In Asia, as early as the first century B.C., suspension bridges were constructed to provide access that averted the need for piers and other forms of centering. In the earlier designs, the structures were suspended with bamboo cables. By the sixth century A.D., the introduction of iron chains displaced the use of bamboo cables in parts of China. (Kranakis, 29)
Suspension bridges are light, strong, aesthetic and span distances from 2,000 to 7,000 feet-- much longer distances than the other types of bridges. It is the most expensive type of bridges to construct. This type of bridge as the names suggest, suspends the roadway from huge bundles of cables, which extend from one end of the bridge to the other. The cables are supported on high sturdy towers and are secured firmly at each end on the ground to firm anchors of solid rocks or massive concrete blocks.
The tower allows the cable to be draped over them for long distances. Most of the weight of the bridge is transferred by way of the cable to the anchorages. At the anchorage, the cables are spread out so that the load is evenly distributed. This provides greater security. (Super Bridges)
What are the uses of the anchorages?
Get two pieces of board an inch in thickness, 4 inches by 6 inches. Place a small nail half way down at the top of the wood. Get a length of string about 36 inches in length and to the middle of the string place a loop around each nail about 10 inches apart. Place the two pieces of wood uprightly facing each other. Allow the string to hang arching between them. Apply a weight about that of a wallet to the loop and notice the result.
Replace the pieces of wood to their original position. This time place the free ends of the string over their corresponding sides and secure each end to a separate secure anchor allowing the length of string between the anchors and the piece of wood to have just a slight arch. Then add the same weight to the loop between the pieces of wood and note the result. Notice that the anchorages on the outside of the pieces of wood help to stabilize the "bridge".
In earlier times some of the cables used were made from twisted grass. During the early part of the he nineteenth century, the cables used on suspension bridges were iron chains. Presently the cables used are made of thousands of individual steel wires bounded tightly together. Steel shows the capacity to withstand great tension making it an ideal choice for cable material.
The Humber Bridge in England, once had the world's longest center span - measuring 4,624 feet. Currently in Japan, the Akashi Kaikou Bridge, linking the islands of Honshu and Shikoku, has a center span of 6,527 feet.
Because of the length, flexibility, and lightweight feature of suspension bridges, wind is always a serious concern. The Tacoma Narrows Bridge opened in 1940, was the third longest suspension bridge in the world -- a span of 2,800 feet. It was noticeably quite unstable even in moderate wind conditions. Attempts were made to address the instability but with little success. On November 7, 1940 barely four months after being opened, it collapsed in a wind speed of 42 mph. It was designed to withstand wind speeds of up to 120 mph. Scientist believe that the wind matched the resonance frequency of the bridge and it produced a destructive displacement. (Super Bridges)
The engineer who designed the structure did anticipate some form of wind displacement, but according to others, he neglected to calculate the effects of multiple pushes of the wind force on the cables of the structure. (Kranakis,155)
Cable-Stayed Bridge
Cable-stayed bridges do look much like the suspension bridges-they all have towers and roadways hanging from cables. However, the difference lies in the manner in which the load of the roadway is supported. In suspension bridges, the cables are draped over the towers and the ends are secured to the anchorages where the load is distributed. In cable-stayed bridges, the cables are secured to the towers that are responsible for bearing the load of the bridge. There are several ways by which the cables can be attached: radial pattern -- the cables extend from several points on the road to a single point at the top of the towers. The parallel pattern attaches the cables from different heights along the tower at an angle parallel to each other.
Demonstrating a cable-stayed bridge
If you are standing up with your arms stretched horizontally, your head now becomes the tower and your outstretched arms the span or roadway of the bridge. Get a partner to tie a piece of rope to each wrist to support your arms in the horizontal position with the middle of the rope resting slightly taut on your head. Get a second piece of rope and repeat the process except this time the ends of the rope are tied to your elbows. Now you have two cable-stayed. Try bringing your arms down to your sides. Where do you feel most of the load?
The concept of cable-stayed bridges might appear to be a relatively new concept, however as early as 1595 there have been published sketches of this type of bridge. It was not until the 20th century that this design became acceptable. In Europe just after World War 11 when steel was scarce, the cable-stayed bridge designs were perfect for reconstructing bombed out bridges which still had standing foundations.
In the United States it is considered a fairly new approach to bridge construction. However, the response has been positive given the aesthetic nature of the feature and the cost effectiveness. For medium length spans between 5000 and 2,800feet, cable-stayed is fast becoming the bridge of choice. Compared to suspensions, they require less cable, can be constructed out of pre-cast concrete sections, and are faster to construct.
In 1988, the Sunshine Skyway Bridge in Tampa, Florida won the prestigious Presidential Design Award from the National Endowment for the Arts.(Super Bridges)
