The Aa'vantage of Suspension Bria'ges over the girder and arch bridges is that the supporting structures (the chains or wire cables) are subjected to strains of tension only, in consequence of which a much lighter con struction for a given load is made possible than with bridges of other types. For the same reason, bridges of this type may be successfully employed for spans of great width where other forms of bridge could not possibly be employed. The number of intermediate piers is reduced to a minimum in the suspension system, which means a saving in cost and a lessening of the obstruction to the waterway. The construction of the suspension bridge, also, -whenever the cables or chains are in place and secured to their anchorages, is a simple matter, as the scaffolding required for the erection of the rest of the structure is comparatively light, and can be sus pended from the cables and moved along as the work progresses.
The Discra'zvantage of Suspension Bridges, on the other hand, is that in high winds or in the passag,e of heavy loads they are liable to deformation and to more or less severe oscillation, in consequence of which they have , been confined principally to ordinary road traffic and are used only to a limited extent for railway traffic.
To stiffen the structure and give it greater stability, several plans have been adopted. One is to make the suspended superstructure a trussed gir der, and, in addition to this, to steady the structure by the use of anchor rods or wire ropes which are attached at different points under the floor, and are carried to the abutments and firmly fastened to them. This plan is adopted in the railway suspension bridge built over the Niagara River by Roebling (1S52-1S55; 4s, fig . 2). A different plan is adopted in the " Point Bridge," at Pittsburgh (fig. 4).. This is a chain bridge, in which the chain itself is stiffened by a truss and jointed at the centre and piers. A somewhat similar plan is employed in the railway bridg-e over the Danube Canal at Vienna. To secure additional lateral stiffness to resist wind pressure, it is usual to draw the cables inward at the centre of the span. In the Niagara Falls Suspension Bridge, which has four cables—two on each side of the bridge—the two upper ones are 37 feet apart where they rest on the towers and onlv 13 feet apart at the centre, while the lower set are 39 feet apart at the towers and 25 feet at the centre.
Where, in addition to the cables at the sides, there are others placed over the axis of the bridge, as is the case in some wide bridges of this type, it is. the practice to draw the outer cables inward at the centre of the span as just noted, and to spreaci the inner cables at the centre and draw them in at the piers. To stiffen the roadway and lessen undulations and relieve the cables of much of the strain they would otherwise have to sus tain, a number of wire rods or cables, called cable-stays, are used. These extend from the saddles obliquely downward, and are attached to the floor. These stays may reach out a considerable distance across the span. They serve an excellent purpose in transmitting directly to the saddles the strain due to the weight of the structure and the load (pi. 4r, fig. -1).
In the Ordish-Leferivre system, illustrated in the Franz-Joseph Suspen sion Bridge, at Prague, the plate-girders of the roadway are supported entirely by such oblique suspension-rods passing directly to the towers; and as, on account of their great leng,th, they require stiffening, they are fastened to a comparatively light chain, passing from tower to tower, by means of vertical suspension-rods.
East Rizrr Suspension greatest existing structure of this type is the suspension briclg,e over the East River joining New York City and Brooklyn CA/. 48, fig. 3). It crosses the river with a single span of 1595 feet. The approaches, which are heavy masonry arches, are 2492.5 feet long on the New York side and 19or feet on the Brooklyn side, mak ing the total length of the structure 5989 feet. The bridge has a clear headway at high tide of 135 feet at centre and of 120 feet at the ends. The cables are four in number, each 15.5 inches in diameter." The piers are 134 feet long and 56 \vide at the water-line, and rise to the height of 28o feet. Each is pierced with two pointed arches for the admission of a rail way track, carriage- and foot-way (Iv 42, fig. II). As above stated, the two outer cables are drawn in toward the centre, and the two inner ones are spread outward, to stiffen the structure against wind-pressure. This splendid structure was designed by John A. Roebling-, and was completed by his son, Washington A. Roebling. It was opened for traffic in 1883. Figures 9 to 12 exhibit a section of the roadway and certain other details of interest.
Other Xotable American Suspension other notable structures of this type, and some of the largest of their kind, are to be seen in the ',Tufted States. Of these may be named the Clifton Bridge,' at Niagara Falls (for foot-passengers), with a span of 1268.3 feet (1869); the Ohio River I3ridge between Cincinnati and Coving,ton, Kentucky (fii. 41, figs. 1, 2), built by the elder Roebling (r867), with a span of 1057 feet between the towers, with two cables only, and a clear headway at low water of 1o3 feet at centre and of 91 feet at the towers. The total length, including approaches, is 2252 feet. Another interesting specimen of this tvpe of structure is the Niagara Falls Railway Suspension Bridge Co/. 48, _fig. 2), completed by the same eminent constructor in 1855. The span is 82r feet and the height above the water-surface 245 feet. There are four cables, which rest on two iron towers at each end, 6o feet high, the towers resting, in turn, on substantial masonry foundations. The superstructure is in the form of a rectangular box 18 feet deep and 24 feet wide, the sides being formed of an open-trussed girder. The top or upper floor of the superstructure carries the railway-track, and the bottom or lower floor is the foot- and carriage-way. The Point Bridge over the Monong-aliela River at Pittsburgh (referred to above) is interesting because of certain peculiarities of construction. This is a chain bridge of three spans, the longest being Soo feet (fi7. 4S, jig. 4).