Federal Bridge Gross Weight Formula


The Federal Bridge Gross Weight Formula, also known as Bridge Formula B or the Federal Bridge Formula, is a mathematical formula in use in the United States by truck drivers and Department of Transportation officials to determine the appropriate maximum gross weight for a commercial motor vehicle based on axle number and spacing. The formula is part of federal weight and size regulations regarding interstate commercial traffic. The formula is necessary to prevent heavy vehicles from damaging roads and bridges. CMVs are most often tractor-trailers or buses, but the formula is of most interest to truck drivers due to the heavy loads their vehicles often carry.
Early 20th-century weight limits were enacted to protect dirt and gravel roads from damage caused by the solid wheels of heavy trucks. As time passed, truck weight limits focused primarily on gross weight limits. By 1974, bridges received special protection from increasing truck weight limits. The bridge formula law was enacted by the U.S. Congress to limit the weight-to-length ratio of heavy trucks, and to protect roads and bridges from the damage caused by the concentrated weight of shorter trucks. The formula effectively lowers the legal weight limit for shorter trucks, preventing them from causing premature deterioration of bridges and highway infrastructure.
Compliance with the law is checked when vehicles pass through a weigh station, often located at the borders between states or on the outskirts of major cities, where the vehicle may be weighed and measured. The one exception to the formula allows a standard five-axle semi-truck configuration to weigh the maximum legal gross weight. This exception was specifically requested by the American Trucking Associations to allow tank trucks to reach the maximum legal gross weight without violating the bridge formula law.

History

The first truck weight limits were enacted by four states in 1913, ranging from in Maine to in Massachusetts. These laws were passed to protect earth and gravel-surfaced roads from damage caused by the steel and solid rubber wheels of early heavy trucks. By 1933, all states had some form of truck weight regulation. The Federal-Aid Highway Act of 1956 instituted the first federal truck weight regulation and authorized the construction of the Interstate Highway System.
In the late 1950s, the American Association of State Highway and Transportation Officials conducted a series of extensive field tests of roads and bridges to determine how traffic contributed to the deterioration of pavement materials. In 1964, the AASHTO recommended to Congress that a bridge formula table be used instead of a single gross weight limit for trucks. The Federal-Aid Highway Act Amendments of 1974 established the bridge formula as law, along with the gross weight limit of. Current applications of the formula allow for up to 7 axles and 86 feet or more length between axle sets, and a maximum load of 105,500 lbs.

Usage

The formula was enacted as law to limit the weight-to-length ratio of a commercial motor vehicle. The formula is necessary to prevent the concentrated weight on a truck's axle from producing stress on bridge members. In simplified form, this is analogous to a person walking on thin ice. When standing upright, a person's weight is concentrated at the bottom of their feet, funneling all of their weight into a small area. When lying down, a person's weight is distributed over a much larger area. This difference in weight distribution would allow a person to cross an area of ice while crawling that might otherwise collapse under their body weight while standing up. For an overweight truck to comply with the formula, more axles must be added, the distance between axles must be increased, or weight must be removed.
A division of the DOT, the Federal Motor Carrier Safety Administration, regulates safety for the U.S. trucking industry. The FMCSA enforces the length, width, and weight limits of CMVs set by the Federal Highway Administration for interstate commercial traffic. Interstate commercial traffic is generally limited to a network of Interstate Highways, U.S. Highways, and state highways known as the National Network. Provided the truck remains on the NN, it is not subject to state limits. These limits come into effect for intrastate commercial traffic, provided the vehicle is not on the NN.
CMVs are defined by the FMCSA as vehicles engaged in interstate commerce that are used to transport passengers or property: vehicles with a gross vehicle weight of or more; those designed or used to transport more than 8 passengers for compensation; vehicles designed or used to transport more than 15 passengers without compensation; or those used to transport hazardous materials in quantities requiring the vehicle to be marked or placarded under hazardous materials regulations.
The weight and size of CMVs are restricted for practical and safety reasons. CMVs are restricted by gross weight, and by axle weight. The federal weight limits for CMVs are for gross weight, for a tandem axle, and for a single axle. A tandem axle is defined as two or more consecutive axles whose centers are spaced more than but not more than apart. Axles spaced less than apart are considered a single axle.
In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles. For example, a three-axle dump truck would have a gross weight limit of, instead of, which is the standard weight limit for five-axle tractor-trailer. FHWA regulation §658.17 states: "The maximum gross vehicle weight shall be except where lower gross vehicle weight is dictated by the bridge formula."

Bridge collapse

The August 2007 collapse of the Interstate 35W Mississippi River bridge in Minneapolis brought renewed attention to the issue of truck weights and their relation to bridge stress. In November 2008, the National Transportation Safety Board determined there had been several reasons for the bridge's collapse, including : faulty gusset plates, inadequate inspections, and the extra weight of heavy construction equipment combined with the weight of rush hour traffic. The I-35 Trade Corridor Study reported that the Federal Highway Administration expressed concern over bridges on the I-35 corridor due to an expected increase of international truck traffic from Canada and Mexico, with the FHWA listing it as "high-priority" in 2005.
As of 2007, federal estimates suggest truck traffic increased 216% since 1970, shortly before the federal gross weight limit for trucks was increased by. This is also the period during which many of the existing interstate bridges were built. Research shows that increased truck traffic shortens the life of bridges. National Pavement Cost Model estimates indicate that one truck does as much damage to roads as 750 cars.
Some smaller bridges have a weight limit indicated by a posted sign. These are necessary when the weight limit of the bridge is lower than the federal or state gross weight limit for trucks. Driving a truck over a bridge that is too weak to support it usually does not result in an immediate collapse. The bridge may develop cracks, which over time can weaken the bridge and cause it to collapse. Most of these cracks are discovered during mandated inspections of bridges. Most bridge collapses occur in rural areas, result in few injuries or deaths, and receive relatively little media attention. While the number varies from year to year, as many as 150 bridges can collapse in a year. About 1,500 bridges collapsed between 1966 and 2007, and most of those were the result of soil erosion around bridge supports. In 1987, the Schoharie Creek Bridge collapsed in upstate New York, due to erosion of soil around the foundation, which sparked renewed interest in bridge design in inspection procedures.
In special cases involving unusually overweight trucks, not observing a bridge weight limit can lead to disastrous consequences. Fifteen days after the collapse of the Minneapolis bridge, a heavy truck collapsed a small bridge in Oakville, Washington.

Formula law

CMVs are required to pass through weigh stations at the borders of most states and some large cities. These weigh stations are run by state DOTs, and CMV weight and size enforcement is overseen by the FHWA. Weigh stations check each vehicle's gross weight and axle weight using a set of in-ground truck scales, and are usually where a truck's compliance with the formula is checked.
FMCSA regulation §658.17 states:
Two or more consecutive axles may not exceed the weight computed by the bridge formula, even if the gross weight of the truck is below otherwise legal limits. Although this means that any two axles must comply with the formula, experience has shown that axles 1 through 3, 1 through 5, and 2 through 5 are critical and must be checked. This means that the axle group which comprises the entire truck and the interior axle groups must also comply with the bridge formula. If these combinations are found to be satisfactory, then all of the other axle groups on this type of vehicle will usually be satisfactory.
Penalties for violating weight limits vary between states, as the states are responsible for enforcement and collection of fines. Some states, such as Connecticut, issue fines on a percentage basis, which means larger trucks pay higher fines. For example, a truck with a legal gross limit of that violates the limit by would pay a fine of $500, while a truck with a legal gross limit of that violates the limit by 5,000 pounds would pay a fine of $250. Other states, such as New York, issue fines on a per-pound basis. Others, such as Massachusetts, impose a less complicated fine schedule whereby a vehicle that violates the limits by less than is fined $40 per, while a violation over pays $80 per .
Some states require overweight trucks to offload enough cargo to comply with the limits. In Florida, any vehicle that exceeds the limits by more than is required to be unloaded until the vehicle is in compliance. Florida also includes a scale tolerance, which allows for violations of less than 10% to be forgiven, and no fine issued. Florida also allows for a load to be shifted for the vehicle to comply with axle weight limits, without penalty.

Exception

There is one exception to the formula: two consecutive sets of tandem axles may carry each if the overall distance between the first and last axles of these tandems is or more. For example, a five-axle truck may carry 34,000 pounds both on the tractor tandem axles and the trailer tandem axles, provided axles 2 and 5 are spaced at least apart.
This exception allows for the standard 5-axle semi-truck configuration to gross up to without being in violation of the bridge formula law. Without it, the bridge formula would allow an actual weight of only to on tandems spaced to apart; compared to with the exception. This exception was sought by the American Trucking Associations so trucking companies could use trailers and weigh. 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 by up to if the vehicle is equipped with an auxiliary power unit 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 550 pounds or the weight certified, whichever is less.

Issues

The bridge formula is based on research into single-span bridges, and fails to consider multiple-span bridges. Two-span bridges may not be fully protected by Formula B, depending on the truck length, span length, and other factors. Shorter wheelbase vehicles have trouble complying with Formula B.
In 1987, the U.S. Congress passed the Surface Transportation and Uniform Relocation Assistance Act, requesting the Transportation Research Board to conduct a study to develop alternatives to Formula B. The study recommended several that were never implemented. It suggested that Formula B was too strict for trucks with shorter axle lengths. One of the alternative formulas was developed in conjunction with the Texas Transportation Institute. TTI HS-20 allowed shorter trucks to have higher weight limits than Formula B. For a 3-axle truck with an axle length of, the weight limit increased from to. TTI HS-20 also failed to address the problem of multiple-span bridges.