It is to be noted that the first application costs rela tively more for material, and less for labor, than subse quent ones are likely to do. On the whole, therefore, it may be assumed that 2.5 per cent is a fair original cost for painting; and if the same amount is expended once in five years, one-half of one per cent only is the annual cost of this item of maintenance.
One manufacturer of a special coating gives a table said to have been compiled from actual experience, in which the following costs are given for his special material: It is claimed for this special material that a new application need be applied only once in ten years. This makes the percentage cost of paint per annum on the basis of $85 per ton of steel as follows: Collecting together now the several points discussed, a recapitulation will be as follows: Yearly cost of inspection should be about equal, for the two classes of structures.
Depreciation and consequent sinking fund require ment for steel, at least 5 per cent; for concrete, not to exceed 2 per cent.
Cost of maintenance of timber, if used : For steel highway structure, about 4 per cent; For steel railway structure, about 1 per cent.
Cost of maintenance of regular pavement or earth road: Surface for concrete structure, less than 1 per cent ; Of regular ballasted railway track less than 1 per cent.
Cost of painting: Steel railway structure, less than 1 per cent.
Steel highway structure, less than 2 per cent.
Total maintenance costs, aside from interest on first cost, for steel structure, from 7 to 11 per cent, depending on character of bridge and kind of flooring; for concrete structure, not to exceed 3 per cent.
Excess of steel over concrete, from 4 to 8 per cent.
This is a rough average percentage figure, therefore, which a company or community, should allow in the first cost figure of a concrete bridge structure over the total cost of a steel structure for foundations and superstruc ture combined, in a true competition between the two classes of bridges. Of course, the interest charge is to be added in each case. In other words, 3 per cent of the cost of the concrete structure should be compared with 7 to 11 per cent of the cost of a competing steel structure, and that type should be selected which will give the lower result.
Or, as a special example, if the lowest responsible bid for a concrete arch is $12,000, the yearly cost to the community will probably be 4 per cent (assumed interest charge), plus 3 per cent for depreciation and mainte nance. Since the yearly costs for a steel structure will amount to from 11 to 15 per cent total (4 per cent as sumed as above for interest) for the steel superstruc ture, and slightly less for the abutments, then, unless the steel bridge complete can be erected for from $6,000 to $8,000, its maintenance and interest charges will be greater than those of a concrete structure.
In order to determine the most economical type of structures for short-span railway bridges, the costs of four different designs were computed for span lengths of from 10 feet to 35 feet. The four types of structures which were estimated are as follows: A—I beam span with standard open wooden floor. B-1—I beams imbedded in concrete and ballast floor.
B-2—Concrete ballast floor supported on I beams. C—Reinforced concrete slab with ballast floor.
The spans were figured for the total dead loads and a Cooper's E-50 live load. The unit allowed stresses in the steel beams were 8,000 lbs. per square inch for live load, and 16,000 lbs. per square inch for dead load. The allowed unit-stresses in the concrete slabs were 750 lbs. per square inch in the concrete, and 13,500 lbs. per square inch in the steel, for dead load; and 500 lbs. per square inch in the concrete, and 9,000 lbs. per square inch in the steel, for live load.
The steel beams were estimated to cost cents per pound; the reinforcing steel, 3 cents per pound; the concrete, 35 cents per cubic foot, or $9.50 a cubic yard. The ballast and floor were estimated at $1.56 per linear foot, and the wooden floor at $3.60 per linear foot.
The costs for each of the four designs for the various spans were plotted as shown in Fig. 82 The spans are laid off on one axis; and the costs in dollars for the structure, exclusive of abut ments, are laid off on the other axis. Fig. 82 shows also the cross-sections of the four types considered. An inspection of the curves will show the following: 1. The I-beam spans with the open wooden floor are the cheapest.