The rapidly increasing capacity of American bridge-building shops and their ability to fabri cate and handle single members of sizes im possible to obtain a few years ago, has enabled the American engineer to design bridges with practically no shop limitations, and he has thus been free to make designs solely with the idea of producing a structure with the maximum strength and rigidity and the minimum weight of material.
The rapid advance thus brought about would have been impossible had not the ingenuity of the erecting engineer kept pace with that of the designing engineer, but this specialty of erection has reached such an advanced stage that spans of 1,800 feet, with single members weighing 120 tons, are now erected as rapidly as any smaller span.
One of the recent tendencies of American practice has been to substitute plate-girder spans for all short bridges, in place of the vari ous types of lattice girders and pin-connected spans heretofore in general use, and very few engineers use any type of steel structure except a plate girder for spans under 90 or 100 feet. Some engineers prefer this type for spans as long as 125 feet. To summarize general prac tice, it can be said that this type of span is used for spans from 20 to 100 feet, spans under 20 feet being either arches or rolled beams, and spans over 100 feet running into riveted trusses. Plate girders are frequently built with solid floors, with the ties bedded in ballast, and the latest floors of this kind are built of reinforced concrete slabs or arches, as the older trough floors and buckle-plate floors have been found unsatisfactory, to the impossibility of preventing rapid deterioration from rust. Plate girder spans over 70 feet long generally rest on pin bearings at each end, to allow for deflec tion, and have cast-steel end shoes.
The usual present practice is to build all spans between 100 feet and 175 feet with riv eted trusses, and the old style, multiple-inter section riveted span has entirely given way to the riveted, single-intersection truss with long panels. For deck spans, the old practice of supporting the ties directly on the upper chord has largely disappeared, and either a solid bal lasted floor is used, or a regular floor system is laid, consisting of cross floor beams resting on the upper chords, and logitudinal stringers framed between floor beams.
For through spans the ordinary practice is to use a floor system with cross floor beams riveted in between the vertical posts and longi tudinal stringers riveted between the floor beams, but in many cases a solid ballasted floor is used, in which case the lower chords are re inforced against bending by supporting them at their middle points from a sub-panel in the truss or by making deep lower chords capable of withstanding the bending in addition to the direct stress.
Spans over 175 feet are usually built with pin connections and, where the span is long, the upper chord is generally curved so as to give a maximum depth at the centre and truss depths are proportioned so as to give a prac tically uniform upper chord section throughout. Upper and lower laterals and transverse brac ing are now universally built of riveted mem bers, and the older form of tension rods used for purposes of wind and vibration bracing has entirely disappeared. Simple truss spans of this character have been built to a maximum length of 710 feet, which is the length of the channel span of the bridge over the Ohio River at Metropolis, Ill.
Such spans could and would be built of much greater length if it were not for the fact that in navigable rivers of a magnitude requir ing such spans, the government will not allow the waterway to be even temporarily obstructed with falsework, so that for long spans it is often necessary to use a cantilever or some other type of structure that can be erected without placing any obstruction in the main channel.
Spans continuous over one or more piers have not been generally adopted in this coun try, though they are used to a considerable extent in Europe. The two notable American examples of this type of construction are the Queensborough cantilever at New York, which is a five-span continuous structure, and the bridge over the Ohio River at Sciotoville, which is built with two spans of 775 feet, continuous over the centre pier.
In draw spans, the recent changes in prac tice have been in the same direction as for through spans, as far as the trusses, floor and bracing are concerned. The practice of coun ter-bracing the trusses by means of diagonal counter-rods has been abandoned, and stiff riv eted diagonals are used, capable of carrying either tension or compression where reverse stresses occur. For draw spans of short span or in cities or other situations where there is not plenty of room for a swing bridge, the ((rolling lifts bridge has been adopted and has proved most efficient. For long draw spans, the centre pier revolving span is still the type in common use, and for spans up to 400 feet a central pivot bearing is generally used. For spans over 400 feet a rim-bearing turntable drum rolling on a circle of conical cast-steel wheels has been found most satisfactory, and for very heavy spans two concentric drums and circles of wheels are used.