The Railways of America—1890

THE most notable invention of latter days in bridge construction is that of the cantilever bridge, which is a system devised to dispense with staging, or false works, where from the great depth, or the swift current, of the river, this would be difficult, or, as in the case of the Niagara River, impossible to make. The word cantilever is used in architecture to signify the lower end of a rafter, which projects beyond the wall of a building, and supports the roof above. It is from an Italian word, taken from the Latin cantilabrum (used by Vitruvius), meaning the 1ip of the rafter. If two beams were pushed out from the shores of a stream until they met in the centre, and these two beams were long enough to run back from the shores until their weight, aided by a few stones, held them down, we should have a primitive form of the cantilever, but one which in principle would not differ from the actual cantilever bridges. This is another American invention, although it has been developed by British engineers-Messrs. Fowler & Baker-in their huge bridge now building across the Forth, in Scotland, of a size which dwarfs everything hitherto done in this country, the Brooklyn bridge not excepted.

The first design of which we have any record was that of a bridge planned by Thomas Pope, a ship carpenter of New York, who, in 1810, published a book giving his designs for an arched bridge of timber across the North River at. Castle Point, of 2,400 feet span. Mr. Pope called this an arch, but his description clearly shows it to have been what we now call a cantilever. As was the fashion of the day, he indulged in a poetical description:

        "Like half a Rainbow rising on yon shore,
        While its twin partner spans the semi o'er,
        And makes a perfect whole that need not part
        Till time has furnish'd us a nobler art."

The first railway cantilever bridge in the world was built by the late C. Shaler Smith, C.E., one of our most accomplished bridge engineers. This was a bridge over the deep gorge of the Kentucky River. The next was a bridge on the Canadian Pacific, in British Columbia, designed by C. C. Schneider, C.E. A very similar bridge is that over the Niagara River, designed by the same engineer in conjunction with Messrs. Field & Hayes, Civil Engineers. This bridge was the first to receive the distinctive name of cantilever.

The new bridge at Poughkeepsie has three of these cantilevers, connected by, two fixed spans, as shown in the illustration. The fixed spans have horizontal lower chords, and really extend beyond each pier and up the inclined portions, to where the bottom chord of the cantilever is horizontal. At these points the junctions between the spans are made, and arranged in such a way, by means of movable links, that expansion and contraction due to changes of temperature can take place. The fixed spans are 525 feet long. Their upper chord, where the tracks are placed, is 212 feet above water. These spans required stagings to build them upon. These stagings were 220 feet above water, and rested on piles, driven through 60 feet of water and 60 feet of mud, making the whole height of the temporary staging 332 feet, or within 30 feet of the height of Trinity Church steeple, in New York. The time occupied in building one of these stagings and then erecting the steel-work upon it was about four months.

The cantilever spans were erected, as shown in the illustration, without any stagings at all below, and entirely from the two overhead travelling scaffolds, shown in the engraving. These scaffolds were moved out daily from the place of beginning over the piers, until they met in the centre. The workmen hoisted up the different pieces of steel from a barge in the river below and put them into place, using suspended planks to walk upon. The time saved by this method was so great that one of these spans of 548 feet long was erected in less than four weeks, or one-seventh of the time which would have been required if stagings had been used.

At the Forth Bridge, all the projecting cantilevers will be built from overhead scaffolds, 360 feet above the water. It contains two spans of 1,710 feet each. When spans of this length are used, the rivets become very long—seven inches—and it would be impossible to make a good job by hand riveting. Hence a power-riveter is used in riveting the work upon the staging. A steam engine raises up a heavy mass of cast-iron, called "the accumulator;" the weight of this in descending is transmitted through tubes of water, and its power increased by contracting the area of pressure, until some twenty tons can be applied to the head of each rivet. One rivet per minute can be put in with this tool.

It will be seen that most of the great saving of time in modern construction of bridges and other parts of railways is due to improved machinery. The engineer of to-day is probably not more skillful than his ancestor, who, in periwig and cue, breeches and silk stockings, is represented in old prints supervising a gang of laborers, who slowly lift the ram of a pile-driver by hauling on one end of a rope passed over a pulley-wheel. The modern engineer has that useful servant, steam, and the history of modern engineering is chiefly the history of those inventions by which steam has been able to supersede manual labor—such as pile-drivers, steam-shovels, steam-dredges, and other similar tools.

Poughkeepsie Bridge | Bridge Page | Contents Page

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