Trestle bridge

“ Trestle Bridge.—When the water is less than 4 feet deep, the trestles may be carried to their places by men wading in the stream; an abutment is formed as for an ordinary bridge; the trestles are placed with their caps parallel to the abutment sill and about 13 feet apart. When the water is too deep or too cold to allow this method to be pursued, the bridge may be constructed as follows:
The abutment sill being placed, the first trestle can usually be placed by hand; the balks are laid and covered with chesses to within 1 foot of the trestle, a roller is laid on the bridge ”

“ A trestle-bridge, then, such as the one before Snap, is a bridge of timber, the beams laid at right angles to the line, and each beam about two feet from its neighbour. Across the beams run the iron rails, and between the beams is nothing at all but emptiness. The whole bridge is supported on a huge scaffolding, which rises from the sides of the canyon crossed, and in some cases these bridges are as much as 150 yards from end to end, and 250 feet above the stream which generally races along below. ”

“ Begun as pioneer works, they undergo, as traffic progresses, many improvements. Ballast is laid down. Iron or steel bridges are substituted for timber. The gorges spanned by trestles are, one by one, filled up, by the use of the steam digger to fill, and the ballast plough to push out, the stuff from the flat bottomed wagons on each side and through the interstices of, the trestles. Sometimes the timber is left in; sometimes it is drawn out and used elsewhere. This trestle bridge plan of expediting the completion, and cheapening the construction, of new railways, wants more study, at home. ”

“ A second trestle is then in like manner fixed in front of the first one, connected to it by a pole just as the first is connected to the bank. And so the thing grows. To all the upper ends of the V's a light pole is tied to form a handrail. In this case, of course, the floor of the bridge is nothing more than a pole, but with the assistance of a handrail it is quite easy to walk along a single pole. ”

“ Trestle bridges may be constructed in rivers whose depth does not exceed 9 feet, and whose velocity is not more than 6 feet. They may be employed with advantage in rivers of moderate depth and gentle current, with hard, even bottoms. When the bed of the river is uneven the adjustment of the trestles to the bottom is very tedious, and if the current is rapid, almost impossible. When the bed is of mud or fine sand, the settlement of the legs is liable to be irregular. ”

“ 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. ”

“ It is no use my reminding you that no railroad goes to the old home place. It isn't there any more, even in outward seeming. Cummins's woods, where you had your robbers' cave, is all cleared off and cut up into building lots. The cool and echoing covered bridge, plastered with notices of dead and forgotten Strawberry Festivals and Public Vendues, has long ago been torn down to be replaced by a smart, red iron bridge. The Volunteer Firemen's Engine-house, whose brick wall used to flutter with the gay rags of circus-bills, is gone as if it never were at all. ”

“ Old bridges, for example, are altogether unsuited to heavy and fast motor-traffic. Often built askew with the line of a main road, they are nearly always very steep, very narrow, and, though often sturdy in appearance, are incapable of bearing the weight of a heavy lorry and trailer moving with the speed of a railway train. Here again is a problem requiring solution. Some people would attempt to adapt the old bridge to modern needs, others prefer an entirely new structure placed parallel with the old one, and, of course, the third alternative is complete demolition. ”

“ The next step was the contrivance of arched beams or bow-string girders, firmly held together by horizontal ties to resist the thrust, instead of abutments. Numerous excellent specimens of this description of bridge were erected by Robert Stephenson on the original London and Birmingham Railway; but by far the grandest work of the kind—perfect as a specimen of modern constructive skill—was the High-Level Bridge, which we owe to the genius of the same engineer. ”

“ But before lattice girder bridges had become so popular, another class had come into use, and afford some splendid specimens of engineering skill. These are suspension bridges, and, perhaps of all kinds, they are the most picturesque. Their graceful sweeps and curves yield perhaps a more pleasing sight for the eye than the solid, rigid, straight lines of the girder bridges.
It was the genius of Thomas Telford which gave a great impetus to this class of bridge. Like Stephenson after him, he had to bridge the surging Menai Straits, but for a carriage road, not a line of rails ”

“ Another passenger, like-minded, was companion of his ramble.
'I fear it is but a utopian scheme to dream of [372] bridging such a flood as this,' observed Holt. 'No piers of man's construction could withstand the force that is in motion on the river to-night. I fear the promoters of the Victoria Bridge are too sanguine.'
'Well, I could pin my faith upon any engineering project sanctioned by Stephenson,' rejoined the other. 'We had him here to view the site, just a mile out of Montreal. He recommended the tubular plan—a modified copy of the English Britannia Bridge. ”

“ But, long before 1862, the Warren and other truss-girders had come into use, and I am inclined to say that, so far as novelty in the principle of girder-construction is concerned, I must confine myself to that combination of principles which is represented by the suspended cantilever, of which the Forth Bridge, only now in course of construction, affords the most notable instance. It is difficult to see how a rigid bridge, with 1,700 foot spans, and with the necessity for so much clear headway below, could have been constructed without the application of this principle. ”

“ The supporting chains, which were brought from old Hungerford Bridge, are thrown over lofty turrets, resting in one case on a projecting bastion of rock, and in the other on a solid pier of masonry. These slender suspenders carry a roadway and two footpaths across a span of 700 feet. The bridge stands 245 feet above high-water level, and its altitude seems to furnish an irresistible temptation to people of a suicidal tendency. ”

“ The Grand Trunk struck into the Prairie Provinces with depots at Fort William, Port Arthur, and many other strategic centres. Mr. Hays had a great idea of the value of bridges. One of his conceptions was a single-span double-track bridge crossing the Niagara Gorge. This was opened in 1897, replacing the suspension bridge opened in 1853. The Gorge Bridge is a splendid piece of engineering, the span of the arch being 550 feet and the width of the railway flooring 32 feet, the lines being 245 feet above water. ”

“ Alexander M. Ross, who had superintended the construction of Robert Stephenson's tubular bridge at Conway, visited Canada, and inspected the site of the proposed structure, when he at once formed the opinion that a tubular bridge carrying a railway was the most suitable means of crossing the St. Lawrence, and connecting Montreal with the lines on the north of the river. ”

“ Hence each arch is complete and independent in itself, the piers having simply to sustain their vertical pressure. The arches are six in number, of 125 feet span each, the two approaches to the bridge being formed of cast-iron pillars and bearers in keeping with the arches.
The result is a bridge that for massive solidity may be pronounced unrivaled. It is one of the most magnificent and striking of the bridges to which railways have given birth, and has been worthily styled "the King of railway structures." ”

“ A cantilever bridge consists of two inverted trussed beams, each balanced on a pier, one part extending over the river and the other to the shore, where it is firmly anchored in solid, heavy masonry. The ends extending over the river toward each other from the opposite piers are joined by a short truss in such a manner as to permit expansion and contraction consequent on changes of temperature, and yet be proof against vertical or lateral pressure. Such a bridge, it is said, sustains scarcely any strain in the center of the span. Each half of the entire bridge is self-balanced on its pier ”

“ Dealing first with reconstruction of the superstructure of railway under-bridges, these, if small, may not give much trouble, though the demand for greater strength will, perhaps, involve some difficulty in working to the limiting construction depth—i.e., the distance from the top of rail to soffit of bridge—particularly as many old bridges have a very niggardly allowance in this respect. It may be, and quite commonly is, necessary to raise the rails a small amount, or, if headway is not restricted, to lower the soffit. ”

“ The Americans have, however, outspanned all the rest of the world in their wire suspension bridges. They have thrown a suspension bridge of 800 ft. span over the Niagara at a height of 260 ft. above the water, to carry not only a roadway for ordinary traffic, but a railway. Suspension bridges are not well adapted for the latter purpose, but there seemed no other solution of the problem possible under the circumstances. ”

“ The contractors have already completed the iron-work for the “first half” of the bridge; with the exception of this face-work, which will be finished somewhat later. It then only remains for the engineer to complete the road and pathway, and open the bridge for traffic. One obstruction will, we fear, remain. The blocks of houses on either side of the river (ultimately destined to be removed) , still look stable as ever—these abut directly upon the new portion of the structure, and will, if not removed before opening the bridge, make a sharp and inconvenient turn in the road necessary. ”

“ The Queensboro Bridge still stood, as did the railway bridges behind them; but much wreckage had fallen into the river, and in one place formed an ugly whirlpool, which Stern had to avoid by some hard work with the paddle.
The whole structure was sagging badly to southward, as though the foundations had given way. Long, rusted masses of steel hung from the spans, which drooped as though to break at any moment. Though all the flooring had vanished centuries before, Stern judged an active man could still make his way across the 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. ”

“ The approach, however, is dangerous, and requires a skilful waterman, who is obliged to pull his boat into the draft or eddy of the dam before he can make the Lock. Though the works of the New Bridge have at present closed several of the Arches of the ancient edifice, yet the 4th and 5th Locks from the Southwark end have been thrown into one, with a strong wooden vaulting, parapet, and roadway above, to increase the water-way beneath. Since the commencement of these works, the fall of the river has also become less dangerous for barges, from the returning tide sooner meeting with resistance ”

“ At Pesth, for instance, the span was 666 ft., and at Fribourg 870 ft. But engineers, having no imagination and but little prudence, went too far, so they had to retreat from their cocksureness. Soon it became evident that a long suspended bridge of metal suffered much from the lateral oscillation caused by wind, and that its flexibility made it unfit for railway traffic. “The platform rose up as a wave in front of any rapidly advancing load, and the masses in motion produced stresses much greater than those which could result from the same weights when at rest. ”

“ The longest tunnel in Europe over land and over water is the Britannia Tubular Bridge built across the Menai Straits, parallel to and some mile and a-quarter from Telford’s beautiful Suspension Bridge opened for road traffic in 1829. It is 1,834 feet 9 inches long, and in fact consists of two independent wrought iron tubes, each placed alongside of the other. There are four spans, two of 460 feet each, and two of 230—that is, the tubes rest upon two abutments and three towers of masonry—at an elevation of 100 feet above high water mark. ”

“ To maintain the level, so as to avoid the winding of the road down a steep descent on one side of the valley and up an equally steep ascent on the other, Telford ran out a lofty embankment from both sides, connecting their ends by means of a spacious bridge. The structure at Pathhead is of five arches, each 50 feet span, with 25 feet rise from their springing, 49 feet above the bed of the river. Bridges of a similar character were also thrown over the deep ravines of Cranston Dean and Cotty Burn, in the same neighbourhood. ”