Weight laws and federal bridge formula: Determining vehicle maximum carrying capacity

From an NTEA member We are a truck equipment distributor. Our medium- and heavy-duty truck customers frequently ask how much load they can carry. We would like to know how federal bridge law regulations can affect maximum payload on a truck chassis. Complex weight laws in both federal highway and state roads systems can impact how much a truck can carry. Many customers assume the gross vehicle weight rating (GVWR) is the amount of load they can carry, but state and federal weight laws limit a vehicle’s maximum amount of weight on the roadway based on number of axles and their placement. Putting this in perspective, the maximum amount of payload weight a truck can carry is set by a group of limitations. First and foremost, chassis ratings — GVWR and gross axle weight rating (GAWR) — must be taken into account before bridge law considerations, where the difference between vehicle curb weight and chassis GVWR sets the upper limit of the potential payload a truck can carry. From this upper limit, GAWR must not be exceeded when the vehicle is loaded to its GVWR. If a GAWR is exceeded, payload must be reduced until all axle loads are within ratings. When axle weight ratings are met, associated payload is used as part of a weight distribution for bridge law purposes. Bridge law requirements may reduce the amount of payload the vehicle can carry in order to remain within roadway limitations. Compliance with bridge formula weight limits used for federal roads and many state roads is determined using the following formula. W = overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds L = distance in feet between the outer axles of any group of two or more consecutive axles N = number of axles in the group under consideration How is the formula used? Various axle configuration weights must be checked to determine Bridge Formula compliance. Three definitions, as cited in NTEA’s Truck Equipment Handbook, must be understood for correct Bridge Formula use. Gross weight — Weight of a vehicle or vehicle combination and any load thereon. The federal gross weight limit on the Interstate System is 80,000 pounds unless the Bridge Formula dictates a lower weight limit. Single-axle weight — Total weight on one or more axles whose centers are spaced not more than 40 inches apart. The federal single-axle weight limit on the Interstate System is 20,000 pounds. Tandem-axle weight — Total weight on two or more consecutive axles whose centers are spaced more than 40 inches apart but not more than 96 inches apart. The federal tandem-axle weight limit on the Interstate System is 34,000 pounds. Bridge formula calculations yield a series of weights. Note, the single-axle weight limit replaces the bridge formula weight limit on axles not more than 40 inches apart, and the tandem-axle weight limit replaces the bridge formula weight limit for axles over 40 but not more than 96 inches apart. At 97 inches apart, for example, two axles may carry 38,000 pounds and three axles may carry 42,000 pounds (see Figure 1, as cited in NTEA’s Truck Equipment Handbook). Federal law states any two or more consecutive axles may not exceed the weight computed by the Bridge Formula, even though single axles, tandem axles and gross weight are within legal limits. As a result, the axle group that includes the entire truck — sometimes called the outer bridge group — must comply with the Bridge Formula. However, interior combinations of axles, such as the tractor bridge (axles 1, 2 and 3) and trailer bridge (axles 2, 3, 4 and 5) in Figure 1 (A), must also comply with weights computed by the Bridge Formula. Axle combinations 1–3, 1–5 and 2–5 are the most critical and must be checked. Typically, if these combinations are satisfactory, the remaining combinations are as well. Application to single-unit trucks The bridge formula table is a quick method many companies use to find values for the inner or outer bridge of a given vehicle. In the single-unit truck example in Figure 2, axles 2–4 have a total distance of 9 feet for the three-axle group (known as the inner bridge in this example). Using the table in Figure 3 for 9 feet and three axles, a maximum weight of 42,500 pounds can be carried on axles 2–5. Total vehicle carrying capacity is determined by the outer bridge (distance from axle 1–4), which is 23 feet in this example. Looking in the four-axle column at 23 feet in the Bridge Formula table, a maximum of 57,500 pounds can be carried on the total vehicle, which includes the weight of the truck chassis, truck body, payload, passengers, fuels and all accessories. This total carrying capacity under the Bridge Formula table will need to be checked against vehicle GVWR and GAWR of the individual axles, as previously noted. NTEA’s Vehicle Center of Gravity and Axle Weight Calculator automatically checks for Federal Bridge Formula conformity and highlights any conditions that need review, including axle overloading, exceeding GVWR and axle spacing. It’s a helpful tool for both weight distribution and Bridge Formula checks.

Weight laws and federal bridge formula: Determining vehicle maximum carrying capacity

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