Features -

The four main factors are used in describing a bridge. By combining these terms one may give a general description of most bridge types.
  • span,
  • material,
  • placement of the travel surface in relation to the structure,
  • form

The three basic types of spans are shown below. Any of these spans may be constructed using beams, girders or trusses. Arch bridges are either simple or continuous. A cantilever bridge may also include a suspended span.

Forces on a bridge -
  • Tension and compression are the two main forces. Tension is pulling. It would be hard to break a straw if you held both ends and pulled apart. Wood also has the ability to resist tension. Tension may be applied parallel to the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong in tension parallel to the grain, but extremely weak in tension perpendicular to the grain.
  • Compression is a pushing (compressing) force. The shorter a piece of wood is, the more compression it can hold. The longer a piece of wood is, the less compression it can hold.