IV.
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 lip 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."
View of Thomas Pope's Proposed Cantilever (1810).
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.[7] 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 (pg. 36). 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 on [page 37], 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.
Pope's Cantilever in Process of Erection. (From his "Treatise on Bridge Architecture.")
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 skilful 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.
General View of the Poughkeepsie Bridge.
After the road-bed of a railway is completed and covered with a good coat of gravel or stone-ballast, and after all the temporary structures have been replaced by permanent ones, that part of the work may be said to be done, requiring only that the damages of storms should be repaired. But the track of a railway is never done. It is always wearing out and always being replaced.
Erection of a Cantilever.
Some of the early English engineers, not appreciating this, endeavored to lay down solid stone walls coped with stone cut to a smooth surface, on which they laid their rails. They called this "permanent way," as distinguished from the temporary track of rails and cross-ties used by contractors in building the lines. But experience soon showed that the temporary track, if supported by a bed of broken stone, always kept itself drained and was always elastic, and remained in much better order than the more expensive so-called "permanent way." When the increase in the weight of our rolling stock began to take place, dating from about 1870, iron rails were found to be wearing out very fast. Some railway men declared that the railway system had reached its full development. But in this world the supply generally equals the demand. When a thing is very much wanted, it is sure to come, sooner or later. The process of making steel invented by, and named after, Henry Bessemer, of England, and perfected by A. L. Holley, of this country, gave us a steel rail which at the present time costs less than one of iron, and has a life five or six times as long, even under the heavy loads of to-day. We are now approaching very near the limit of what the rail will carry, while the joints are becoming less able to do their duty. Bad joints mean rough track. Rough track means considerably greater expenditure both for its maintenance and that of all the rolling stock, as the blows and shocks do reciprocal damage, both to the rails and to that which runs on them. Hence all railway managers are now devoting more care and attention to their tracks.
In laying track on a new railway, if it be in an old-settled country where other railroads are near and the highways good, the ties are delivered in piles along the line where wanted, and the haul of the rails is comparatively short. The ties are laid down, spaced and bedded, adzed off to a true bearing, and the rails laid upon them; the workmen being divided into gangs, each doing a different part of the work. After the track is laid, the ballast-trains come along and cover the roadbed with gravel. The track is raised, the gravel tamped well under the ties, and the track is ready for use.
Spiking the Track.
Rail Making.
The road is then divided into sections about five miles long. On each section there is a section-boss, with four to six laborers. Their duty is to pass over the track at least twice a day in their hand-car, to examine every joint, and where one is found low or out of line, to bring it back to its true position by tamping gravel under it and moving the track. They have also to see that all ditches are kept clear of water, a most essential point, as without good drainage the ground under gravel ballast becomes soft, and the mud is churned up into the gravel, and the whole soon gets into bad order.
They have to see that the fences are all right, that trees and telegraph poles do not fall across the track, that wooden bridges do not burn down, that iron and stone bridges are not undermined by freshets, and always to set up danger signals to warn the trains.
Track Laying.
It is admitted by competent judges, that the track of the Pennsylvania Railroad is the best in this country, and one of the best in the world. It is kept up to its high standard of excellence by a system of competitive examinations.
About the first of November, in each year, after the season's work has been done, a tour of inspection is made over all the lines, on a train of cars expressly prepared, consisting of two or more cars not unlike ordinary box cars with the front end taken out. Each car is pushed in front of an engine, and goes slowly over the line, by daylight only, so that the inspecting party may have a full view of the road.
The Pennsylvania road is divided into Grand Divisions, Superintendents' Divisions, of about 100 miles long, Supervisors' Divisions, of about 30 miles, and Subdivisions, of 2½ miles.
The examining committee for each Supervisor's Division consists of the supervisors of other divisions. As they pass along, they mark on a card. One sub-committee marks the condition of the alignment and surfacing of the rails; another the condition of the joints and the spacing of the ties; another the ballast, switches, and sidings; another the ditches, road-crossings, station grounds. The marks range from 0 to 10, 0 being very bad, 5 medium, and 10 perfection. When the trip is done these reports are all collected and the average is taken for each division.
As an inducement to the supervisors and the foremen of the Subdivisions to excel on their division, premiums are given as follows:
$100 to the supervisor having the best yard on his Grand Division.
$100 each to the supervisors having the best Supervisor's Division on each Superintendent's Division of 100 miles.
$75 to the foreman having the best subdivision of 2½ miles on each Grand Division.
$60 to each foreman having the best subdivision on his Superintendent's Division, including yards.
$50 to the foreman having the best subdivision on each Supervisor's Division.
In addition to the above there are two premiums of honor given by the general manager, which bring into competition with each other those parts of the main line lying on either side of Philadelphia, viz.:
$100 to the supervisor having the best line and surface between Pittsburg and Jersey City.
$50 to the second best ditto.
If a supervisor or foreman of subdivision receives one of the higher premiums, he is not allowed to be a competitor for any others premiums, except the premiums of honor.
The advantages of these inspections and premiums are these: Every man knows exactly what the standard of excellence is, and strives to have his section reach it. Under the old system, a man never got off of his own section, and had no means of comparison, and like all untravelled persons, became conceited.
The standard of excellence becomes higher and higher every year. Perfect fairness prevails, as the men themselves are the judges. The officers of the road make no marks, but usually look on and see that there is fair play.
This brings the officers and men nearer together, and shows the men how all are working for the common good. An agreeable break is made in the monotony of the men's lives. They have something to look forward to better than a spree.
It is by the adoption of such methods as these that strikes will be prevented in the future. It encourages an esprit de corps among the men, and educates them in every way.
This system was first devised and put in operation on the Pennsylvania Railroad in 1879, by Mr. Frank Thomson, General Manager, to whom the credit of it is justly due.