This is the VOA Special English Development Report.
Bridges are an important part of transportation systems. Theymake it possible to travel over rivers and between hills ormountains.
The most common kind of bridge iscalled a beam bridge. It is made of wood that has been speciallytreated. 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 fifteenmeters long. The wood should be able to support about one-hundredkilograms for each square centimeter of area.
Beam bridges can also be made from concrete. These can also befrom three to fifteen meters long.
Carpenters can also build beam bridges made from concrete becausewooden forms need to be built. The forms decide the shapes of theconcrete beams. But in addition to carpentry skills, people whobuild such bridges also must know how to mix cement with soil andwater to form concrete.
It is important not to remove the forms until the concrete hasdried. Concrete beam bridges can take more weight than wood beambridges, up to two-hundred kilograms for each square centimeter.
A bridge going over a valley between two hills should be as longas the valley is wide. But a bridge crossing a river presents a morecomplex problem. The bridge must be high enough to be above thehighest point water has reached during periods of flooding.
If it is not, the bridge and its supporting structures can reducethe water flow of the river. If this happens, flooding may takeplace upstream from the bridge. Also, the water flowing under thebridge will travel faster. This may cause the soil holding thebridge in place to be washed away. Then the bridge may be damaged ordestroyed.
Bridges need to be inspected often to be sure they are notdamaged. From time to time, wooden bridges need chemical treatmentspainted on the beams. Concrete bridges also need to be repaired ifpieces of concrete fall out or become weak.
You can get more information about building small bridges fromthe group Volunteers in Technical Assistance. VITA is on theInternet at vita.org.
This VOA Special English Development Report was written by GaryGarriott.