in certain locations with tensile ones in others so no overwhelming force, especially gravity, overcomes the structure at any time, leading to damage or collapse. The complicating factor is that compression and tension on a bridge are constantly shifting because of stressors like:.

It would be easy to build bridges if the loads on them stayed static. The forces on them would never change. The reality is that the loads can vary dramatically and dynamically throughout the day and over time. Bridges carry everything from trains, cars, trucks, and pedestrians to water lines and other utility infrastructure. The amount of traffic and utility volume shift throughout the day, causing significant variations in the live load , which can increase and decrease tensile and compressive forces across the structure.

Example: When a railroad travels over a bridge, the structure bends and flexes, then returns to its original relaxed state once the train passes by. In transferring force, a design moves stress from an area of weakness to an area of strength.

As we'll dig into on the upcoming pages, different bridges prefer to handle these stressors in different ways. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar.