Suspension Bridge

Lumacad, Eugine A. English 3 (Technical Communication) Mendez, Charles Wayne M. I. Introduction Suspension bridges in their simplest form were originally made from rope and wood. Modern suspension bridges use a box section roadway supported by high tensile strength cables. In the early nineteenth century, suspension bridges used iron chains for cables. The high tensile cables used in most modern suspension bridges were introduced in the late nineteenth century. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0. 1 inch thick, can support over half a ton without breaking. Light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet far longer than any other kind of bridge. They are ideal for covering busy waterways. With any bridge project the choice of materials and form usually comes down to cost. Suspension bridges tend to be the most expensive to build. A suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and have to be securely anchored into the bank at either end of the bridge. The towers enable the main cables to be draped over long distances. Most of the weight or load of the bridge is transferred by the cables to the anchorage systems. These are imbedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free. II. Body A. Advantages and Disadvantages of a Suspension Bridge A. a Advantages of a Suspension Bridge The center span may be made very long in proportion to the amount of materials required, allowing the bridge to economically span a very wide canyon or waterway. It can be built high over water to allow the passage of very tall ships. Neither temporary central supports nor access from beneath is required for construction, allowing it to span a deep rift or busy or turbulent waterway. Being relatively flexible it can flex under severe wind and seismic conditions, where a more rigid bridge would have to be made much stronger and so also heavier. A. b Disadvantages of a Suspension Bridge Lacking stiffness the may become unusable in turbulent and strong wind conditions and so require temporary closure to traffic. Being flexible in response to concentrated loads the structure is generally not used for regional rail crossings, which concentrate the maximums “live” loading at the location of the locomotives. Under sever wind loading, the towers exert a large torque force in the ground, and thus require very expensive foundation work when building on soft ground. B. Structural Analysis of a Suspension Bridge The main forces in a suspension bridge are tension in the main cables and compression in the pillars. Since almost all the force on the pillars is vertically downwards and they are also stabilized by the main cables, they can be made quite slender. Assuming a fairly negligible cable weight compared to the deck and vehicles being supported, a suspension bridge’s main cables will form a parabola (very similar to a catenary, the form the unloaded cables take before the deck is added). This can be seen from the constant gradient increase with linear (deck) distance, this increase in gradient at each connection with the deck providing a net upward support force. Combined with the relatively simple constraints placed upon the actual deck, this makes the suspension bridge much simpler to design and analyze than cable-stayed bridge design, where the deck is in compression. C. Suspension types The suspension in older bridges may be made from chain or linked bars, but modern bridge cables are made from multiple strands of wire. This is for greater redundancy; a few flawed strands in the hundreds used pose very little threat, whereas a single bad link or eyebar (metal bar) can eliminate the safety margin or bring down the structure outright. III. Conclusion

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