Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/423

 BRIDGES 379 brick, and concrete structures will be found in the Zeitschr. dcs which is 141 feet above Trinity H. W. The clear width of the two Ocsterreichen Ing. uvd Arch. Vereines, 1895. shore spans is 270 feet. The total length of the bridge is 940 feet, Recently steel arches of great span have been constructed, partly and that of the approaches 1260 feet on the north and 780 feet on because they have a better appearance than girder bridges, but the south. The width of the bridge between parapets is 60 feet, mainly because they can be erected without false works. The first except across the centre span, where it is 49 feet. The main towers very important bridges of this kind were the St. Louis Bridge (for consist of a skeleton of steel, enclosed in a facing of granite and description, see article Bridges in Ency. Brit. vol. iv. p. 339), Portland stone, backed with brickwork. There are two high-level and the Douro Bridge (Fig. 18). Amongst other examples the footways for use when the bascules are raised, the main girders Washington Bridge over the Harlem river, New York, may be men- of which are of the cantilever and suspended girder type. The tioned (see Plate). It has a length of 2375 feet, with a roadway 80 cantilevers are fixed to the shore side of the towers. The middle feet in width, 133 feet above the river. Two principal arches are girders are 120 feet in length and attached to the cantilevers by 510 feet span, springing from granite piers. Each steel arch has six links.. The main suspension chains are carried across the centre ribs resting on steel pins 34 inches long and 18 inches in diameter, span in the form of horizontal ties resting on the high-level footway carried on steel bearings. The ribs, of a nearly uniform depth of girders. These ties are jointed to the hanging chains by pins 20 13 feet, are formed of a web plate and flanges of steel plate. The inchesindiameter with a ring in halves surrounding it 5 inches thick. Niagara arched Railway Bridge, replacing a suspension bridge, was One half ring is rigidly attached to the tie and one to the hanging erected without stopping the traffic. The span is 550 feet and the chain, so that the wear due to any movement is distributed over rise 114 feet. The hinges are formed of two steel castings. Between the length of the pin. A rocker bearing under these pins transmits the load at the joint to the steel columns of the towers. The abutment towers are similar to the river towers. On the abutment towers the chains are connected by horizontal links, carried on rockers, to anchor ties. The suspension chains are constructed in the form of braced girders, so that they are stiff against unsymmetrical loading. Each chain over a shore span consists of two segments, the longer attached to the tie at the top of the river towrer, the shorter to the link at the top of the abutment tower, and the two jointed together at the lowest point. Transverse girders are hung from the chains at distances of 18 feet. There are fifteen main transverse girders to each shore span, with nine longitudinal girders between each pair. The trough flooring, the convex face of the upper and concave face of the lower casting § inches thick and 6 inches deep, is riveted to the longitudinals! are 45 segmental rollers set radially to the centre of movement. The anchor ties are connected to girders embedded in large concrete The pressure on the rollers is one ton per lineal inch. The blocks in the foundations of the approach viaducts. Garabit Viaduct carries the railway near St Flour, in the Cantal The two bascules are each constructed with four main girders. Department, France, at 420 feet above low water. The deepest Over the river these are lattice girders, with transverse girders 12 part of the valley is crossed by an arch of 541 feet span, and 213 feet apart, and longitudinal and subsidiary transverse girders feet rise. The bridge is similar to that at Oporto, also designed dividing the floor into rectangles 3 feet by 3J feet covered with by Mr Seyrig. It is formed by a crescent-shaped arch, continued buckled plates. The roadway is of pine blocks dowelled. The on one side by four, on the other side by two lattice girder spans, bascules rotate through an angle of 82°, and their rear ends in the on iron piers. The arch is formed by two lattice ribs hinged at the bascule chambers of the piers carry 365 tons of counterweight, the abutments. Its depth at the crown is 33 feet, and its centre line total weight of each being 1070 tons. They rotate on steel shafts follows nearly the parabolic line of pressures. The two arch ribs 21 inches in diameter and 48 feet long, and the bascules can be are 65| feet apart at the springings and 20 J feet at the crown. The lifted or lowered in one minute, but usually the time taken is one roadway girders are lattice, 17 feet deep, supported from the arch and a half minutes. They are worked by hydraulic machinery. ribs at four points. The total length of the viaduct is 1715 feet. The lattice girders of the side spans were first rolled into place, so The main girders of girder bridges consist of three as to project some distance beyond the piers, and then the arch principal members, a tension and compression boom or ribs were built out, being partly supported by wire-rope cables from the lattice girders above. The total weight of ironwork was 3200 chord, and a web. When the booms are parallel airder tons and the cost £124,000 (Annales des travauxgmbliqucs, 1884). the web resists the shear. If one or both brld es ^ In suspension bridges the principal member is in tension, booms are curved the straining action on the and can be made of material of great tenacity, such as wire. web is diminished. At first the girders had plate webs The compression member required in girder bridges is absent. even for large spans, but now plate webs are not used for Hence, suspension bridges should be economical in cost. spans over 50 to 75 feet. The web in modern girders is But, on the other hand, expensive piers and anchorages are an arrangement of struts and ties termed bracing. In necessary, together with stiffening members to counteract some girder bridges the members are jointed or connected the flexibility of the chain. The Ordish Bridge, built at by pins, in others they are more rigidly connected by Prague in 1868, had oblique chains supporting riveting. The pin connexion system first used in England in the Crumlin and Newark Dyke Bridges is now much 011 bridges.' ^ie stiffening girders at intermediate points of more commonly adopted in America, so that it has come the span. A curved chain supports the oblique chains and keeps them straight. Cables are now made of to be regarded as a distinctively American feature. With wire either in twisted strands or laid parallel. In the East pin connexions some weight is saved in the girders, and River Bridge, New York (see Plate), the individual wires erection is easier, but the girders are less rigid than forming the cable were led over the piers one by one, thus riveted girders and suffer more from oxidation. Commonly avoiding the severe straining to which heavy cables are now the pin connexion is restricted to tension members. subjected from flexure over the saddles and elsewhere A fundamental difference in girder bridges arises from the method of support. In the simplest case the main during erection. girders are supported at the ends, and if there are several The Brooklyn Bridge, New York, including approaches, is 5989 feet long. The centre span is 1595^ feet, and the clear height over spans, are discontinuous. Next, the main girder may be the water 135 feet. The tower piers are built on caissons. The supported at more than two points so as to be continuous roadway is suspended from four steel-wire cables. Number of wires over two or more spans. This economizes weight, but the in strands of cable, 278 ; strands in each cable, 19; wires (7 B.W. G.) objection is that very small alterations of level of the in each cable, 5296 ; strength of cable per square inch, 71 tons ; total weight of bridge, 14,680 tons ; approximate cost, £3,067,000. supports involve comparatively large differences in the The Tower Bridge, London, is a suspension bridge with a second- straining action. Settlement of one of the piers might ary bascule bridge in the centre span to permit the passage of perhaps render the girder unsafe. A girder may be ships (see Plate). Two main towers in the river and two supported at the centre and then forms a pair of cantitowers on the shore abutments carry the suspension chains. The opening bridge between the river towers consists of two leaves levers. Lastly, some bridges are composed of cantilevers or bascules, pivoted near the faces of the piers and rotating in a and suspended girders. The main girder is then virtually vertical plane. When raised, the width of 200 feet between the a continuous girder hinged at the points of contrary main river piers is unobstructed up to the high-level foot-bridge, flexure so that no ambiguity can arise as to the stresses.