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112 lengths which would be less onerous in their demands upon the bridges despite the fact that their gross weights might materially exceed 40,000 pounds.

Interstate system design loading

These are the reasons that underlie the recommendation of the American Association of State Highway Officials which proposes to limit the gross weight of vehicles by the limitation of constituent group axle loads conforming to a table of weights corresponding to a range of axle group spacings. The values in the table have been calculated in such amounts as to permit the operation of vehicles of various lengths and corresponding weights, all safe in respect to the capacity of H15 bridges.

Bridges of H15 design now so greatly predominate among existing bridges, on even the principal highways, that no higher limits of vehicle gross weight can be reasonably permitted. The design standard proposed for bridges on the National System of Interstate Highways is H20-S16. Bridges of this standard are designed to support loading equivalent to that of a tractor-semitrailer combination of 28-foot over-all wheel base, weighing 72,000 pounds. Such a bridge would be equally safe for a combination vehicle weighing 100,000 pounds if its over-all wheel base length were 54 feet and the constituent axles were appropriately loaded. The statement that it would be equally safe means that it would support such a vehicle without stress in excess of the design assumption, which involves an ample factor of safety to allow for uncertainties of material and construction and for the effects of fatigue induced by repetition of stress. In war or other emergency such H20-S16 bridges will support without danger the infrequent application of vehicles of appropriate length and weight distribution grossing from 160,000 to 200,000 pounds, depending upon the vehicle wheel base, the length of bridge span, and the number of load repetitions.

PROTECTION OF ROADS AND BRIDGES AGAINST OVERLOADING

For reasons stated in the preceding pages it is proposed to build road foundations and surfaces on the interstate highway system for the support of 18,000-pound axle loads, and to build bridges of H20-S16 design. Costs of improving the system have been estimated for roads and bridges conforming to these standards.

If heavier loads are to be permitted, the contemplated design of roads and bridges should be strengthened and the estimated costs should be correspondingly increased. The roads and bridges actually built should be designed for the safe support of loads to be permitted. When so built, they must be protected against overloading by appropriate laws adequately enforced. Unnecessary and costly damage will result from failure to heed this injunction.