Exception
There is one exception to the formula: two consecutive sets of tandem axles may carry 34,000 pounds (15,000 kg) each if the overall distance between the first and last axles of these tandems is 36 feet (11 m) or more. For example, a five-axle truck may carry 34,000 pounds both on the tractor tandem axles (2 and 3) and the trailer tandem axles (4 and 5), provided axles 2 and 5 are spaced at least 36 feet (11 m) apart.
This exception allows for the standard 5-axle semi-truck configuration to gross up to 80,000 pounds (36,000 kg) (the legal limit) without being in violation of the bridge formula law. Without it, the bridge formula would allow an actual weight of only 66,000 pounds (30,000 kg) to 67,500 pounds (30,600 kg) on tandems spaced 36 feet (11 m) to 38 feet (12 m) apart; compared to 68,000 pounds (31,000 kg) with the exception. This exception was sought by the American Trucking Associations so trucking companies could use 40-foot (12 m) trailers and weigh 80,000 pounds (36,000 kg). It was the only way tank truck operators could reach 80,000 pounds without adding axles to their fleets of trailers already in operation.
A CMV may exceed the bridge formula limits (or gross weight and axle weight limits) by up to 400 pounds (180 kg) if the vehicle is equipped with an auxiliary power unit (APU) or idle reduction technology. According to the FMCSA, this is permitted "in order to promote reduction of fuel use and emissions because of engine idling". To be eligible, the vehicle's operator must prove the weight of the APU with written certification, or—by demonstration or certification—that the idle reduction technology is fully functional at all times. Certification of the APU's weight must be available to law enforcement officers if the vehicle is found in violation of applicable weight laws. The additional weight allowed cannot exceed 400 pounds or the weight certified, whichever is less.
Read more about this topic: Federal Bridge Gross Weight Formula
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