Building Big

October 10, 2000

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BRIDGE FACTS
 

BridgeBridges are all around us - but how do they work? What makes them hold our weight? Why are there different kinds of bridges? Is one kind of bridge better than another kind? Wow - there is so much to find out about bridges! Read on and learn about the different bridge types and why one might work better than another!

There are five basic types of bridges: arched, cable-stayed, cantilevered, post and beam, and suspension.

Arched Bridges: Arched Bridges are one of the oldest forms of bridges. An arched bridge channels the weight from above to the foundations below. Pretty cool huh! Arch bridges don't need additional supports or cables - they will stand on their own for a long time. In fact, there are arch bridges still standing today that were built by the Romans!

An arch bridge has a semicircular design with abutments on each end. An arch is known as a compression structure. Stone arches are built over a temporary wooden form called a centering. Once the last piece of stone, called the keystone, has been set in place, the arch is pretty much finished. But if the centering is removed before the keystone is set, the uppermost stones would want to fall, and this would push the sides of the arch outward. To prevent this from happening, more material must be built up against the sides of the arch before the centering is removed. This prevents movement between the stones, and also compresses them together. The more the wedge-shaped stones are compressed, the stronger the arch.

Post and Beam Bridges: Beam bridges are the most commont types of bridges. To understand how a beam works, imagine a board placed over two supports, creating a bridge. If you stand on the middle of the board, it will bend. The top surface of the board grows slightly shorter because it is being compressed; the bottom surface is being stretched because it is in tension. If the board cannot withstand one or both of these forces, it will break. In addition to bending, beams must also be able to handle twisting.

Early post-and-beam bridges were actually tunnels: two parallel tubes made of wrought iron and supported on high stone piers. Later designs created open sides, usually employing trusses. The open design resisted torsion caused by wind by allowing the wind to pass through. A truss is a collection of interconnected triangles, the sides of which carry the tension and compression forces. Today, steel trusses are among the most common bridges in North America. They are very strong and can span long distances.

Suspension BridgeSuspension Bridges: The main parts of a suspension bridge, other than the roadway, are the towers, the cables and the cable anchorages. The roadway itself actually hangs from the cables. If the cables stopped at the tops of the towers, their own weight in addition to the weight of the roadway and that of the traffic would bend the tops of the towers toward each other. To prevent this from happening, the cables pass over the tops of the towers and are connected to concrete anchorages cast into solid rock. The action of the cables pulling down on both sides of a tower creates a strong vertical force that is carried to the foundations.

Cantilevered Bridges: A cantilever bridge is used for crossing large spans, especially ones with heavy loads. A cantilever is a projecting beam supported or fixed at only one end, like a shelf bracket or diving board. By increasing the depth of a beam, it's possible to reduce the amount of bending in the beam. Since most of this bending happens in the center of the beam, that is where the depth should be the greatest. Because this becomes the heaviest part of the structure, the pier is usually in the center of the beam.

Cable stayed Bridges: In a cable-stayed bridge, the roadway is supported by a series of cables extending in one or two planes from the tower or towers. Unlike the main cable of a suspension bridge, cable stays are straight and anchor directly into the roadway itself. Each forms the third leg of a triangle. The cable stays are in tension while the road and tower are in compression.

DOMESDome

A dome is a curved roof which encloses a circular space. A dome must support its own dead load as well as the live load of wind, rain, snow, or ice. Geodesic domes are made of interlocking geometric shapes--often triangles. Because loads are spread over many triangles, these domes are especially strong. It is hard to distort a triangle; compression at one joint is balanced by tension along the opposite side. The geodesic dome’s design distributes loads over all of the different triangles.

SkyscraperSKYSCRAPERS

A skyscraper is a very tall building, generally greater than 40 or 50 stories. Many forces are at work on tall buildings, or skyscrapers. Gravity and the dead load of the tower push down, the ground pushes back up, and small air movements push from the side. The foundation distributes the load into the surrounding ground material and can help balance the sideways wind force. The size of the foundation depends on the strength of the supporting ground. A foundation placed in rock can be smaller than a foundation placed in sand or mud, for example. Check out these sites about skyscrapers!

 

DAMS

A dam is a barrier built across a stream or river to stop or check the flow of water. Generally done to capture water for human irrigation, flood control or to generate elecrical power. The most important load that a dam must be able to support is the water behind it. How much the water pushes on the dam is called water pressure. A dam’s shape can help it to withstand the pressure.

How does a dam withstand the crushing pressure of a lakeful of water? Water pressure increases with the depth of the water. In deep water, there is more water "piled up," which causes the pressure to be greater at the bottom than at the surface. A dam’s design must enable it to withstand greater pressure at the bottom than at the top. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the top of the dam can be built thinner so as not to use unnecessary costly materials.

There are different types of dams based on what materials used, and dam design. The factors in determining the type of dam built include the size, foundation conditions, temperature, pressure changes, water analysis, and location.

Dam

The two basic types of dams include earth filled dams and masonry dams. Gravity dams utilize the downward force of the weight of the construction materials to resist the horizontal force of the water. Concrete-buttress dams have less material in the wall itself but use support buttresses around the outside for support. The arch dam is built in a curved arch facing the water.

One of the most important parts of a dam is the spillway, which allows excess water to be discharged.

TUNNELSTunnel

Tunnels can be divided into four general categories, depending on the type of ground that they pass through: soft or solid rock, soft ground or underwater. Before starting to dig a tunnel, test holes have to be drilled and soil samples need to be taken. All tunnels need ventilation to provide air to workers and to draw out any dangerous fumes. In order to prevent soft ground tunnels from collapsing, a support structure is built. A circular or arch-shaped design has been found to be the best to support the load over the tunnel.

Engineers and tunnel diggers are always faced with new challenges as they bore into the earth, blasting through mountains and digging under waterways. Modern tunneling and mining was born in 1849 with the invention of the rock drill. Along with dynamite, the rock drill made it possible to excavate large amount of earth.

Tunnels built through solid rock don't need as much support as soft rock or soft ground tunnels. To build a tunnel in solid rock, explosives are used to blast out the rock, or a rock mole which cuts through the rock is used. Immersed tube tunneling is one of the most common methods for tunneling under water. This type of tunnel is built by cutting a trench and dropping in a pre-assembled tube.

To learn more about different kinds of structures check out these super links!

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