(B) Floors

When the strength of the superstructure will admit of such loading, the roadway floor should be reconstructed with a reinforced-concrete slab and brick wearing surface, and the sidewalk should be of reinforced-concrete. In some cases, where new steel stringers are required, buckle plates may be used with some form of comparatively-permanent wearing-surface. Where the foot traffic is heavy, the sidewalk can be given an asphalt wearing-surface. Few of the older bridges are heavy enough for the loading indicated, having been originally floored with plank. The time for use of a timber floor with the side of the grain exposed to wear has passed in most places. The spikes work up and cut automobile tires, and the floor soon requires renewal. Lumber is now so costly that it is not usually economical to employ untreated timber in so exposed a place as a bridge floor, and the use of treated lumber only, with wood-block wearing-surface, is recommended.

We have scores of iron and steel bridges over rivers and small streams, which structures are too light to carry a concrete and brick roadway floor or a concrete sidewalk. The weight of trucks now traversing these bridges makes the use of wooden stringers very undesirable because of the presence of knots and other defects, hence steel stringers are employed. The practice in some places is to lay planks directly on the steel stringers with only occasional fastenings, but this does not seem to be satisfactory. Bolting at each bearing point is expensive, hence our practice is to bolt a surfaced 24' X 5" nailing piece on top of each stringer, nailing the planks with two nails at each intersection, as in the case of wooden stringers. It has been found that such solid nailing distributes the load so that a wheel load is carried by at least two stringers. On such bridges the stringers should be from 24" to 28" center to center; and planks of uniform width, at least 10 inches wide and surfaced to 24 inches, should be laid parallel with the back-walls, even though the skew necessitates extra-long planks. This prevents pointed ends, which are hard to support. In extreme cases the planks may be in more than one length across the floor, but the joints in adjacent planks should not come over the same stringer or adjacent stringers, but should lap for a distance equal to at least two spaces between stringers.

In case the bridge carries street-car traffic, there seems to be no better construction than to employ ties to support the rails and planks. Seven-inch grooved rails and half-inch tie plates are used. Planks at least 10 inches wide and surfaced to 34 inches give good results. The blocks should then have sufficient depth to come flush with the rail. A shaped wood-filler on each side of the rail is much lighter and more permanent than concrete filling. It is very important, however, that each tie shall have but two bearings and that the rails be as nearly over the stringers as possible, otherwise the springing and warping of the ties will give an irregular bearing for the rails, resulting in future trouble. In this connection it might be suggested that all street-car rails be painted, when laid, like all other struc-