Arch Bridges

An architectural structure that relies on a curved, semi-circular shape for support. Most arches distribute weight equally and are an attractive option for bridges because they can span great distances without needing posts or other weight-bearing mechanisms to be sunk into the ground. Common across wide rivers, gorges, and other places where it may not be practical to set up intermediary supports.

Source of Strength Strength comes from its shape. As the arch presses downward from its top, the ground pushes upward with an equal force. Extra support is also often placed along the abutments, or sides, of the structure to keep the arch from bending outwards when heavy loads press down on it, but in most cases nothing needs to touch the ground at any point along the span. As a result, the arch is always under compression, causing it to become rigid and maintain its strength.

Engineering Design Process Building Bridges Step 1: Research

How They’re Built These sorts of bridges have to be built in such a way so as to handle the strong compression forces that help them keep their shape, which often takes a bit of engineering expertise. There are a couple of different techniques, but most involve starting with a basic form and then adding supports along the way. Some builders start with a wooden frame, for instance, which helps maintain the arch shape during construction. This is most common for bridges made of stone or concrete. Once the complete arch is in place, the temporary wooden structure can be removed. Ancient stone bridges relied on the final top stone, or keystone, to be put into place for the bridge to experience the compression that would make it strong. Supports along the side of the bridge can also help keep its shape as loads press down from above.

Types of Building Material Many of the oldest arch bridges are made of cut stone. Modern bridges that rely on reinforced poured concrete can often maintain their strength from the concrete itself, which means that no additional supports are needed along the sides. Ancient Romans and Greeks built a number of these structures as did civilizations throughout Asia, and a number of these bridges still exist today. Stones are some of the best materials for withstanding compression because of the way they absorb and transfer energy. Stone and brick are still certainly used in modern bridge construction, although steel and poured concrete, which can be more easily shaped, are also quite common. Supports are very common in metalworking, though, particularly if a bridge is designed to support great weights. Train trestles, for instance, often need to be a lot stronger than bridges used primarily by pedestrians or even passenger cars.

Variations and Uses Most popular over water It’s also common to see these bridges over steep gorges, mountain valleys, or anywhere else cars or trains need to go but where it might be difficult to sink supports. There are many types of arch bridges, and many different ways in which they can be used. They are perhaps most popular over water since they don’t require posts or supports to be sunk midway, which can be both disruptive to water flow and expensive to install and maintain. Depending on how high the arch is they may also allow easy passage of water vessels like boats. It’s also common to see these bridges over steep gorges, mountain valleys, or anywhere else cars or trains need to go but where it might be difficult to sink supports. When built properly they can usually support great loads without disturbing much of the surrounding environment.

Beam Bridges

sometimes called a girder bridge a rigid structure that consists of one horizontal beam supported at each end, usually by some kind of pillar or pier. In structural terms, it is the simplest type of bridge and is a popular selection because of its inexpensive construction costs. Today, it is commonly made from reinforced concrete or steel beams for everything from pedestrian bridges to highway overpasses.

Source of Strength This type of bridge works on the principles of compression and tension so a strong beam is needed to resist twisting and bending under the weight it must support. When a load — like a group of traveling cars — pushes down on the beam, the weight of the beam pushes down on the piers. The beam's top edge is pushed together as the result of compression, and tension causes the bottom edge to stretch and lengthen. This works in the same way that a wooden plank supported by blocks on each end can only hold a certain amount of weight before it buckles. The top reaches maximum compression, and the bottom snaps under too much tension.

How They’re Built Many beam bridges used in road construction are made from concrete and steel because these materials are strong enough to withstand the forces of compression and tension. The distance a beam can span is directly related to its height, because higher beams offer more material to dissipate tension. To create taller beams, trusses may be added for reinforcement. A truss is a construction of lattice work that supports a beam, creating rigidity and increasing the beam’s ability to dissipate the compression and tension. This technique only works to a certain degree, because eventually the weight of the bridge and trusses will be too heavy to be supported.

Limitations In spite of reinforcements like concrete, steel, and trusses, the biggest limitation of this type of bridge is still its length, so they rarely span more than 250 feet (76.2 m). Longer distances can be reached by daisy-chaining bridge sections together to create a continuous span. One of the world's longest bridges is a continuous span beam bridge that was created this way. Located in Louisiana, the Lake Pontchartrain Causeway is a pair of parallel bridges that measure almost 24 miles (38.5 km) long and are supported by 9,500 concrete pilings.

Lake Ponchatrain Causeway

Chesapeake Bay Bridge Tunnel

Key West

Key West

Suspension Bridges

suspension bridge, bridge with overhead cables supporting its roadway. One of the oldest of engineering forms, suspension bridges were constructed by primitive peoples using vines for cables and mounting the roadway directly on the cables. A much stronger type was introduced in India about the 4th century ad that used cables of plaited bamboo and later of iron chain, with the roadway suspended.

In modern times, the suspension bridge provided an economical solution to the problem of long spans over navigable streams or at other sites where it is difficult to found piers in the stream. British, French, American, and other engineers of the late 18th and early 19th centuries encountered serious problems of stability and strength against wind forces and heavy loads; failures resulted from storms, heavy snows, and droves of cattle. Credit for solving the problem belongs principally to John Augustus Roebling, a German-born American engineer who added a web truss to either side of his roadways and produced a structure so rigid that he successfully bridged the Niagara Gorge at Niagara Falls, New York, the Ohio River at Cincinnati, and, finally, in his masterpiece, the East River between Brooklyn and Manhattan at New York City.

Cable Stayed Bridge http://science.howstuffworks.com/engineering/civil/bridge7.htm