How Bridges Work

There are short bridges and there are long bridges, but all bridges have two things in common. No matter what size the bridge is, they have to deal with compression and tension. All bridges are engineering marvels, because they can hold such heavy loads while spanning such long distances. Every bridge has to have good support, and the bridge design will determine how far apart that support can be.

The three bridge types are the beam bridge, the suspension bridge, and the arch bridge. The beam bridge is designed to span a short distance of about two hundred feet. The suspension bridge can span up to seven thousand feet. The arch bridge can span one thousand feet. The span is the distance between supports.

Compression and tension are the main things that all bridges have to deal with, but there are many other forces that must be taken into consideration when designing a bridge. The location of the bridge will determine how it is designed most of all.

Weather is always a factor to take into consideration also. Weather can be very powerful, and bad weather can bring a bridge down if it is not designed correctly. Rain, wind, salt spray, and ice can be very hard on bridge materials. Over time lessons have been learned on just which materials stand up best to certain forces of nature. Steel and prestressed concrete are used in most highway bridges today.

Torsion, which is the twisting force, is almost completely eliminated in most bridges today. The very largest suspension bridges still have to deal with torsion. Resonance, or the vibration that can occur in a bridge, is another strong element to deal with.

Resonant waves can destroy a bridge, so dampeners are built into most bridges today. These dampeners will interrupt these dangerous waves or vibrations. Weather still remains the most unconquered of all the forces a bridge has to deal with. Engineers continue to learn from bridge failures of the past and try to improve on the bridge design each time. It is amazing that weather-related failures far outnumber design failures.

As the earth moves, the bridge will move. Bridges are put together in sections and some bridges actually float on their supports to accommodate those movements. A bridge has to move or it will break. I areas where there are earthquakes, the bridges must be flexible in order to remain intact. The longer the distance the bridge has to span, the more the bridge will tend to sway and ripple.

Cables and trusses are added above and sometimes below the deck of the bridge to take care of this problem. Suspension bridges have long cables that usually run between towers. The deck of the bridge is suspended from these cables. The towers are under compression and the cables are under tension, therefore holding up the bridge deck.

Arch bridges are under compression all the time. Arch bridges can be seen in older bridges going back hundreds of years. The arch itself will dissipate the weight from the center of the bridge deck to the end abutments. This type of design is only for shorter bridges because the longer the arch is, the weaker it becomes.

Highway overpasses are bridges that use the beam design. The steel beams height determines the distance that the span of the bridge can be. The stronger a beam is, the more it will dissipate the compression and tension that is produced.

So, bridges that can distribute weight evenly, take the weight of their load without breaking, and handle bad weather of all kinds, are well-designed bridges. There have been bridge designs that use triangles and arcs very successfully. In the future engineers will learn more about bridge design and will design even stronger bridges that can span longer and longer spaces.

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