Bridge

a construction by which a passage is secured across a body of water or an open space. The simplest bridge is a log or slab of stone with an end on either shore. The first step in advance is to lay two or more beams parallel and cover them with flooring. All bridges of this sort depend on the stiffness of the material and its ability to stand the strain of bending. A great many small bridges of the present day, floors, and flat roofs are of this sort.

Another kind of bridge may be represented by a spider's thread by means of which the spinner crosses an open space. The ends are made fast to the opposite shores and the thread sustains the weight of the spider by reason of its tensile strength, or ability to stand the strain of a stretching force. The circular orb web of the spider seen in dewy meadows is a bridge of this sort. The natives of farther India bridge streams by means of long canes of rattan. The ancient Aztecs of Mexico and the Peruvians understood the art of bridging chasms by means of vines and cables. Some of the finest bridges in the world, as the Brooklyn Bridge, the Suspension Bridge at Niagara Falls, and that at Menai Strait are suspended on wire cables of great tensile strength.

A third sort of bridge is built of short pieces of material, disposed in the form of an arch. The strength of a well built arch depends on the ability of the material to withstand crushing. The Romans were the first to use material of this sort extensively in bridging. Bridges of stone and brick are common in all civilized countries. Before the introduction of iron construction, the bridges of cities and important roadways were built on arches. The bridges over the Seine in Paris and the famous London Bridge are of this description. Roman aqueducts and military roads were laid over arched bridges.

If we add the ability to resist pressure from the ends, ability to resist doubling-up force such as is possessed by braces, piling, or the legs of a chair, we have the four kinds of strength utilized by the modern bridge builder. The iron pillars that support warehouse floors and the wooden trestlework of railroad bridges afford examples of material resisting a stress of this sort.

The truss is an admirable invention. It is a frame designed to take the place of the beam required in the first kind of bridge described, or of the arch of masonry. By a very simple but shrewdly devised plan, light beams, braces, and tierods are so combined that when the ends of the truss are supported it cannot bend without pulling the tie rods in two. A truss is much lighter and less expensive than a beam of equal strength. Iron trusses longer than any possible beam are entirely practicable and are able to sustain enormous loads.

The cantalever bridge consists of two balanced trusses approaching each other from opposite shores or piers, like the branches of two trees reaching out from opposite sides of a walk. When the trusses have grown out far enough to meet, they are bolted together.

The Romans were the great bridge builders of antiquity. Their first bridges were of wood. A stone bridge was built over the Tiber about 127 B. C. The Emperor Trajan built a bridge over the Danube forty-six feet above the water. The first stone bridge of which we have any record in England was built near' Stratford in 1087. The first stone London bridge was built in 1176. An iron bridge was thrown over the Severn in 1776. It had a 100 foot span and 45 foot rise. Iron is now the prevailing material.

The longest bridge in the world is said to be one crossing the Danube, having a length of 12,705 feet. There is a very long bridge across Galveston Bay, Texas. Brooklyn Bridge is said to be the ninth in length. The highest bridge in the United States is a railroad bridge crossing the Pecos River, Texas. It is 321 feet high. The highest bridge in the world is a new cantalever bridge crossing the gorge of the Zambezi River at Victoria Falls, South Africa. It is 650 feet long and 420 feet in height. A fine cantalever steel bridge crosses the River St. Lawrence six miles above Quebec. It has a span of 1,800 feet. It is high enough to clear the tallest ship. The so-called tubular bridges require mention. They are enormous wrought iron tubes, so large that railroad tracks may be laid within them. A bridge of this sort crosses the Menai Strait, Wales; another crosses the St. Lawrence at Montreal.

In calculating the required strength of a bridge, builders find it necessary to allow quite as much for wind as for the weight of the bridge itself and that of passing trains. A bridge across the Tay, near Dundee, Scotland, was 10,780 feet long. It was considered the most remarkable bridge in the world. It blew down in a hurricane in 1879.

See Arch; Architecture