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110 weighed during 1947 in two States in which different transportation practices have developed in the presence of materially different laws:

It is apparent that in California, where the law prescribes an axle load limit of 18,000 pounds, a maximum gross vehicle weight of 76,800 pounds, and a maximum length of 60 feet, the average gross vehicle weight of vehicles in use is greater, while the average axle load is less than the corresponding weights of vehicles in use in New Jersey where the law prescribes no limit of axle load, a maximum gross vehicle weight of 60,000 pounds, and a maximum length of 50 feet.

Since an axle-load limit of 18,000 pounds is now, and for years to come will be, essential for the protection of existing roads; since a large mass of evidence exemplified by the above comparison indicates clearly that such an axle-load limitation need not unduly limit either the gross weight or pay loads of vehicles; and since the necessities of efficient design and administration of the highway system require fixation of axle loading, the conclusion is strongly supported that 18,000 pounds should be adopted as the maximum axle load permis- sible under the laws of all States, that this limit without future in- crease should be rigidly enforced, and that highways built in the future should be designed for the normal support of axle loads of that magni- tude in the frequency of their probable occurrence.

AXLE-LOAD COMBINATIONS AND SPACINGS DETERMINE REQUIRED BRIDGE DESIGN

Unlike roads, bridges are affected by the gross weight of vehicles as well as by the load on each axle. The effect of gross weight, however, is not that of a weight applied at a single point, but rather that of a weight distributed over a significant length—the length of the over-all wheel base—and applied to the bridge within that length at points separated by the distances between axles. Some of the stresses generated in a bridge are responsive to separate axle loads; others are determined in their amount by the magnitude and spacing of all the axle loads applied by a vehicle. Stresses generated by single axle loads are critical mainly in the floor system of the bridge; stresses generated by groups of axle loads are critical in the larger supporting fabric, such as the trusses or main girders. In the latter parts, vehicles of widely different total weight may generate identical stresses if the lengths and axle spacings of the vehicles differ appropriately in relation with their weights.

Distribution of weight important

So, for the design of a bridge it is not sufficient to know the maxi- mum gross weight of the vehicles that are expected to use it. Some assumption must also be made of the manner in which the gross weight will be distributed and applied through axles of definite spacing. Similarly, whereas an existing bridge may be adequately protected