In 1956 the federal aid highway legislation was issued applying the AASHO 1946 policy (18,000 pounds single axle limits, tandem axle limits of 32,000 pounds and gross vehicle weight of 73,280 pounds)
Also a first clause was applied which allowed for the operation of trucks with maximum limits that were legal before the 1956 changes.
In order to save bridges from too heavy loads, in 1975 the Congress published truck weight regulations limiting the gross weight of trucks to 80,000 pounds with the single-axle weight now 20,000 and the tandem-axle weight now 34,000 pounds.
Although the weight of a truck is important, the length of the truck is also very important. Under a constant load, a shorter truck will cause more deterioration and damage to bridges due to load concentration.
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In Figure 2.1A, the stress on bridge members as a short truck is much more than that caused by a longer truck as shown in Figure 2.1B, even though both trucks have the same axle weight and total weight. The weight of the shorter truck is concentrated on a small area, while the weight of the longer truck is spread