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The quest for travel has always been one that man has tried to conquer. First, he traveled on foot, and then developed methods of transportation such as boats, but one of the first obstacles that he had to overcome was getting across rivers, waterways, and canals. Today, bridges have become a part of everyday life; hence without them, people would have a much more difficult time going from one place to another. In the United States alone, there are more than half a million bridges (History of Bridges 1). Although bridges have been in existence since ancient times, they were not first made by man. Nature actually made the first bridges by toppling logs across streams.
This was considered the first beam bridge as the flat rocks were used to support the log. Then, the water eroded the rocks and eventually a stone bridge was formed. The ancient Romans built arched bridges because they needed roads and bridges to connect the sections of their empire. The original swing bridge, from which the suspension bridge was produced, was a simple vine that swung people over the water.
From these beginnings, men created four main types of bridges. To begin with, beam bridges, also known as girder bridges, are one of the easiest and simplest types of bridges to build. In ancient times, people used to make these bridges out of granite. Beam bridges can be made by putting a wooden plank over an area needing to be crossed. To get across larger areas, piers or supports can be fabricated up from the river bed to hold up a couple of beams. The piers of the girder bridge have constant pressure on them.
Therefore, the beams, which are usually made out of concrete or steel, have a lot of strength so they do not crumble due to the heavy load. Many beam bridges are made of pre-stressed concrete. This concrete has steel rods inside of it, which help resist tension. When the weight pushes down on a bridge, the top of the beams are pressed together creating compression, while the bottom edge is stretched and causes tension. Beam bridges are not practical in cities with a lot of high rises, because the bridge has very low clearance. The biggest limitation of the girder bridge is its length.
The farther apart the piers are, the weaker the bridge gets. Beam bridges are good to use on spans of up to two hundred feet (How Bridges Work 1). A bridge of greater distance leads to the next type of bridge, the arch bridge. Arch bridges are half circle structures with abutments at each end. There are different types of arch bridges such as Roman, Baroque, and Renaissance.
These different types of bridges are similar in structure, but not in architecture. Arch bridges are a natural form of bridge, they do not need any supports or cables. Arch bridges made from stone do not even need cement to be held together. Ancient Romans have built arch bridges that are still up to this day. The world's first iron arch bridge, which was built in 1779, is still in tact (Wilkinson 24). Arch bridges have pressure at all times.
The design naturally puts all of the weight from the bridge to the abutments, which are usually the ground. There is almost no tension on the arch bridge, and whatever tension there is, the curve naturally dissipates it to the under side of the bridge. The bigger the under side, the more tension there will be. Therefore, size limits the strength of the bridge. The arch bridge must be made of materials that perform well under compression. While the bridge is being built, it is very shaky until the two ends meet in the middle.
Arch bridges can cross a span of up to eight hundred feet and can be used to get across large areas where piers cannot be placed, unlike the suspension bridge where piers are mandatory. (Bridges 2) Suspension bridges are aesthetic, light, and strong. They are the longest type of bridge spanning from two thousand feet to seven thousand feet (Bridges 2). The longest suspension bridge is one point six miles long and is located in Michigan (Wilkinson 25). Suspension bridges are also the most expensive type of bridge to build. They are made with heavy-duty cables. The main cable is draped between the towers.
The roadway is hanging, or suspended, from huge main cables. These cables stretch from the weight of the bridge. Most of the weight is carried on the cables to the anchorage. The anchorage is imbedded in solid rock or large concrete blocks.
Inside the anchorage, cables are spread over large areas to distribute the load and not break free. Steel wires connect the road to the main cable. Cables are made from thousands of small steel wires that are tightly fastened together. A single strand of wire is a tenth of an inch thick and can hold a half-ton of weight without breaking. Todays steel is an ideal material for cables because it is very strong under tension; however, earlier suspension bridges were made out of twisted grass. Later on, people started making them out of thick iron chains.
There are two different types of suspension bridges, one with 'M's have towers, and one with 'A's have towers. The one with 'M's have towers requires four anchorages. Underneath the roadway lies a truss system. The trusses are there to resist bending and twisting of the bridge. Weight, or compression, pushes down on the bridges deck.
The cables take the weight to the towers, which in turn are compacted straight into the earth. The suspension bridges main concern is wind, because this bridge is so flexible. Cable stayed bridges look very similar to suspension bridges, but they support the weight in very different ways. The main difference is in the way the wires are connected.
In a cable-stayed bridge, the cables are connected straight to the tower, as opposed to the suspension bridge, where the cables are connected to a main cable running between the towers. The cables in a cable-stayed bridge can be attached to the road in a variety of ways. The four main configurations are radiating, harp, fan, and star systems. The most common of the four is the radial pattern. In this pattern, the cables are parallel to each other. Cable stayed bridges also look very futuristic, but their ideas go back a long way (Bridges 3).
A cable-stayed bridge is used to cross medium lengths, which are approximately five hundred feet to two thousand eight hundred feet (Bridges 3). The cable-stayed bridge requires less cable than a suspension bridge, and is much faster to build in comparison to suspension bridges. Cable stayed bridges are supported by stave towers. These towers are A shaped, and absorb almost all of the bridges compression. Unlike suspension bridges, cable stayed bridges only have two towers and do not need any anchorages.
Now that man has overcome the obstacle of getting across rivers, canals, and waterways efficiently, it is now time to perfect these human creations so no more mishaps happen. Even though people keep hearing about collapsing bridges, they still use them because it has become a part of their everyday life style. Without bridges, people would have a difficult time getting from one place to another. It could be done by means of a ferry or other boat, but that is time consuming and a real pain in the neck because once a person gets across the river or canal, they would have to walk everywhere. So, without bridges, things would not be the way they are today.
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