The central distance is just as important in bridge construction as the weight of the axle. In Figure 1A, the load on the bridge girders when rolling on a longer truck is much lower than on a short vehicle, as shown in Figure 1B, even though both trucks have the same total weight and individual axle loads. The weight of the longest vehicle is distributed, while the shortest vehicle is concentrated in a smaller area. The collapse of the Interstate 35W bridge over the Mississippi River in Minneapolis in August 2007 once again drew attention to the problem of truck weight and its relationship to bridge loading. [12] In November 2008, the National Transportation Safety Board determined that there were several reasons for the bridge collapse, including (but not limited to): defective gussets, inadequate inspections, and the additional weight of heavy construction equipment combined with the weight of rush hour traffic. [13] The I-35 Trade Corridors Study found that the Federal Highway Administration (FHWA) expressed concern about bridges on the I-35 corridor due to an expected increase in international truck traffic from Canada and Mexico, with the FHWA classifying them as “high priority” in 2005. [14] In fact, 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 a 63-foot (19.2 m) five-axle tractor. [1] FHWA Order § 658.17 states: “The maximum permissible gross laden weight is 80,000 pounds (36,000 kg), unless the lower permissible gross laden weight is prescribed by the bridge formula.” [9] The first weight restrictions on trucks were enacted by four states in 1913, ranging from 18,000 pounds (8,200 kg) in Maine to 28,000 pounds (13,000 kg) in Massachusetts. These laws were enacted to protect dirt and gravel roads from damage caused by the steel and solid rubber wheels of old heavy trucks. In 1933, all states had some form of truck weight regulation. The Federal-Aid Highway Act of 1956 introduced the first federal truck weight regulations (set at 73,280 pounds or 33,240 kilograms) and authorized the construction of the interstate highway system. [2] This exemption allows the standard configuration of a 5-axle tractor-trailer to reach up to 36,000 kg (80,000 pounds) gross (the legal limit)[9] without violating the Transition Formula Act.

Without it, the bridge formula would allow for 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 (11.6 m) apart; compared to 68,000 pounds (31,000 kg), with the exception. This exemption was requested by the American Trucking Associations to allow trucking companies to operate 40-foot (12.2 m) trailers weighing 80,000 pounds (36,000 kg). This was the only way for tanker operators to reach £80,000 without adding axles to their already in service trailer fleets. [26] The weights of the Federal Highway Administration and the Federal Bridge Formula determine the correct bridge weight formula, that is, the weight a vehicle should have for a particular bridge. The law states that the weight of a vehicle depends on the number of axles and the distance between the axles. Each state has its own regulations for weights on roads and bridges. In most cases, states have adopted the USDOT standards for interstate highways mentioned above, otherwise they could lose funds. However, there are some exceptions. Information and links to each state`s rules can be found on these two websites: Federal Highway Administration and Ovesize.io.

Ultimately, truck and fleet owners need to make sure their trucks comply with the laws of the states they visit. You can ensure compliance by ensuring that your weight does not exceed the limits of the federal or state formula. The formula was enacted as law to limit the weight-to-length ratio of a commercial vehicle (CMV). [4] The formula is necessary to prevent the axles of the concentrated truck from overloading the roadways and bridge components (which could lead to bridge collapse). [5] In simplified terms, this is analogous to a person walking on thin ice. When standing, a person`s weight is concentrated on the underside of their feet, directing all their weight into a small area. In the supine position, a person`s weight is distributed over a much larger area. This difference in weight distribution would allow a person to crawl through an ice surface that might otherwise collapse under their body weight while standing. In order for an overweight truck to meet the formula, additional axles must be added, the distance between the axles increased or the weight removed.

[1] [6] Often, the shipper or consignee is aware of the limitations associated with road-facility bridges. When in doubt, ask. If you`re going to a new area, ask anyway. To understand the bridge formula, you must first understand the terms. Federal law stipulates that two or more consecutive axles may not exceed the weight calculated by the bridge formula, although individual axles, tandem axles and total weight are within the legal limits. Therefore, the axle group comprising the entire truck – sometimes referred to as the “outer deck group” – must conform to the bridge formula. However, the internal axle combinations, such as the “towing axle” (axles 1, 2 and 3) and the “trailer axle” (axles 2, 3, 4 and 5), must also correspond to the weights calculated by the bridge formula (Figure 3).