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Bridges are crucial transportation links that carry road and rail traffic across rivers, gorges or other roads. When a bridge collapses or closes for repairs, it can cause massive traffic problems or strand people altogether, if they live on an island.

As we go through this list of reasons why bridges collapse, keep in mind that most bridge collapses are the result of multiple factors. For example, a flood that damages bridge piers might not have caused a collapse — except for a design flaw and poor maintenance. Remove one of those factors and the bridge may have remained upright.

• Earthquake: Fortunately, earthquake-triggered bridge collapses are relatively rare. In addition, builders can construct bridges in earthquake-prone areas to withstand tremors.
• Flood: Floods cause bridge collapses in a few different ways. Severe floods can cause rivers and creeks to overflow, picking up debris. Flooding can collapse bridges in a far more insidious way — by gradually wearing away the earth around and underneath the bridge piers.
• Poor Maintenance: When a bridge is designed, the engineers assume a certain level of maintenance that is necessary for the bridge to live out its intended lifespan.

Full list at HowStuffWorks.com.

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