Shawneen Thompson, age 7, of Pittsburgh, Pa., for her question:
HOW ARE BRIDGES BUILT ACROSS WIDE RIVERS?
Man's first bridge most likely was a log that had fallen across a river. And it probably didn't take him too long to discover that with several logs placed side by side it was easier to take a heavy load from one side of the river to the other.
Today, building a bridge is a job for an engineering team that is involved in beams, columns, tension bars and cables.
Bridges used for crossing rivers are designed in a great variety of ways. The distance to be crossed, in most cases, determines the type of bridge to be built, although also important is the load that will be carried and the type of foundations that are feasible.
Almost all modern bridges must be built with steel and sometimes with concrete covering. They have to be able to stand firm under their own weight (called static load) plus be able to take a live load of people, vehicles, rain and wind.
If you were to build a big bridge across a river today, you'd make it either a truss, suspension, arch or cantilever bridge or several of these types combined.
A truss bridge is often used by railroads. It consists of fabricated trusses, placed side by side, which are made of girders and tie rods riveted together for great stiffness.
Suspension bridges are most dramatic and are used to jump over long waterways where central supports are impracticable. High pillars at the ends are built to hold cables which are hung to make a carefully calculated curve downward from both ends toward the middle. Massive end columns must bear the load of the whole structure. The deck of this bridge is somewhat arched, converting some of the downward load into endwise thrust.
An arch bridge is a form of suspension bridge with a massive steel arch replacing the suspension cables. The deck of the bridge is supported by steel rods or bars dropping down from the arch.
The cantilever bridge is commonly found in lift bridges.
The main thing necessary to solve in building large bridges across rivers is to correctly handle stresses that is the forces tending to stretch, compress,bend, twist and shear the steel members of the bridge. Weights must be transmitted to and borne by the end abutments and any pillars set in between.
Much engineering work must be done on paper before ground is broken for the actual bridge construction.
Careful analysis must be made of the rock upon which the weight of the bridge will rest. Footings must be absolutely sound. Then come complicated mathematical sessions to determine and analyze the strains that will be faced.
It takes a tremendous amount of brainpower to build a large bridge.