Sidewalks are usually reinforced slabs of granitoid or specially-hard concrete, or else untreated planks. The latter are the cheapest, even when the cost of maintenance is considered; for they last fairly well; but, like all other timberwork, they are subject to destruction by fire, involving also injury to the metallic portion of the structure.

Sub-Paving or Base

As previously stated, the reinforced-concrete slab makes the best and most economic base for pavement. The reinforcing must be arranged to suit the steel floor-system, which will be described later. A timber base, either plain or creosoted is not truly economic construction because of the fire risk, nevertheless it is frequently used because of limited funds, or for lack of appreciation of true economy in design. Comparing creosoted and plain planking, the former is more expensive and heavier, but the plain plank as a base rots so rapidly that its use is uneconomic. Modern concentrated loads require that the distances between supports for the planks be small-say two feet for 3-inch planks and two and a half feet for 4-inch ones. The former thickness is totally unfit to support heavy traffic, especially after either abrasion or decay has started. With the live loads adopted in the standard bridge-specifications of today, timber joists or stringers are not truly economic; for very heavy members would be required for panels of any length, even down to the shortest ever likely to be considered. The cheapest timber floor is obtained by plank carried on ties supported by widely-spaced steel stringers. Common practice, however, spaces the stringers closer and puts nailing-strips thereon. This increases materially the cost of the stringers.

Curbs

The best kind of curb to adopt depends essentially upon the style of deck. With a concrete deck or a pavement on a reinforced concrete base, a concrete curb properly faced with a steel angle is the appropriate construction; but for timber decks, wooden guards are the proper thing. True economy demands that the drainage of the deck be correctly taken care of, either by openings beneath the curbs or by an effective grade on the roadway.

Waterproofing

When the steelwork is encased in concrete, or when no dripping of water on the space below the deck is allowable, it becomes necessary to provide waterproofing between the paving and the sub-base. The membrane type of waterproofing, previously described for railroad bridges, is the best and most economical.