Railroad bridge

“ Suspension bridges of large span have been generally considered as entirely unfit for railroad purposes; but John A. Roebling has proved the contrary by erecting a wire suspension railroad bridge of eight hundred feet clear span across Niagara River; which with heavy loads and violent gales has shown itself to be both stiff and strong to any desired amount. The construction of a bridge upon any other plan would have been hardly possible at the site of Mr. Roebling’s Niagara bridge, there being no opportunity for scaffolding or for piers, pontoons or hydraulic presses. ”

“ Nonetheless, the covered bridge era might have lasted until the coming of the automobile were it not for a combination of events that led bridge designing into the modern era. The newly developed truss and the fledgling analytic methods found themselves a new client, the railroad. Trains being unable to ford even small streams or tolerate the sinuous alinement and steep grades used by pedestrians and wagon traffic, a great many bridges were needed, and not only more bridges, but stronger and more durable ones. This accelerated the growth of design technology. ”

“ Thus the railroad crossing the Delaware at Trenton has, in this way, typified step by step every stage of the development of American bridge-building.
The timber bridges developed the steel truss bridge, the typically American construction, of to-day. In an earlier day the timber bridges were the glory of the engineer. Sometimes you see one of these old fellows remaining, like the long structure that Mr. Walcott built across the Connecticut River at Springfield, Mass., in 1805, and which still does good service; but the most of them have passed away. Fire has been their most persistent enemy. ”

“ In the construction of the new bridge it became necessary to dismantle the draw of the old one, and to erect temporary supports while the timbers and draw of the new bridge were being put in place. To prevent the stoppage of its trains while this building was going on, the company constructed a temporary bridge adjoining the old one, for their transportation; expecting to have the new bridge completed before the winter rise, which usually began near the close of December, should render the river navigable. ”

“ 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. ”

“ Next on the journey the cantilever bridge is reached. This bridge was constructed in 1882. Just below this is the steel arch bridge, both being railroad bridges. The second one was first constructed as a suspension bridge by John A. Roebling, being the first railroad bridge of its kind in the country. It has been several times replaced, the present structure having been erected in 1897. Just below the railroad bridges several persons have made the trip across the gorge on ropes. ”

“ This bridge and its approaches cost eighty millions of dollars, and is used by ten or twelve railroad companies. Above the tracks is a big street with carriage-roads, street-cars, and walks for foot-passengers.
The honor of building the finest bridge in the [31] world would have satisfied most men, but not ambitious Captain Eads. ”

“ The plans for the new bridge to replace the old one are not shown by any allegations of fact to constitute a 'plan for the development of any water front.' The construction or replacement of a railroad bridge across a bay or river is nto necessarily a 'water front development.' The company was empowered under the act of 1860 not only to construct and maintain its railroad and the existing double-track bridge across the bay, but also to replace that bridge by another having additional tracks, whenever the company found it expedient so to do. ”

“ We set up this spindly tower on rails, that we call the ‘traveller’ and it moves backwards and forwards over the trusses and the timber falsework that we build before the steel really begins to be set up. When the steel—the trusses—is up and riveted, then away with the falsework. Our bridge stands by itself. You can put up a 500-foot span in no time at all by using the falsework.”
You make bold to ask what the engineer does when the river is too deep to admit of falsework. ”

“ This bridge, a view of which is given in the cut below, is about two and a half miles below the falls, and spans the river near the head of the rapids, above the whirlpool. From pier to pier it is eight hundred feet long, and in breadth eight feet. It is suspended on eight wire cables, four on each side, which pass over towers fifty-four feet high, built of heavy timber. The present structure is only the scaffolding for constructing a larger bridge, intended for the passage of railroad cars. The towers for the large bridge will be of solid masonry, each eighty feet high. ”

“ As has often been said by this court, bridges are merely connecting links of turnpikes, streets, and railroads; and the commerce over them may be much greater than on the streams which they cross. A break in the line of railroad communication from the want of a bridge may produce much greater inconvenience to the public than the obstruction to navigation caused by a bridge with proper draws. ”

“ The bridge will upset, turning about its lower edge, when the whole pressure of the wind, multiplied by half the height of truss, overbalances the whole weight, multiplied by the half width. In very exposed places the rod A D, fig. 74, answers a very good end; when the road is upon the upper chord, and a rod from B to the masonry, when upon the lower.
177 OBLIQUE BRIDGES. 201. The effect of running a train over a skew bridge, is to depress one side before the other; as the load comes upon the centre of one truss before it does upon the opposite one. ”

“ In Great Britain not a single case of disaster of any description has been reported as occurring at a draw-bridge since the year 1870, when the present system of official Board of Trade reports was begun. The lesson clearly to be drawn from a careful investigation of all the American accidents reported would seem to be that a statute provision making compulsory the interlocking of all draws in railroad bridges with a proper and infallible system of signals might have claims on the consideration of an intelligent legislature ”

“ To make accidents from this cause, if not impossible at least highly improbable, it is only necessary to make the ties and flooring of all bridges between the tracks and for three feet on either side of them sufficiently strong to sustain the whole weight of a train off the track and in motion, while a third rail, or strong truss of wood, securely fastened, should be laid down midway between the rails throughout the entire length of the bridge and its approaches. ”

“ The track should also be provided with guard-timbers well fastened, and the width between the trusses should be so great that the wheels of a derailed train will be stopped by [Pg 65] the guard-rail before the side of the widest car can strike the truss.
The importance of a substantial floor system has been very fully recognized by the railroad commissioners of Massachusetts, who have recently issued a very suggestive circular, accompanied by numerous examples of track construction for railway bridges. ”

“ The clear water-way of this bridge is 5,800 feet; and the entire length, including piers and abutments, one and one fourth miles. There are three sets of arches, which allow of two carriage roads and one railroad, the whole width being thirty feet.
192. An arch to support a passing railroad train must be very rigid. It is customary to connect them with a light truss, which effectually counter braces the arch, and prevents that change of form which would otherwise take place; depending entirely upon the arch for strength. ”

“ Soon after the railroad era started in 1827, bridge engineers realized that the timber truss-arch bridges were not serving satisfactorily under the speed and weight of railroad traffic and sought various methods to introduce iron members into the truss arch. ”

“ The earliest of the very distinguished list of American bridge-builders erected great timber structures for the highroads and the post-roads. Some of them went back many centuries and came to the stone bridge, in many ways the most wonderful of all the artifices by which man conquers the obstructive power of a running stream. ”

“ In one State a late annual report covers a whole railroad with the remark, "All of the bridges on this line are in excellent order;" and yet there were at that very time, and are now, on that road, several large wooden bridges with a factor of safety referred to the breaking-weight of not over two under a fair load, assuming the iron rods to be of the very best material,—a point upon which there is no evidence whatever. ”

“ Both arches and arch braces, however, are very usefully applied to bridges which have been made too light.
194. The manner of applying arches is well shown in the bridges of the Pennsylvania Central Railroad, built by Hermann Haupt, Esq.
These bridges are on Howe’s plan, to which have been added strong wooden arches. The systems are connected by adjusting the counter braces against the arch by set screws. The arrangement is simple and effectual. The name of the builder is sufficient to warrant the stability of the bridge. ”

“ One example is 234 feet high. It is the great Portage Bridge spanning the Genesee River, in the State of New York, on a railroad between New York and Buffalo. It looks like a miracle of carpentry, this wonderful bridge of frames; its length is 240 metres, and the piers—sixteen romantic scaffoldings—form immense triangles with flattened [Pg 354] summits upon which a double gallery rests as a firm support for the railway. Each scaffolding rises from a pile of masonry nine metres higher than water-level, so that floods do not break their force against the timber frames. ”

“ A span of this plan, upon the New York and Erie Railroad, of forty feet, and which weighed only three tons, supported a load of fifteen hundred pounds per lineal foot for two days; when the bridge had settled nearly one half inch. A load of rails weighing 1318 lbs. per foot (of bridge) was then rolled over, upon a truck without springs, thus making the whole load upwards of 2,800 lbs. per foot, when the whole deflection was three fourths of an inch. Upon removing the load the bridge returned to its original position, within one fourth of an inch. ”

“ Fig. 101.
Every part of a bridge not resisting some force is worse than useless, as it adds to the weight. Lightness not only increases the economy directly, but indirectly by removing a part of the permanent load.
222. Foremost in class number two stands Wendel Bollman’s Iron Suspension and trussed bridge. For simplicity of construction and directness of action, this bridge is unsurpassed. The weight at each post is transferred at once to the abutment or pier. The upper chord is of cast iron, hollow, octagonal without, and circular within. ”

“ 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 ”

“ Cast-iron girder, thirty feet span, load thirty-three tons.
Mr. W. H. Barlow, (Eng.) observed, “that in case of a timber viaduct, a freight train, at a low speed, produced a certain deflection; but an extra train, with a much lighter engine, seemed to push the bridge like a wave before it.”
The Britannia tubular bridge was depressed three fourths of an inch by two locomotives and a train of two hundred and eighty tons standing still; but at seventy miles per hour, the deflection was sensibly less.
192 CHAPTER IX.
IRON BRIDGES.
“A little knowledge is a dangerous thing.” ”

“ It will not do to assume, that, because no defects are very evident in a wooden bridge, therefore it has none. When a wooden bridge, originally made of only fair material, has been in use under railroad trains for twenty-five or [Pg 63] thirty years, and in a position where timber would naturally decay, we are bound to suspect that bridge. To assume such a bridge to be all right until we can prove it to be all wrong, is not safe. To assume a bridge to be all wrong until we can prove it to be all right, is a safe method, though not a popular one. ”

“ The towers, however, were designed to sustain a heavy train and locomotive and to withstand the terrific wind of the monsoon. The pressure of such a wind on a 320-foot tower is tremendous. The bridge was completed within the specified time and bore without flinching all the severe tests to which it was put. Heavy trains—much heavier than would ordinarily be run over the viaduct—steamed slowly across the great steel trestle while the railroad engineers examined with utmost care every section that would be likely to show weakness. ”

“ Roebling’s system of trussed suspension bridges. The cost of material alone in one span of the Britannia bridge, of 460 feet, exceeds the entire cost of the Niagara bridge of 800 feet span; add to this that we are sure of the strength of wire cables, but not of tubes, and that the 800 feet span of the Niagara bridge weighs only 1,000 tons in itself against 1,400 in a 460 feet span of tube, and it will not be difficult to prove the superiority of the suspension over the tubular system; thus,
A suspension bridge of 800 feet span costs $400,000. ”