Bridges. Some are small and insignificant, while others are quite extraordinary. You may even feel that some should come under the category of "Wonders of the World." However you may feel about them, all bridges are made for specific reasons. Convenience and money are reasons that top the charts.
What is a bridge? A bridge is a structure used by people and vehicles to cross-areas that are obstacles to travel. Engineers build bridges over lakes, rivers, canyons, and dangerous highways and railroad tracks. Without bridges, people would need boats to cross waterways and would have to travel around such obstacles as canyons and ravines.
Bridges range in length from a few feet or meters to several miles or kilometers. A bridge must be strong enough to support its own weight as well as the weight of the people and vehicles that use it. The structure also must resist various natural occurrences, including earthquakes, strong winds, and changes in temperature.
Most modern bridges have concrete, steel, or wood framework and an asphalt or concrete roadway. The roadway is the part of a bridge on which people and vehicles travel.
The majority of bridges are held up by at least two supports set in the ground. The distance between two adjacent supports is called a span of a bridge. The supports at each end of a bridge are called abutments, and the supports that stand between the abutments are called piers. The total length of the bridge is the distance between the abutments. Most short bridges are supported only by abutments and are known as single-span bridges. Bridges that have one or more piers in addition to the abutments are called multi-span bridges. Most long bridges are multi span bridges. The main span is the longest span of a multi span bridge.
The unit is set up according to the following outline:
-Types of bridges -Focus -Objectives -Background History of the bridge
-Vocabulary -Lesson Plan -Activity/Field Trip
For further information you can go to the library and surf the net, get bridge construction videos, and more books.