Bridges

Bridges


The beam bridge...(also known as a girder bridge)
consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 80 metres.
By increasing the height of the beam, the beam has more material to dissipate the tension. To create very tall beams, bridge designers add supporting latticework, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force spreads through the truss.

beam_bridge_1.jpg
beam_bridge_1.jpg


The truss bridge...(a beam bridge with braces)
consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. Rigid arms extend from both sides of two piers. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. These "diving boards," called cantilever arms, support a third, central span.
A single beam spanning any distance undergoes compression and tension. The very top of the beam gets the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. This is why we have I-beams, which provide more material on the tops and bottoms of beams to better handle the forces of compression and tension.
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bridge-thru.gif
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bridge-deck.gif

And there's another reason why a truss is more rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area.

The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge

firthofforth2_bridge_1.jpg
firthofforth2_bridge_1.jpg


The arch bridge...
has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steelor concrete, and they can span up to 250 metres.
Tensional force in arch bridges, on the other hand is virtually nil. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch.
But as with beams and trusses, even the mighty arch can't outrun physics forever. The greater the degree of curvature (the larger the semicircle of the arch), the greater the effects of tension on the underside of the bridge. Build a big enough arch, and tension will eventually overtake the support structure's natural strength.
aqueduct_bridge_1.jpg
aqueduct_bridge_1.jpg


The suspension bridge...
can span 600m to over 2 km-- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
As the name implies, suspension bridges, like the Golden Gate Bridge or Brooklyn Bridge, suspend the roadway by cables, ropes or chains from two tall towers. These towers support the majority of the weight as compression pushes down on the suspension bridge's deck and then travels up the cables, ropes or chains to transfer compression to the towers. The towers then dissipate the compression directly into the earth.
The supporting cables, on the other hand, receive the bridge's tension forces. These cables run horizontally between the two far-flung anchorages. Bridge anchorages are essentially solid rock or massive concrete blocks in which the bridge is grounded. Tensional force passes to the anchorages and into the ground.
external image bridge-suspension.gif
external image bridge-suspension.gif



In addition to the cables, almost all suspension bridges feature a supporting truss system beneath the bridge deck called a deck truss. This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.

But not every suspension bridge is an engineering marvel of modern steel. In fact, the earliest ones were made of twisted grass. When Spanish conquistadors made their way into Peru in 1532, they discovered anIncan empire connected by hundreds of suspension bridges, achieving spans of more than 150 feet (46 meters) across deep mountain gorges.
goldengate_bridge_1.jpg
goldengate_bridge_1.jpg




Cable-Stayed Bridge

The cable-stayed bridge, like the suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.

Cable-stayed bridges, like the Sunshine Skyway in Florida, require less cable and can be built much faster than suspension bridges. Cable-stayed bridges are becoming the most popular bridges for medium-length spans (between 150m and 1km).
he tower of a cable-stayed bridge is responsible for absorbing and dealing with compressional forces. The cables attach to the roadway in various ways. For example, in a radial pattern, cables extend from several points on the road to a single point at the tower, like numerous fishing lines attached to a single pole. In a parallel pattern, the cables attach to both the roadway and the tower at several separate points.
external image bridge-cable-stay.gif
external image bridge-cable-stay.gif



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sunshineskyway_bridge_1.jpg


Sunshine Skyway Bridge- Florida


Forces and Motion

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external image bridge-ct.gif

Tension and Compression: Two Forces Every Bridge Knows Well

What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.

Tension: What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress.

Compression: What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length. Compressional stress, therefore, is the opposite of tensional stress.
Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.

Buckling occurs when compression overcomes an object's ability to endure that force. Snapping is what happens when tension surpasses an object's ability to handle the lengthening force.

The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon.
In transferring force, a design moves stress from an area of weakness to an area of strength.

Torsion, for instance, is a particular concern for engineers designing suspension bridges. It occurs when high wind causes the suspended roadway to rotate and twist like a rolling wave. As we'll explore on the next page, Washington's Tacoma Narrows Bridge sustained damage from torsion, which was, in turn, caused by another powerful physical force
The natural shape of arch bridges and the truss structure on beam bridges protects them from this force. Suspension bridgeengineers, on the other hand, have turned to deck-stiffening trusses that, as in the case of beam bridges, effectively eliminate the effects of torsion.
In suspension bridges of extreme length, however, the deck truss alone isn't enough protection. Engineers conduct wind tunnel tests on models to determine the bridge's resistance to torsional movements. Armed with this data, they employ aerodynamic truss structures and diagonal suspender cables to mitigate the effects of torsion.

Shear: Shear stress occurs when two fastened structures (or two parts of a single structure) are forced in opposite directions. If left unchecked, the shear force can literally rip bridge materials in half. A simple example of shear force would be to drive a long stake halfway into the ground and then apply lateral force against the side of the upper portion of the stake. With sufficient pressure, you'd be able to snap the stake in half. This is shear force in action.


Resonance: is like the vibrational equivalence of a snowball rolling down a hill and becoming anavalanche. It begins as a relatively small, periodic stimulus of a mechanical system, such as wind buffeting a bridge. These vibrations, however, are more or less in harmony with the bridge's natural vibrations. If unchecked, the vibration can increase drastically, sending destructive, resonant vibrations traveling through a bridge in the form of torsional waves.

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