Combination Briakes, constructed in part of wood and in part of iron, are in vogue in the United States to a considerable extent for railroad bridges on account of their cheapness. The practice is to construct the members subject to tension of wrought iron and the compression-members of wood, the connections being either of cast or wrought iron. They are adapted for spans up to 200 feet.
St. Paul Highway structure shown in Figure (y5/. 44) is a truss bridge which spans the Mississippi River at St. Paul. It is a highway bridge, anci is constructed of a series of trusses, of which the largest, the channel span, is of iron, the remainder being of wood. The peculiar feature of this bridge resides in the fact that its roadway is built upon a grade oft : zo up to the channel span, where it is : 2S—an arrangement that is made necessary by reason of the bluffs (125 feet high) on the west bank of the river. The eastern approach is formed by an embankment 15oo feet long, followed by 375 feet of trestle-work in bents 3o feet apart. Then follow seven spans of 14o feet each, succeeded by the channel spans of 24o feet and a short span of So feet, over the St. Paul and Omaha Rail road to the top of the bluff. The piers are built in steps, so that there is a difference of 7 feet in the height of each succeeding; one, and the roadway is supported on the top chords by bents. The short spans are of wood. The channel span is of iron, and g,ives a clear headway of 63 feet above high-water level.
Girara' AZ'CillIC of the most celebrated roadway struc tures of its kind is the Girard Avenue Bridge across the Schuylkill River at Philadelphia. It has five spans, with a total length of 865 feet, and the width of the roadway is Da) feet, divided into one carriage-way of 67Y, feet and two sidewalks each of r6; feet. This bridge is remarkable as the first attempt in this country to combine the American system of pin connection bridges with a solid roadway of stone.
Arch Bridges, in use some thirty or forty years for road way bridges, have of late come into vogue for railway purposes; they are, as a class, remarkably handsome structures (pi. 43, fig. 2o). The arch is constructed either as a plate girder or as an open girder. Provision is made for the slight distortion and lateral displacement of the arch girder by reason of stress produced by the load and by changes of temperature. For this purpose the feet of the arches, which rest upon the piers, are not rigidly joined to the latter, but by some form of interposed bearing which will permit of a slight degree of play (fig. 22). By introducing- a third
joint at the crown of the arch the latter may without resistance adjust itself to the changes of temperature by expanding or contracting, in consequence of which the crown of the arch will rise and fall very slightly.
St. Louis most important structure of this type, and one 0/ the most imposing bridge structures in the world, is the steel tubular arch bridg-e over the Mississippi at St. Louis. It has three spans, each formed with ribbed arches of cast steel. The centre span is 52o feet wide, and the side spans are 5o2 feet (fil. 47, fig. 2). The piers and abutments rest at coftiderable depth on rock-foundation, which was secured by the use of caissons with air-chambers and locks. The bridge was completed at a cost of ten million dollars, and opened for traffic in 1874.
Douro River iron-arch bridge of bold and noble design is that over the Douro River near Oporto, Portugal (fig. 4). This structure was finished in 1877. It is 1511 feet long, and spans the river with a single arch 52o feet wide, with a height above the water of 198 feet. The piers are iron trestles resting on granite foundations. The lower plat form is divided into five bays by the suspension-rods. It is formed of two lattice girders spaced 291/ feet apart from axis to axis and ioY; feet wide between the chords. The main girders have been strengthened, with the view of resisting the wind; so that the platform alone, without the aid of the rigidity of the suspension-rods, is amply strong for the purpose, the construction being such that the stress due to the wind is transmitted directiv to the abutments, where, against the masonry, are arranged shoes which permit of expansion, but prevent all lateral motion.
ilarlem River the present time a steel-arch viaduct of two spans is being constructed over the Harlem River, New York (fig. 3). These arches are plate girders of steel, having- each a span of 5ro feet. They rest on massive masonry piers, which rise to the level of the roadway, which, in turn, is supported on vertical posts erected on the arches. The height is 152 feet above the water-surface. The approaches are viaducts of granite carried on stone arches.