Railway bridge

“ Messrs Fowler and Baker were the engineers of the Forth railway bridge. It was begun in 1883 and finished in 1890. It is built nearly all of steel, and is one of the most stupendous works of the kind. It crosses two channels formed by the island of Inchgarvie, and each of the channel spans is 1710 feet in the clear and a clear headway of 150 feet under the bridge. Three balanced cantilevers are employed, poised on four gigantic steel tube legs supported on four huge masonry piers. The height of the bridge above the piers is 330 feet. ”

“ As a rule, these road-bridges are simple and inexpensive in character, except in towns, or in cases where the line crosses the roads very obliquely, or where the road is situated at the top of a deep cutting, or bottom of a high embankment. Away from towns and out in the open country, permission is generally obtained to divert the roads to a moderate extent, so as to obtain a more favourable angle and height for the bridge; but in towns, where the roads become streets, sometimes of great width, with houses and shops on each side, little or no diversion can be allowed. ”

“ 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. ”

“ 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." ”

“ Other bridges are the Obermainbrücke of five iron arches, opened in 1878; an iron foot (suspension) bridge, the Untermainbrücke; the Wilhelmsbrücke, a fine structure, which from 1849 to 1890 served as a railway bridge and was then opened as a road bridge; and two new iron bridges at Gutleuthof and Niederrad (below the city) , which carry the railway traffic from the south to the north bank of the Main, where all lines converge in a central station of the Prussian state railways. ”

“ By one o'clock we had reached the railway bridge at the head of Second Lake. Upon the trestlework were perched three boys and a man, fishing. They had that listless air and unkempt appearance which are so characteristic of the little groups of humanity often to be found on a fair day angling from piers, bridges, and railway embankments. Men who imagine the world is allied against them will loll away a dozen hours a day, throughout an 37 entire summer season, sitting on the sun-heated girders of an iron bridge ”

“ This scheme also proved abortive, and for forty years more the travelling public put up with what they could get, but at last in 1860 the North British Railway fixed upon a site about six miles to the west of South Queensferry as suitable for the construction of a railway bridge. The bridge was to have consisted of a number of large spans of 500 ft. each in the centre, with approaches in shorter spans at either end. ”

“ The line not only threads knots of mountains, but also spans numerous rifts. Indeed, so much bridging became necessary that the railway has become known as “The Bridge Line.” In completing this road nearly every type of structure known to the engineer was adopted. The largest structure is the Wingatui Viaduct,197 where the rail is carried about 146 feet above the floor of a broken, winding gorge on a creation of steel comprising three spans, each measuring 196 feet in length, and five smaller spans, each of 66 feet, supported on pyramidal steel towers. ”

“ The bridge is for a single line of railway of 5 ft. 6 in. gauge. The back guys are the most heavily strained part of the structure, the stress provided for being 1200 tons. This is due to the half weight of centre girder, the weight of the cantilever itself, the rolling load on half the bridge, and the wind pressure. The anchors are built up of steel plates and angle, bars, and are buried in a large mass of concrete. The area of each anchor plate, normal to the line of stress, is 32 ft. ”

“ 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 central girder portion is simply an ordinary double-line railway bridge of 350 ft. span with girders of a type intermediate between the girder of parallel depth and the bowstring. This is an economical type, and many Continental bridges have been so constructed.
When lecturing some years ago, Mr. Baker, with a view of presenting in a form easily understood and popularly remembered, a simple diagram of the manner in which the principal stresses of a cantilever bridge are distributed, devised the following arrangement of a human cantilever, or a living model of the Forth Bridge. ”

“ Three of the channel-iron cross-girders are extended outwards, and to the ends of these are riveted tee-iron stiffeners to steady the main girders. In some cases stamped, or ribbed, wrought-iron plates are used for a footway, but, although more durable, they do not give such a [Pg 149] secure or agreeable foothold as timber. The ascent or descent of the bridge may consist either of steps and landings, or of ramps, according to circumstances or expediency. Sometimes these bridges are made with curved tops, terminating in steps when nearing the steps, or ramps. ”

“ In railway work an “overbridge” is a bridge over the railway, and an “underbridge” is a bridge carrying the railway. In all countries there are legal regulations fixing the minimum span and height of such bridges and the width of roadway to be provided. Ordinarily bridges are fixed bridges, but there are also movable bridges with machinery for opening a clear and unobstructed passage way for navigation. Most commonly these are “swing” or “turning” bridges. “Floating” bridges are roadways carried on pontoons moored in a stream. ”

“ On a working railway, the train service must be kept going, irrespective of renewals, and very often the best that can be done is to reduce the double line to single line working at the site of the operations. It is not always expedient or possible to make a temporary bridge and diverted line for traffic purposes, as the expenditure to be incurred might be too great to warrant the outlay, or there may be local difficulties to effectually prevent the introduction of a provisional structure. ”

“ The splendid high-level bridge over the Tyne at Newcastle and the Victoria bridge at Berwick were two of his earliest and most successful examples of this branch of engineering. When the act was passed in 1844 for the Chester and Holyhead line, Stephenson gave long and anxious consideration to the best type of bridge for crossing the Conway and the Menai Straits. Eventually he decided upon the tubular girder form, the type of railway bridge which will always remain inseparably connected with his name. ”

“ Many railway bridges over strategical rivers can be displaced by tunnels, but many others must be armoured with cone-shaped roofs as a protection against overhead wars from airships and aeroplanes, 358. See Albi Railway Bridge, the plate facing page 8, and Cannon Street Railway Bridge, the plate facing page 48. ”

“ The gradients are easy and the curves are of wide radius. At one point the location of the line ran through two private cemeteries, but as the graves of the ancestors could not be disturbed under any considerations whatever, the line had to make a wide swerve to avoid this obstruction.
All the materials for the railway were obtained from Great Britain, and the solidity of the construction is an outstanding feature. Every bridge is carried out either in steel, concrete, brick or stone, whichever was found to be the most economical. ”

“ 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. ”

“ Every bridge, from the simplest to the most complicated, including the then novel structure of the "skew bridge," iron girders, siphons, fixed engines, and the machinery for working the tunnel at the Liverpool end, had all to be thought out by his own head, and reduced to definite plans under his own eyes. Besides all this, he had to design the working plant in anticipation of the opening of the railway. He must be prepared with wagons, trucks, and carriages, himself superintending their manufacture. ”

“ 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. ”

“ To guard against disaster from tampering or misuse, the bridge, when either raised or lowered, is locked. To lower the bridge it is only necessary to reverse the winding direction of the winch. The span of steel moved in this manner weighs 55 tons, and the whole operation of opening and closing occupies about half-an-hour, eight men under a native superintendent sufficing for the movement of the winches. ”

“ The new high-level bridge, carrying the railway only, was opened by King Edward VII. in 1906; it consists of four steel spans on granite piers. The hydraulic swing bridge, on the low level, was built to replace a stone structure erected in 1781 on the site of a bridge dating from 1250, and destroyed by a flood in 1771. The Roman bridge, the Pons Aelii, is said to have spanned the river at the same point. The hydraulic bridge (1876) consists of one large centre pier, two midstream piers and two abutments; and its foundations are iron cylinders resting on the solid rock, 60 ft. ”

“ 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. ”

“ In this particular bridge the girders and roadway form a solid framework, which rests on the abutments during the passage of the trains. Strong chains, secured to the corners of the framework, pass over large sheaves on the top of the iron standards, and then round drums placed below the level of the rails, and terminate by attachment to heavy counter-weights suspended in iron cylinders. The counter-weights are adjusted to approximately balance the bridge, so that a moderate power applied to the wheel-gearing on the drums is sufficient to raise the roadway to the required height. ”

“ To open the bridge, all that the natives have to do is to wind a winch which rotates the sprocket wheel, and as the counterweight descends the whole span rises vertically and horizontally, being guided in its movement by the rack on each tower. The counterweight is the full width of the286 bridge, and when the span has been lifted to its fullest extent the counterbalance weight lies across the track, to form a high barrier to any one attempting to cross the bridge when the span is open. ”

“ The bridge has three spans, each formed of arches of cast steel. The centre span is 520 ft. and the side spans 502 ft. in the clear. The rise of the centre arch is 471/2 ft., and that of the side arches 46 ft. Each span has four steel double ribs of steel tubes butted and clasped by wrought iron couplings. The vertical bracing between the upper and lower members of each rib, which are 12 ft. apart, centre to centre, consolidates them into a single arch. The arches carry a double railway track and above this a roadway 54 ft. ”

“ Nor must the bridge be looked upon as a thing standing by itself, but rather as a portion—certainly a somewhat expensive portion—of a gigantic system of railway lines converging from all directions upon the capital of Scotland, affording means not only of more speedy and more comfortable travelling, but also giving facilities for the provision of a larger number of those through trains which are constantly becoming more necessary by the yearly addition of many miles of both main lines and branch lines. ”

“ An essential part of a switch is a frog. (See Frog.) On bridges and trestles the track construction also varies somewhat from that on embankments and in cuts. Sometimes the floors of bridges are made solid and carry ballast on which the ties and rails are supported in the usual manner, but more commonly the ties are laid on the bridge stringers and carry the rails without any ballast. An essential part of railway track on bridges is a wooden or metal rail laid parallel to each of the track rails and a little distance away from them. ”