VOA Special English 慢速英语听力材料

This is the VOA Special English Development Report.

Bridges are an important part of transportation systems. They make it possible to travel over rivers and between hills or mountains.

The most common kind of bridge is called a beam bridge. It is made of wood that has been specially treated. The treatment keeps the wood from aging and breaking apart. Carpenters, people with woodworking skills, can build beam bridges.

Small beam bridges made from wood can be from three to fifteen meters long. The wood should be able to support about one-hundred kilograms for each square centimeter of area.

Beam bridges can also be made from concrete. These can also be from three to fifteen meters long.

Carpenters can also build beam bridges made from concrete because wooden forms need to be built. The forms decide the shapes of the concrete beams. But in addition to carpentry skills, people who build such bridges also must know how to mix cement with soil and water to form concrete.

It is important not to remove the forms until the concrete has dried. Concrete beam bridges can take more weight than wood beam bridges, up to two-hundred kilograms for each square centimeter.

A bridge going over a valley between two hills should be as long as the valley is wide. But a bridge crossing a river presents a more complex problem. The bridge must be high enough to be above the highest point water has reached during periods of flooding.

If it is not, the bridge and its supporting structures can reduce the water flow of the river. If this happens, flooding may take place upstream from the bridge. Also, the water flowing under the bridge will travel faster. This may cause the soil holding the bridge in place to be washed away. Then the bridge may be damaged or destroyed.

Bridges need to be inspected often to be sure they are not damaged. From time to time, wooden bridges need chemical treatments painted on the beams. Concrete bridges also need to be repaired if pieces of concrete fall out or become weak.

You can get more information about building small bridges from the group Volunteers in Technical Assistance. VITA is on the Internet at vita.org.

This VOA Special English Development Report was written by Gary Garriott.