them. For eight-foot or ten-foot sidewalks three stringers are economic. Sidewalks on deck, plate-girder bridges reduce, or even eliminate, the economy of the stringerless type of floor-system.

With half-through, plate-girder spans, the positions and numbers of girders are generally fixed, two being just outside of the curb-lines; but, in wide roadways it is economic to use three lines of girders, although the splitting of the traffic by the middle girder is not a desirable feature. In the case of a very shallow floor, though, such splitting cannot be avoided.

The stringerless type of floor-system is nearly always cheaper in half-through, plate-girder spans than the standard type with its stringers and floor-beams. With electric-railway tracks the advantage is smaller than where there are none. With no such tracks the saving of metal amounts to some 200 lbs. per lineal foot of span for a structure of any ordinary width; and the excess cost of the thicker slab is small. Again, the stringerless type is shallower than the other-which is often very important. By spacing the beams closely a very shallow floor can be obtained at moderate cost; but too shallow sections are not desirable, as they do not afford sufficient lateral support for the top flanges of the main girders.

The floor-systems of half-through, plate-girder bridges very often must be encased in concrete. This is to the advantage of the stringerless type, which has smaller area to cover and involves more simple work.

When a concrete slab is employed, it should be carried over to the webs of the girders and supported on shelf angles. If the floor is encased, the detail at the girder web needs careful attention, in order to prevent water from entering and thus causing rusting of metal and splitting off of encasement.

Sidewalks on half-through, plate-girder spans are carried on cantilever brackets, but outer steel stringers will be needed. The inner edge is supported on a shelf angle. A close spacing of these brackets is advantageous, as generally there is difficulty in properly taking care of large top-flange tension.

With truss spans, the standard floor-system of stringers and floor-beams is nearly always adopted, because it would usually be uneconomic to support the cross-girders by the chords between panel points. As previously indicated, the economic panel length for the floor-system per se is from fifteen to seventeen feet, which is too short for the economics of the trusses, hence longer panels than that limit generally have to be adopted. For a bridge of practically any width of roadway, the metal in the floor-system with 25-foot panels weighs about 70 pounds per lineal foot more than that required for the economic panel length, and for 30-foot panels about 150 pounds per lineal foot more. Divided panels in spans of moderate length permit the use of the economic panel length, but the secondary truss- members increase the weight of metal in trusses enough, or more than enough, to offset the saving in the floor-system. With long spans, however, this uneconomic feature disappears.