These are the men who are building the bridge. In a little time there will be no advancing ends, finding their path from pier-top to pier-top. There will be, instead, a long and slender path for the railroad; the bridgemen will have done their work well; a great river will have once again been conquered.
The bridge problem is always different, it constantly has the fascination of variety. That variety will come into play at unexpected turns. Once, down in a deep Colorado cañon, whose walls rose precipitously for a thousand-odd feet, and which was all but filled by a deep and rapid river, the engineers of the Rio Grande & Western found absolutely no ledge whatsoever upon which they might rest their rails. They puzzled upon the problem for a little while, and then they swung a girder bridge parallel with the river. The bridge was supported by braced girders, that fastened their feet in the walls of the cañon, hardly wider there than a narrow city house. The railroad has been running over that construction for more than thirty years; it is one of the scenic wonders of the land, and a triumph for the engineer that built it. In constructing the expensive West Shore Railroad up the Hudson River, similar difficulties were experienced south of West Point, and truss bridges were built parallel with the steep river banks to carry the tracks from ledge to ledge. It is not an unusual matter for the construction engineer to spend a quarter of a million dollars to span some deep, waterless gully in the mountains, which could not be filled for more than twice that sum.
Many times, in these days of increasing weight of equipment, it becomes necessary to replace a bridge, without interrupting the traffic. The construction engineer never fails to meet the problem. Years ago, he took Roebling’s famous suspension bridge at Niagara Falls, removed the stone towers and replaced them with towers of steel, without delaying a single train; and a little later he took that bridge itself, and substituted a heavy cantilever for it, while all the time a heavy traffic poured itself over the structure. The rebuilder of bridges works like the original builder—with plentiful falsework. He timbers in and around his structure, and then step by step and with exceeding caution removes the old and substitutes the new. An old girder is taken out between trains; before another train of cars shall roll over the structure a new one is ready, temporarily bolted until the riveters can make it fast. It sounds complicated, but it is remarkably simple, under the careful plans of a patient engineer, who has that infinite thing that we call genius.
Sometimes a bold engineer strikes out into a new method, quicker and less expensive than these piecemeal efforts. Of such was the job at Steubenville, O., where a 205-foot double-track span was erected on heavy falsework alongside the old bridge. In a carefully chosen interval between a service of frequent trains, both the old and the new spans—together weighing 1,300 tons—were fastened together and drawn sideways a distance of twenty-five feet in one minute and forty seconds. The new span was then in place, and the old one—ready to be dismantled—stood on falsework at the side. The entire job had been accomplished in an interval of seventeen minutes between trains.
That is not unusual. The floating method is sometimes adopted with remarkable success—especially in the case of draw-bridge spans. There the problem complicates itself exceedingly, for both the water and the land highways must be kept open for traffic; yet it is a matter of record that the Pennsylvania Railroad, operating a fearfully heavy suburban service in and out of Jersey City, recently substituted one draw for another on its Hackensack River Bridge without delaying a single train.
But even in this high noon of the day of steel, the stone bridge holds its own. The big chiefs of railroad construction look upon it with favor. Higher priced than a steel bridge of equal capacity it requires initial outlay. But forever after, it represents a saving—a saving chiefly in that very important figure, maintenance. A steel bridge requires constant attention and constant expense. A stone bridge requires little of either; and therein lies its strength in its old age. Engineers point to such structures as the Thomas Viaduct down at Relay, or to the wonderful stone bridges that have stood through the centuries in older lands; they bear in mind the constant battle that a steel bridge must make against the ravages of weather and against the sinister thefts of corrosion, and ofttimes they rule in favor of the oldest type of sizable bridge.
Two things are all-important in the choice between the steel bridge and the arch bridge of stone or concrete. The first is the accessibility of the quarries. If they are not very near the solid bridge will cost four times that of one of steel and the average American railroad is not able to spend money in that fashion, even in the hopes of future economies in maintenance. If the quarries are close at hand, as they were years ago when Kirkwood built the Starucca Viaduct for the Erie, the cost of a masonry bridge will hardly exceed that of steel trusses, and the concrete structure may cost a little less. Then there comes into play the second consideration. The stone or concrete bridge has tremendous weight, no ordinary foundation work will serve it. If the river bed and banks be of sand or poor earth, the engineer had best give up his hopes of the Roman form of structure. He can build steel towers and trusses on piles of caissons—hardly solid stone piers and abutments and aides.
All these things considered, the stone bridge is still more than holding its own in modern railroad construction. The Boston & Albany Railroad began building these splendidly permanent structures along its lines through the Berkshires more than twenty years ago. More recently both the Pennsylvania and the Baltimore & Ohio have been looking with favor upon this type of bridge. The Baltimore & Ohio has just finished building its massive Brandywine Viaduct, near Wilmington, a splendid double-track structure, 764 feet in length, and composed of two 80-foot, two 90-foot, and three 100-foot arches.