_NOtable Eitrofican Susfiension the noteworthy Euro pean suspension bridp-_,es may be named the cable bridge over the valley of the Saone at Freiburg (1832), with a span of 195.7 feet; the Danube I3ridge. between Pesti] and Ofen (1845), with a central span of 666 feet and two lateral spans of 29S feet; the Hungerford Bridge over the Thames at Lon don (1845), with three spans of 676 feet and two of 329.5 feet; the cable bridge over the Vilaine at Roche-Bernard, with a span of 664 feet; the chain bridge over the Menai Straits at Bangor (1S26), with a span of 578.5 feet (fi/. 43, jig. S); the Franz-Joseph Bridge over the Moldau at Prague (1868), with a central span of 4So.3 feet and two lateral spans of 156.5 feet each; the Franzcns Chain Bridg-e over the same river at Prague (1842), with spans of 433.4 and io9 feet respectively; the Albert Suspension Bricig-e over the Thames at Chelsea, with a centre span of 400 feet and two side spans of 155 feet each; the Hammersmith Bridge over the Thames in Lon don (1827), with three spans of 399.5 feet each and two of 147.3 feet; the Dordogne Bridge at Cubzac (1839), with spans of 357.6 feet; the Maas Bridge at Serainp; (1S43), with a span of 344.5 feet; and the Conway Chain Bridge over the Menai Straits (1826), with a span of 327 feet.
Royal ingenuity with which American bridge engineers overcome obstacles of an unusual character by departing from traditional custom is well illustrated in the original design of the bridge carrying the line of the Denver and Rio Grande Railroad over the Royal Gorge of the Arkansas River in Colorado (./5/. 44, .fig. 3). At this point the walls of the canon rise almost vertically to the height of 1800 feet, and the difficulties of mak-ing a roadway into the mountain-side by blasting were so great that it was determined to bridge the space to be crossed by the railroad. This was accomplished by suspending the superstructure by means of rods from two "arch-braces" or rafters braced against the walls of the gorge. The superstructure is a plate girder 7 feet deep, and in three spans with a total length of 275 feet. It spans the gorge at a height of 47 feet above the water-surface. The structure was designed and built by C. Shaler Smith.
13ridgc modern times piers of iron are not infrequently used in situations where it is desired to economize space beneath the bridge, or where it is important to reduce to a minimum the load upon the founda tion, or, filially, where their use affords the most convenient mode of obtaining a g-ood foundation. (See p. 286.) Where the stnicture is raised to a considerable height above the surface, iron piers are cheaper than those of stone.
Piers of Cast Iron are either in the form of coluiniis, of which one or two rows resting on low stone foundations are placed directly under the girders, or they consist of separate tubular pillars or columns driven into the ground to a solid foundation either by arming the foot of the column with a screw (pi. 50, Jig. IT) and screwing them down, or by washing out the earth, sand, etc., beneath them with the aid of a water-jet, etc.; or, filially, they are tubular piers formed of cylindrical sections fastened together with screw-bolts passing through inside flanges. These sections are added one after another as the cylinder is sunk to its foundation. This is accomplished by either the vacuum or the plenum process.
Sinking Piers: UaC1/11111 the vacuum process the air is exhausted from the interior of the cylinder, when its own weight, aided by atmospheric pressure, causes it to sink a certain depth, and the external pressure forces the soft soil around the open bottom of the cylinder up into its interior. The air is now admitted and this soil is taken out. The cyl inder is then closed above, again exhausted of air, etc., and this is repeated until the bottom of the cylinder has reached a sufficiently solid foundation.
The Plenum Process is the reverse of this. The water is forced out from around the bottom of the cylinder by forcing air into it; and when empty, men are sent down into it (through an air-lock, fi/. 5o, jig. 12, to keep the pressure intact), who remove the soil from around the bottom and to some distance below it. When they have left the chamber, the pressure is with drawn, and the cylinder, no longer sustained by the upward pressure of the compressed air under its closed top, sinks to the depth of the excavation made below its foot. Air is again forced into the chamber and the same series of operations repeated until the desired solid foundation is reached. This principle—modified in details—was adopted in the construction of the South Street Bridge over the Schuylkill at Philadelphia G6/. 5o, Ag-. i3). The diameter of the cylinders should be such that for a double-track bridge each pier should be formed of two, or at most of three, such cylinders. To diminish the liability to further settlement under the weight of the bridge, as well as to increase the resistance of the iron to concussion, the interior of the cylinders is filled with concrete. Filially, it may be mentioned that in certain cases the towers of suspension bridges (the Dordogne Bridge at Cubzac, 1839) aud the piers of viaducts (St. Gall Bridge, 134.8 feet high) are constructed of cast iron in the form of tiers or trestles.