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. 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 , feet m.
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. Bridge failures are a relatively rare occurrence. So, what is it that keeps them from tumbling down due to the force of gravity?
A beam bridge has its deck beam in tension and compression. The beam can be squeezed and stretched depending on conditions. The abutments are in compression, which means they are always being squeezed. An arch bridge supports loads by distributing compression across and down the arch. The structure is always pushing in on itself.
The towers piers of a suspension bridge are in compression and the deck hangs from cables that are in tension. The deck itself is in both tension and compression. A cable-stayed bridge is similar to a suspension bridge. However, the deck hangs directly from the piers on cables. The piers are in compression and the cables are in tension.
The deck experiences both forces. A truss bridge is a variation of a beam structure with enhanced reinforcements. 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. 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. However, if you pull a cable too much, it will break. The Inka understood this and used the engineering concept of tensile strength. The tensile strength of the grass cables, or how much they can be pulled from opposite directions before they break, is critical.
The bridge builders also knew how much the cables could be stretched by the weight of the expected foot traffic on the bridge. The tensile strength of a grass rope depends on the type of grass, how much grass is used to make it, and how it is twisted and braided together with other ropes.
Can you guess how big a load the largest cable of the Q'eswachaka can hold before it breaks? Each main cable, as thick as a man's thigh, can hold 5, pounds, or 2, kilograms, more than the weight of an average automobile or the combined weight of 12 llamas! By using tension, the Inka engineers were able to span much longer distances than the bridges in Europe at the time. In Spain and the rest of Europe at the time of the Inka Empire, most bridges were built in the shape of short arches that used a pushing force, or compression.
By using a pulling force, or tension, the Inka engineers were able to build bridges that were much longer than the bridges in Europe at the time. Read additional quotes and paraphrased quotes from Spanish historians, contemporary engineers, and cultural experts to get insights about the innovative engineering qualities of the Inka Road and the Q'eswachaka bridge.
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