How are bridges made?

Question

Very simple (like grade 5 level)

Answer 1

Don't they need to be supported from under (or over) the platform? The arch is a good example where the keystone can distribute the pressure from the platform and exert it down the arch. Same with diagonals and crosses. But resonance should also be kept in mind.

Answer 2

Like all structures, a survey of the location of the bridge will be must be done.  When the elevations have been determined, material must be chosen for the location of the bridge for example, if the bridge is short, like going over a small creek, you dont design it with segmental type bridge but may more basic like a small steel bridge or reinforced concrete.  If it is going over traffic, then the bridge may have to be arched, and steel or concrete may be the first choice, if it is a long span, the prestressed concrete may be used or suspension type structure.  Once the structure type is determined, design of layout for where piers will be located and height of the roadway will be layed out.  Beam layout and load calcuation are done to determine the size and the number of beams that will be needed for the structure and then the pier are design.  Next come lateral load due to earth quake and wind loads are applied to the structure and the structure must be further designed to resist these loads.  Finally, these loads are applied to the piers and foundation of the bridge and the foundation has to analysis to resist dead load, live load, and lateral load. Construction documents are created and submitted for approval by the architect or the county engineer for its approval.  Hopefully this answers the question.