But stone bridges even then cost money, and so the timber structure still remained the most available. Many men can still remember the tunnels, into whose darkness the railroad cars plunged every time they crossed a stream of any importance whatsoever. They have nearly all gone. The wooden bridge was ill suited to the ravages of weather and of fire—ravages that were quickened by the railroad, rather than hindered. A substitute material was demanded. It was found—in iron.
The first iron bridge in the United States is believed to be the one erected by Trumbull in 1840 over the Erie Canal at Frankfort, N. Y. Record is also held of one of these bridges being built for the North Adams branch of the Boston & Albany Railroad, in 1846. About a year later, Nathaniel Rider began to build iron bridges for the New York & Harlem, the Erie, and some others of the early railroads. His bridges—of the truss type, of course, that type having been worked out in the timber bridges of the land—were each composed of cast-iron top-chords and post, the remaining part of the structure being fabricated of wrought-iron. The members were bolted together. Still, the failure of a Rider bridge upon the Erie in 1850, followed closely by the failure of a similar structure over the River Dee, in England, influenced officials of that railroad to a conclusion that iron bridges were unpractical, and to order them to be removed and replaced by wooden structures. For a time it looked as if the iron bridge were doomed. That was a dark day for the bridge engineers. A contemporary account says:
“The first impulse to the general adoption of iron for railroad bridges was given by Benjamin H. Latrobe, chief engineer of the Baltimore & Ohio Railroad. When the extension of this road from Cumberland to Wheeling was begun, he decided to use this material in all the new bridges. Mr. Latrobe had previously much experience in the construction of wooden bridges in which iron was extensively used; he had also designed and used the fish-bellied girder constructed of cast and wrought-iron.”
Under the influence of the really great Latrobe, an iron span of 124 feet was built in 1852 at Harpers Ferry. In that same year, the B. & O. built its Monongahela River Bridge, a really pretentious structure of 3 spans of 205 feet each, and the first really great iron railroad bridge in all the land. The path was set. The conquest of iron over wood as a bridge material was merely a problem of good engineering. The iron bridge quickly came into its own. The Pennsylvania Railroad began building cast-iron bridges of from 65 to 110 feet span at its Altoona shops for the many creeks and runs along the western end of its line. The other railroads were following in rapid order. Squire Whipple, Bollman, Pratt—all the others who could design and build iron bridges—were kept more than busy by the work that poured in upon them.
And in the day when the iron bridge was coming into its own, Sir Henry Bessemer, over in England, was bringing the steel age into existence, first making toy cannon models for the lasting joy of Napoleon III, and then making a whole world see that steel—that dead thing with the living muscle—was no longer to be limited for use in tools and cutting surface. Steel was to become the very right-hand of man. And so steel came to the bridge-builders, at first only in the most important wearing points such as pins and rivets, finally to be the whole fabric of the modern bridge. The transition was gradual. The early engineers began using less and less of cast-iron and more and more of wrought, until they had practically eliminated cast-iron as a bridge material. Then there came a quick change; there was another dark day for the railroad bridge engineers of America. In 1876—that very year when the land was so joyously celebrating its Centennial—a passenger train went crashing through a defective bridge at Ashtabula, Ohio. There was a great property loss—thousands and thousands of dollars, and a loss of lives that could never be expressed in dollars. An outraged land asked the bridge-builders if they really knew their business.
Out of that Ashtabula wreck came the scientific testing of bridges and bridge materials, and the abolition of the rule-of-thumb in the cheaper sorts of construction. Out of that miserable wreckage came also the use of steel in the railroad bridge. Steel had found itself; and how the steel bridges began to spring up across the land! They spanned the Ohio, and they spanned the Mississippi, and they spanned the Missouri; a great structure threw itself over the deep gorge of the Kentucky River. When the day came that fire destroyed the famous wooden viaduct of the Erie over the Genesee River at Portage, N. Y. (you must remember the pictures of that tremendous structure in the early geographies), steel took its place.
All this while the bridge engineer attempted more and more. He built over the deep gorge of the Niagara. He conquered the St. Lawrence in and about Montreal. He laughed at the mighty Hudson and flung a dizzy steel trestle over its bosom at Poughkeepsie. He built at Cairo, at Thebes, and at Memphis, on the Mississippi, and again and again and still again at St. Louis. The East River no longer halted him or compelled him to resort to the alternative of the very expensive types of suspension bridge. He has finally thrown a great cantilever over it, from Manhattan to Long Island. The steel bridge has come into its own.
Let us study for a moment the construction of the different types of railroad bridge. For the tiny creeks—the little things that are mad torrents in spring, and run stark-dry in midsummer—where they cannot be poured through a pipe or a concrete moulded culvert, the simplest of bridge forms will suffice. And the simplest of bridge forms consists of two wooden beams laid from abutment to abutment and holding the ties and rails of the track-structure. As the first development of that simplest idea comes the substitution of steel for wood, giving, as we have already seen, protection against fire and a far greater strength. The steel beam has greater strength than a wooden beam of the same outside dimension and yet in its design it effects for itself a great saving of material, by cutting out superfluous parts and becoming the structural standard of to-day, the I beam. When the I beam becomes too large to be made in a single pouring or a single rolling, it may be constructed of steel plates and angles firmly riveted together, and thus still remains the possibility of the simplest form of bridge. That single span may be further increased, or the bridge developed into a succession of increased spans by the substitution of the lattice-work girder, effecting further saving in weight without material loss of strength for the solid-plate girder. The track may be laid atop of such girders or—to save clearance in overhead crossing—swung between them at their bases.
The limit in this form of bridge is generally in a 65-foot or a 100-foot span. It is not practical to build the girders up outside of a shop; and the 65-foot length represents the two flat-cars that must be used to transport any one of them to the bridge location. Some railroads have used three cars for the hauling of a single girder, and so increased these spans to 100 feet; but as a rule, over 65 feet, and the truss, the most common form of railroad bridge in this country, comes into use.