While riveted bridges are now quite gener ally used for spans from 100 to 150 feet, they have been employed to some extent up to 425 feet. The recent forms of riveted trusses do not, however, conform to the general charac ter of European designs, but embody the dis tinctively American feature of concentrating the material into fewer members of substantial construction. With but rare exceptions the trusses are of the Warren, Pratt and Baltimore types with single systems of webbing. At a distance where the riveted connections cannot be distinguished, the larger trusses have the same general appearance as the corresponding pin bridges.
When a bridge extends over a valley it often consists of a series of plate-girder spans sup ported by towers, while the stream may be crossed by one or two plate-girder or simple truss spans. Such a structure is known as a viaduct.
The recent examples of viaduct construction with their stiff bracing of built-up members and riveted connections exhibit a marked contrast to the older and lighter structures with their adjustable bracing composed of slender rods. The longest as well as the highest viaduct in America is located on the Crow's Nest line of the Canadian Pacific Railway at Lethbridge, Alberta. Its length is 5,327.6 feet and its height 314 feet from the base of the track-rail to the bed of the river. The tower spans are 67 feet and the intermediate spans 100 feet in length. It was completed in 1909 and accommodates a single track. The longest double-track rail road viaduct is 2,558 feet long and carries the Chicago and Northwestern Railway across the valley of the Des Moines River. It was built in 1901. Along with this viaduct should be men tioned the viaduct terminal of the Chesapeake and Ohio Railroad at Richmond, Va., whose length, including the depot branch, is 3.13 miles. A large part of this is not very much higher than an elevated railroad in cities. The excel lent details and clean lines of this substantial structure give it a character which is surpassed neither in this country nor abroad.
While the elevated railroads which have been built recently also embody many of the charac teristics of the best viaduct construction, spe cial study has been given to improve their zsthetic effect. The use of curved brackets, of connecting plates whose edges are trimmed into curves so as to reduce the number of sharp angles, and of rounded corners of posts, con stitute some of the means employed. The re sults are seen in the structures of the Boston Elevated Railroad and in some of the latest construction in Chicago.
The longest simple truss-bridge span (not continuous over supports) having trusses with riveted connections throughout is in the ter minal railroad bridge at Kansas City over the Missouri River. The length of span is 425 feet
6% inches between centres of bearings. This is a structure with a highway on the upper deck and a double-track railroad on the lower deck. The bridge was completed in 1911.
The longest simple truss-bridge span with pin-connected trusses is 720 feet long. It is in a bridge to accommodate the Burlington, Illi nois Central, Louisville and Nashville and Frisco railroads in crossing the Ohio River at Metropolis, Ill., and was completed in 1917. It would have been slightly more economical to use a cantilever bridge for three of the spans, but the deflections for the simple trusses are smaller and determined the choice. Next in size are the spans of the Saint Louis Municipal Bridge over the Mississippi River. The three river spans are each 668 feet long and were erected in 1912. The bridge is designed to carry a double-track railroad on the lower deck and a highway on the upper one. This is one of the earliest bridges in which nickel steel was employed, the loading being very heavy. In the same year (1912) another bridge was completed at Louisville, Ky., replacing a canti lever bridge over the Ohio River to New Al bany, Ind., the longest span in which is 615 feet 254 inches. It accommodates a double track railroad between the trusses and high ways outside of the trusses, supported by canti lever brackets.
The recent changes in the details of pin connected truss bridges have been mainly the result of efforts to eliminate ambiguity in the stresses of the trusses, to reduce the effect of secondary stresses and to secure increased stiff ness as well as strength in the structure. Double systems of webbing have been practically aban doned so far as new construction is concerned. The simplicity of truss action thus secured per mits the stresses to be computed with greater accuracy and thereby tends to economy. Be fore 1900 very few through bridges, and those only of large span, were designed with end floor beams in order to make the superstruc ture as complete as possible in itself and inde pendent of the masonry supports. Now this improved feature is being extended to bridges of small spans. Similarly dropping the ends of all floor beams in through bridges so as to clear the lower chord and to enable the lower lateral system to be connected without producing an excessive bending movement in the posts has likewise been extended to the smaller spans of pin bridges and is now the standard practice. The expansion bearings have been made more effective by the use of large rollers and of bed plates so designed as to properly distribute the large loads upon the masonry. In the large spans of through bridges the top chord is curved more uniformly, thereby improving the aesthetic appparance.