REINFORCED CONCRETE BRIDGE Most of the carpenter and other labor is hired in the town.
Gravel, sand, stone, cement, lumber, teaming, board— money for all these goes into the business of the town.
When the bridge is built, it increases in strength as it grows older ; it is the most graceful bridge made; there is no maintenance expense; it is quiet under traffic.
When maintenance of a steel bridge is considered, a reinforced concrete bridge shows economy of investment.
It is the last claim in which the present interest cen ters and by the foregoing presentation, the only points now left open for comparison are costs of roadway re newal and of re-painting. The former is a most difficult matter for determination since a concrete structure will always carry the regular railroad ballast and track, or the highway or street surfacing, which has to be main tained only to the same extent as the adjoining thor oughfare. It has already been stated that concrete can not usually compete with steel highway bridges (as to first cost), when the latter are to be designed with plank flooring. The life of this material is extremely variable, depending almost entirely on the density of traffic. On Brooklyn Bridge the 21/2-inch spruce plank of the wear ing surface must be replaced about every four months, while the writer has known of country highway bridges in which the flooring rotted away so as to need renewal before the traffic had produced enough wear to necessi tate relaying. Between these two extremes, the con ditions are so wide as to make this item too indefinite to be entertained except in cases or locations where the wear and cost of renewal are known. In suck an in stance it is only necessary to estimate the yearly ex pense for renewal, and compute from it the percentage of the total cost of the structure. As an example, as sume a bridge 100 ft. long and 30 ft. wide, of which the planking has to be renewed once a year. Assume a 3-in. yellow pine wearing surface, which costs, including tear ing up, spikes, labor, material, and supervision, $40 a thousand. This item then aggregates 30X100X3÷1000 X40=-$360. If the initial total cost of the structure were
$10,000, the yearly percentage cost of roadway renewal would be slightly less than four. This figure is known to be very modest, and when it (representing a yearly cost of 12 cents per square foot in this instance) is compared with $1.35 per square foot, which is about the yearly cost of renewal of the planking of the Brooklyn Br'dge, this point is made all the clearer.
The floors of railroad bridges (unless constructed with buckle plates or similar material, on which the regular ballast roadbed is carried), are largely of tim ber, which has a relatively short life. In general it may be estimated that the wooden portion of such a floor will last about ten years in the northern part of the United States, decreasing to about six in the Southern States. The cost of a floor is almost purely a function of the length of the bridge alone, so that it is impossible to assign to it a percentage value. A rough estimating figure of $3 per ft. per track is sometimes given for the timber work. Table V gives some estimated costs of steel work and railroad bridge floors for different spans, for the sake of comparison.
The painting of a steel structure is a great question. There are several hundred "best paints" on the market, some of which are good, and some of which will hardly last until the paint salesman can collect his bill. The best paints obtainable will last about six years, after which period a new painting is necessary for good maintenance.
The Bridge Department of New York makes a prac tice of painting Brooklyn Bridge about once in three years, with one good coat at a cost of about $20,000.
The total cost of a steel bridge varies widely from year to year, prices for similar work at different times within the writer's experience running from $70 to $105 per ton. Of this amount, about $2.50 should cover the cost of painting an average railroad bridge; while high way bridges run somewhat higher, because of their rela tively lighter structural members.