and most approved specifications. Paved floors are always to be supported by steel stringers, preferably of rolled I beams, spaced generally not to exceed three (3) feet six (6) inches between centres. For asphalt or stone-block pavements, a buckled-plate floor, with concrete thereon, shall be used. The surface of the pavement must be thoroughly drained so as not to retain water, and the upper surface of the buckled plate, before it is covered with the concrete, must be protected from rusting by a liberal use of the best obtainable preservative coating.

When wooden-block paving is adopted, it may rest on a timber floor from four (4) to five (5) inches thick, which in turn rests on and is spiked to timber shims that are bolted effectively to the steel stringers.

All paved floors must be pitched so as to drain transversely to the structure; but plank floors need not be pitched, as the water will drain through the quarter-inch openings.

CLEARANCES.

The smallest allowable clear roadway shall be twenty (20) feet, measured between inclined end posts, excepting for cheap country bridges, where it may be reduced to eighteen (18) feet, or even to fourteen (14) feet, in case that the bridge be so short that no provision need be made for teams passing thereon.

The smallest allowable clear headway shall be fourteen (14) feet, except for bridges in cities where the ordinances require a greater height. The corner brackets may, however, encroach on the specified clear headway, provided they do not extend either laterally or downward more than five (5) feet.

EFFECTIVE LENGTHS AND DEPTHS.

See Specifications for Railroad Structures.

STYLES OF BRIDGES FOR VARIOUS SPAN LENGTHS.

In general, spans of and below twenty (20) feet are to consist of rolled beams or simply wooden joists; spans from