The American Railway: Its Construction, Development, Management, and Appliances

by Thomas Curtis Clarke

After 1870, the weights both of locomotives and other rolling stock began to be increased very rapidly. This, together with the development of the manufacture of iron, and especially the invention of rolled beams and of eye-bars, gave a great impetus to the construction of iron bridges. At first cast-iron was used for the compression members, but the development of the rolling-mill soon enabled us to make all parts of rolled iron sections at no greater cost, and rolled iron, being a less uncertain material, has replaced cast-iron entirely. Iron bridges came in direct competition with the less costly Howe truss, and during the first decade of their construction every attempt was made to build them with as few pounds of iron as would meet the strains.

Old Burr Wooden Bridge.

S. Whipple, C.E., published a book in 1847 which was the first attempt ever made to solve the mathematical questions upon which the due proportioning of iron truss-bridges depends. This work bore fruit, and a race of bridge designers sprang up. The first iron bridges were modelled after their wooden predecessors, with high trusses and short panels. Riveted connections were avoided, and every part was so designed that it might be quickly and easily erected upon staging or false works, placed in the river. This was very necessary, for our rivers are subject to sudden freshets, and if we had adopted the English system of riveting together all the connections, the long time required before the bridge became self-sustaining would have been a serious element of danger.

Following the practice of wooden bridge building, iron bridges were contracted for by the foot, and not by the pound as is now the custom. To this accidental circumstance is greatly due the development of the American iron bridge. The engineer representing the railway company fixed the lengths of spans, and other general dimensions, and also the loads to be carried and the maximum strains to be allowed. The contracting engineer was left perfectly free to design his bridge, and he strained every nerve to find the form of truss and the arrangement of its parts that should give the required strength with the least number of pounds weight per foot, so that he could beat his competitors. When the different plans were handed in, an expert examined them and rejected those whose parts were too small to meet the strains. Of those found to be correctly proportioned, the lowest bid took the work.

By the rule of the survival of the fittest all badly designed forms of trusses disappeared and only two remained: one the original truss designed by Mr. Whipple, and the other, the well-known triangular, or "Warren" girder, so called after its English inventor.

It speaks well for the skill and honesty of American bridge engineers that many of their old bridges are still in use, designed for loads of 2,500 pounds per lineal foot, and now daily carrying loads of 4,000 pounds and over per foot. Sometimes the floor has been replaced by a stronger one, but the trusses still remain and do good service. The writer may be permitted to point to the bridge over the Mississippi River at Quincy, Ill., built in 1869, as an example. Most bridge-accidents can be traced to derailed trains striking the trusses and knocking them down. Engineers (both those specially connected with bridge works, and those in charge of railways) know much better now what is wanted, and the managers of railways are willing to pay for the best article. The introduction of mild steel is a great step in advance. This material has an ultimate strength, in the finished piece, of 63,000 to 65,000 pounds per square inch, or forty per cent. more than iron, and it is tough enough to be tied in a knot, or punched into the shape of a bowl, while cold. With this material it is as easy to construct spans of 500 feet as it was spans of 250 feet in iron.

Bridges are now designed to carry much heavier loads than formerly. The best practice adopts riveted connections except at the junction of the chord-bars and the main diagonals, where pins and eyes are still very properly used. Plate girders below the track are preferred up to 60 or 70 feet long, then riveted lattice up to 125 feet. The wind strains also are now provided for with a considerable excess of material, amounting in very long spans to nearly as much as the strains due to gravity. Observing the rule that no bridge can be stronger than its weakest part, a vast deal of care and skill has been applied in perfecting the connections of the parts of a truss, and many valuable experiments have been made which have greatly enlarged our knowledge of this difficult subject. The introduction of riveting by the power of steam or compressed air is another very great improvement.[5]

Kinzua Viaduct;
Erie Railway.

Valleys and ravines are now crossed by viaducts of iron and steel, of which the Kinzua viaduct, illustrated here, is an example. A branch line from the Erie, connecting that system with valuable coal-fields, strikes the valley of the Kinzua, a small creek, about 15 miles southwest of Bradford, Pa. At the point suitable for crossing, this ravine is about half a mile wide and over 300 feet deep. At first it was proposed to run down and cross the creek at a low level by some of the devices heretofore illustrated in this article. But finally the engineering firm of Clarke, Reeves & Co. agreed to build the viaduct, shown above, for a much less sum than any other method of crossing would have cost. This viaduct was built in four months. It is 305 feet high and about 2,400 feet long. The skeleton piers were first erected by means of their own posts, and afterward the girders were placed by means of a travelling scaffold on the top, projecting over about 80 feet. No staging of any kind was used, nor even ladders, as the men climbed up the diagonal rods of the piers, as a cat will run up a tree.