Usage
The formula was enacted as law to limit the weight-to-length ratio of a commercial motor vehicle (CMV). The formula is necessary to prevent the concentrated weight on a truck's axle from producing stress on bridge members (possibly causing a bridge collapse). In simplified form, this is analogous to a person walking on thin ice. When standing upright, a person's weight is concentrated at the bottom of their feet, funneling all of their weight into a small area. When lying down, a person's weight is distributed over a much larger area. This difference in weight distribution would allow a person to cross an area of ice while crawling that might otherwise collapse under their body weight while standing up. For an overweight truck to comply with the formula, more axles must be added, the distance between axles must be increased, or weight must be removed.
A division of the DOT, the Federal Motor Carrier Safety Administration (FMCSA), regulates safety for the U.S. trucking industry. The FMCSA regulates the length, width, and weight limits of CMVs for interstate commercial traffic. Interstate commercial traffic is generally limited to a network of Interstate Highways, U.S. highways, and state highways known as the National Network (NN). Provided the truck remains on the NN, it is not subject to state limits. These limits (which can be lower or higher than federal limits) come into effect for intrastate commercial traffic, provided the vehicle is not on the NN.
CMVs are defined by the FMCSA as vehicles engaged in interstate commerce that are used to transport passengers or property: vehicles with a gross vehicle weight of 10,001 pounds (4,536 kg) or more; those designed or used to transport more than 8 passengers (including the driver) for compensation; vehicles designed or used to transport more than 15 passengers (including the driver) without compensation; or those used to transport hazardous materials in quantities requiring the vehicle to be marked or placarded under hazardous materials regulations.
The weight and size of CMVs are restricted for practical and safety reasons. CMVs are restricted by gross weight (total weight of vehicle and cargo), and by axle weight (i.e., the weight carried by each tire). The federal weight limits for CMVs are 80,000 pounds (36,000 kg) for gross weight (unless the bridge formula dictates a lower limit), 34,000 pounds (15,000 kg) for a tandem axle, and 20,000 pounds (9,100 kg) for a single axle. A tandem axle is defined as two or more consecutive axles whose centers are spaced more than 40 inches (100 cm) but not more than 96 inches (240 cm) apart. Axles spaced less than 40 inches (100 cm) apart are considered a single axle.
In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles (see table below). For example, a 25-foot (7.6 m) three-axle dump truck would have a gross weight limit of 54,500 pounds (24,700 kg), instead of 80,000 pounds (36,000 kg), which is the standard weight limit for 63-foot (19 m) five-axle tractor-trailer. FMCSA regulation ยง658.17 states: "The maximum gross vehicle weight shall be 80,000 pounds except where lower gross vehicle weight is dictated by the bridge formula."
Read more about this topic: Federal Bridge Gross Weight Formula
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