Construction Site

Different things about the construction site influence the design of a bridge.  These include the ground and/or water at the construction site, the climate and existing human uses of the area.

Ground/Water at Site

The ground itself needs to be able to support both the dead load and live load of the bridge without allowing the bridge to slowly sink into the ground.  The foundations of the bridge must also be strong enough to resist the wear and tear of water that may be flowing past them.  It is important that bridge engineers make sure that they plan for times when the water may be very high due to a flood; the stress on the bridge will be highest then.  If there is a possibility of large pieces of ice flowing down the river then they need to plan for this too.

Climate

Depending on what part of the world the bridge is being built, it may have to withstand extreme hot and cold temperatures, high winds, temperatures that may cause it to expand or contract, chemicals such as acid rain and corrosion damage such as rust.

Existing Human Uses of Area

Finally the engineering team has to make sure that existing human uses of the area under and around the bridge are not affected too much by its construction.  An example of this would be a bridge that is built overtop of the entrance to a harbor, such as the San Francisco Golden Gate Bridge.  Large ocean-crossing ships need to be able to fit underneath the bridge with room to spare.

If that was not enough engineers also have to think about human-caused accidents.  Could the bridge survive if a huge ship accidentally hit one of the supports? What if a large truck hit a supporting cable? Accidents like these have and will happen, and the bridge should not collapse as a result.

See also environmental impact.