Until 1982, federal law set only upper limits (or ceilings) for the weight limits of the intergovernmental system. Some states maintained significantly lower weight limits, which eventually became a barrier to long-distance truck traffic. In 1982, federal law was amended to establish weight limits for the interstate system, including bridge formula limits, both the maximum and minimum weight (i.e. floors and ceilings) that states must allow on the interstate system. The 1974 federal highway amendments increased the weights allowed on the interstate system to 20,000 lbs on a single axle, 34,000 lbs on a tandem axle, and 80,000 pounds gross weight (23 U.S.C. 127). But Congress balanced this productivity concession by adopting the bridge formula. Consequently, motor vehicles may be laden to the maximum weight only if each group of axles of the vehicle and its distance also meet the requirements of the formulae. This prevents the vehicle from overloading the bridges in the same way that a person lying on thin ice would minimize the risk of breakthrough. The weight limits of the intergovernmental system in some States may be higher than these figures because of acquired rights.
When the total weight and axle limits of the interstate system were adopted in 1956, states were allowed to keep those or « grandfathers » that were higher. In 1975, states were also allowed to maintain « grandfather » bridge limits that were higher than the limits set for the interstate system. The central distance is just as important in the construction of the bridge as the weight of the axle. A bridge is analogous to thin ice on a pond. Walking on ice concentrates a person`s weight on the small area covered by the individual`s feet, and the ice can break. However, lying down distributes the same weight over a much larger area and the ice breaks less often. Consider the example of trucks crossing a bridge: In Figure 1(A), the load on the bridge elements when they are driving on the longer truck is much lower than that of the short vehicle in Figure 1(B), 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 concentrates all the weight in a small area. In addition to the weight limits of the bridge formula, federal law states that the gross weight of the vehicle is capped at 80,000 pounds. State law requires county officials to provide TxDOT with technical analysis if a county`s road load zone or bridge boundaries change. This analysis must be signed by an engineer. The bridge formula shall be calculated by multiplying the distance in feet between the outer axes of any group of two or more consecutive axes by the number of axes of the group considered.
This number is then divided by the number of axes of the group considered minus one. After obtaining this quotient, add 36 and the product of 12 times the number of axes of the group considered to the quotient. This final number is then multiplied by 500 to bring the total gross weight on a group of two or more consecutive axles to the next 500 pounds. This formula limits the weight of axle groups to reduce the risk of damage to highway bridges. The permissible weight depends on the number of axles in a vehicle and the distance between those axles. However, the weight limits for single or tandem axles replace the limits of the bridge formula for all axles up to 96 inches apart. Bridges on highways in the interstate system are used by a variety of traffic. They are designed to withstand the intended loads. However, as trucks became heavier in the 1950s and 1960s, something had to be done to protect the bridges.
The solution was to link the permissible weights to the number and distance of the axles. 2The following laden vehicles shall not be driven on bridges H15-44: 3-S2 (5 axles) with a wheelbase of less than 38 feet; 2-S1-2 (5 axles) with wheelbase less than 45 feet; 3-3 (6 axles) with wheelbase less than 45 feet; and 7-, 8- and 9-axle vehicles, regardless of wheelbase. Centre distance is another consideration that must be taken into account when maintaining the federal weight. To protect the bridges, it is necessary to calculate the number and distance of the axles that support the load of the vehicle. For example, a bridge weight formula is also applied to commercial vehicles to determine their compliance with federal weight limits. The federal bridge formula applies when the gross weight on two or more consecutive axles exceeds the constraints of the formula, except that two successive sets of tandem axles can support a gross load of 34,000 pounds each if the total distance between the first and last axles is 36 ft or more. In 1975, the United States Congress adopted the bridge formula to limit the weight-to-length ratio of a vehicle crossing a bridge. This is done by increasing the centre distance or distributing the weight on additional axles.
The reason for this formula is that in the 1950s and 1960s, trucks became heavier and began to put too much pressure on bridges on highways. Federal law states that two or more consecutive axles cannot exceed the weight calculated according to the formula, although single axles, tandem axles and gross weight are within the legal limits. In other words, the axle group comprising the entire truck – sometimes referred to as the « outer deck group » – must conform to the bridge formula. However, the internal combinations of axles, 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 formula (Figure 3). The method described above can be used to check any combination of axes, but multiple axes close together usually create the most critical situation. Single axle weight – The total weight of one or more axles with centres not more than 40 inches apart. The federal uniaxial weight limit on the interstate system is 20,000 lbs. This is a violation because the actual weight exceeds the weight allowed by the Bridge formula. To correct the situation, a certain load must be removed from the vehicle or the center distance (35 feet) increased. Some definitions are necessary to use the Bridge formula correctly.