Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/252

234 RAILWAY has, however, as a material for railway structures, been very gen- erally superseded by wrought -iron, forming plate girder bridges. Timber is now almost unheard of for railway bridges on account FIG. 19. Ordinary bridge over or under a railway. The left-hand side is for a cutting, and a bridge over the railway ; the right-hand side is for an embank- ment and a bridge under the railway ; the difference is in the foundations. of its want of durability and stiffness ; and such bridges as have formerly been built of timber in Great Britain are being rebuilt of stone, brick, or plate-iron. Viaducts. The longest viaduct in England is perhaps the Congleton, on the Manchester and Birmingham Railway ; it is of stone, 1026 yards or more than half a mile in length and 106 feet high, and it cost 113,000, or 113 per yard run. The Dane viaduct, on the same line, is of brick, 572 yards long and 88 feet high, and it cost 54,000, or 96 per yard run, having 23 arches of 63 feet span. The Avon viaduct, on the Midland Railway, is of brick, 240 yards in length and 51 feet high, with 11 arches of 50 feet span ; it cost 14,000, or 60 per yard run. For comparison it maybe stated that the Britannia Tubular Bridge across the Menai Straits, 616 yards long and 104 feet high, cost 600,000, or 974 per yard run. On the different lines entering London there are several miles of brick viaducts in the approaches to termini, and .also at Manchester and other large cities and towns. Many interesting details might be given as to bridges and viaducts of the larger kind, but we must here confine ourselves to some account of the Forth Bridge, now in course of construction at Queensferry, referring the reader for other examples to the article BRIDGES. Fio. 20. The Forth Bridge. The Forth Bridge, designed by Mr John Fowler and Mr Benjamin Baker, is thfc largest and most remarkable railway bridge in the world. One of its spans is shown in elevation and plan in fig. 20. The bridge consists of 2 spans of 1700 feet each, 2 of 675 each, 15 of 168 each, and 5 of 25 each. Including the width of the piers, there is almost exactly 1 mile of main spans and half a mile of approaches by viaducts, making together about 1 miles of total length. The clear headway under the centre of the bridge is 150 feet above the level of high water, and the highest part of the bridge is 361 above the same level. Each of the three main piers consists of a group of four cylindrical piers of masonry and concrete, 49 feet in diameter at the top and from 60 to 70 in diameter at the bottom. The deepest pier is about 70 feet below low water, and the rise of the tide is 18 feet at ordinary spring tides. In the piers there are about 120,000 cubic yards of masonry and in the superstructure 44,500 tons of steel. The contract was let for the sum of 1,600,000, being at the rate of 645 per lineal yard. An impression of the magnitude of the bridge is derived from a comparison with the largest railway bridge in England, the Britannia Bridge, which has a span of 465 feet, the ratio of which to that of the Forth Bridge 1700 feet is as 1 to 3 '65. The site of the Forth Bridge is at Queensferry. At this place the Firth of Forth is divided by the island of Inchgarvie into two channels, which, being as much as 200 feet in depth, precluded the construction of intermediate piers. Hence the adoption of two large spans of 1700 feet each, between which the central pier is founded on the island midway across. The bridge is composed of three double lattice-work canti- levers, like scale-beams, 1360 feet in length, poised on three sub- structures, and connected at their extremities by ordinary girders 350 feet long, which complete the main spans. The bridge is taper in plan, varying from a width of 120 feet the distance apart of the lower members of the cantilevers at the piers to a minimum of 31 at the ends, in order to confer a degree of stiffness laterally, for resisting irregular stresses, wind - pressure in particular. The columns above the piers, forming the basis of the cantilevers, are 12 feet in diameter. The lower booms, as well as the struts of the cantilevers, being the members in compression, are circular in cross section, this form of section having been selected as the most effective for resisting compressive stress. The lower boom is at the piers 12 feet in diameter, constructed of plates 1J inches in thickness. The size is gradually reduced towards the ends, where the diameter is 5 feet, made of plates three-eighths of an inch in thickness. Correspondingly the upper member of each cantilever is a tapering box-lattice girder, rectangular in section, 12 feet deep by 10 wide at the piers, and 5 feet by 3 at the ends. The central girders are 32 feet apart. The wind-pressure is assumed for calcu- lation at a maximum of 56 tt> per superficial foot. It is calculated that the maximum possible stress on any member of the bridge is at the rate of 7^ tons per square inch of section. The required ultimate strength of steel under compression is from 34 to 37 tons per square inch, and under tension from 30 to 33 tons. Between the two main girders the double line of way is to be earned on an internal viaduct (see smaller figure in fig. 20), supported by trestles and cross girders. The way will consist of heavy bridge rails, Brunei section, laid on longitudinal sleepers bedded in four steel troughs, into which the wheels will drop in case of derailment, and then run on the sleepers. Railway Stations. Railway stations are either "terminal" or Station "intermediate." A terminal station embraces (1) the passenger station ; (2) the goods station ; (3) the locomotive, carriage, and waggon depots, wher'e the engines and the carrying stock are kept, cleaned, examined, and repaired. At many intermediate stations the same arrangements, on a smaller scale, are made ; in all of them there is at least accommodation for the passenger and the goods traffic. The stations for passengers and goods are generally in different and sometimes in distant positions, the place selected for each being that which is most convenient for the traffic. The passenger station abuts on the main line, or, at termini, forms the natural terminus, at a place as near as can conveniently be obtained to the centre of the population which constitutes the passenger traffic. The goods station is approached by a siding or fork set off from the main line at a point short of the passenger station. Terminal branches of the railways where, for example, there is a sharp incline are sometimes worked by stationary engines and ropes to the point where the locomotive joins the train. The loco- motive station is placed wherever the ground may most conveni- ently be obtained, at or near to the terminus ; in some cases it is found at a distance of 3 or 4 miles. An abundant supply of good water and ample means of drainage are important at stations. There should be ample area of land to admit of the greatest possible extension of accommodation, and the erection of buildings on land adjacent to the station grounds should be discouraged. Companies have been compelled to repurchase at greatly advanced cost land originally disposed of by them as "surplus," and generally with a view to building operations. When this course is adopted prudent managers should take care to secure in the conveyance power to repurchase the freehold at original prices, with allowance for out- lay in building or otherwise, by valuation. In laying out the approaches and station-yard of passenger A rrar stations ample width and space should be provided, with well- inent defined means of ingress and egress to facilitate the circulation of term: vehicles, and the setting -down pavement should be as long as possible, to admit of several carriages discharging passengers and luggage at the same time. The pavement should be wide and sheltered from the weather by a roof, overhanging beyond the kerb, or spanning the roadway, but in all cases free from columns. The position of the main buildings relative to the direction of the lines of rails is the distinguishing feature in terminal stations. When space permits, the usual course is to place them on the departure side parallel to the platform, but they are frequently placed at the end of the station at right angles to the rails and platforms. Or these two systems are combined in a third arrangement, in which the offices are placed in a fork, between two or more series of lines and platforms. Of the metropolitan termini, the Great Northern