Weight laws and the federal bridge formula: How is payload affected?

From an NTEA member We are a truck equipment distributor, and our medium- and heavy-duty customers are always asking how much weight their trucks can carry. We would like to know how federal bridge law regulations can affect the maximum payload on a truck chassis. These complex weight laws can impact how much a truck is able to carry. Many customers assume gross vehicle weight rating (GVWR) is the maximum payload, but state and federal weight laws limit the weight a vehicle can carry based on number and placement of axles. First and foremost, chassis ratings (GVWR and gross axle weight rating, or GAWR) must be taken into account prior to 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 limit, GAWR must not be exceeded when the vehicle is loaded to its GVWR. If so, reduce payload until axle loads are within their ratings. When these limitations have been addressed, the amount of payload that can be used to meet these criteria is considered part of a weight distribution for bridge law purposes, which may further reduce the amount of payload a vehicle can carry to remain within roadway restrictions. Bridge formula sample Compliance with bridge law weight limits is determined with the following formula: W = the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds. L = the distance in feet between the outer axles of any group of two or more consecutive axles. N = the number of axles in the group under consideration. How is the bridge formula used? The weight on various axle configurations must be checked to determine compliance with the bridge formula, and the following definitions are needed to aid in its proper use: Gross Weight — the 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 — the 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 — the 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 inches but not more than 96 inches apart. At 97 inches apart, for example, two axles may carry 38,000 pounds (see A in Figure 1) and three axles may carry 42,000 pounds (see B in Figure 1). Federal law states that 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. Interior combinations of axles, such as the “tractor bridge” (axles 1, 2 and 3) and “trailer bridge” (axles 2, 3, 4 and 5), must also comply with weights computed by the bridge formula. Axle combinations 1-3, 1-5 and 2-5 are the most critical and should be checked. If these combinations are satisfactory, those remaining should be as well. Before determining bridge formula compliance, check the vehicle’s single-axle, tandem-axle and gross weight. The truck shown in Figure 2 satisfies the single-axle weight limit (12,000 pounds is less than 20,000 pounds), the tandem-axle limit (30,000 pounds is less than 34,000 pounds) and the gross weight limit (57,000 pounds is less than 80,000 pounds). With these restrictions met, a check is performed for bridge formula requirements for axles 1–4: Actual weight = 12,000 + 15,000 + 15,000 + 15,000 = 57,000 pounds Maximum weight (W) = 57,500 pounds (bridge table for “L” of 23 feet and “N” of 4 axles) Since axles 1–4 are satisfactory, check axles 2–4: Actual weight = 15,000 + 15,000 + 15,000 = 45,000 pounds Maximum weight (W) = 42,500 pounds (bridge table for “L” of 9' and “N” of 3 axles) This is a violation, because the actual weight exceeds the weight allowed by the bridge formula. You must either reduce the load, add axles or increase space to comply with the bridge formula.

Weight laws and the federal bridge formula: How is payload affected?

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