Federal Bridge Gross Weight Formula

Federal Bridge Gross Weight Formula

wheelbase trucks. The shorter truck is damaging the bridge because all of its weight is concentrated in a smaller area.] The Federal Bridge Gross Weight Formula is a mathematical formula used by truck drivers and United States Department of Transportation (DOT) officials to determine the appropriate maximum gross weight for a commercial motor vehicle (CMV) based on axle spacing. This is necessary to prevent damage to roads and bridges. CMVs are most often semi-trucks or buses, but the formula is of most interest to truck drivers due to the heavy loads their vehicles often carry.

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. There is one exception to the formula which allows the common five-axle semi-truck configuration to weigh the maximum legal gross weight without violating the bridge formula law.


The first truck weight limits were enacted by four states in 1914, ranging from a low of convert|18000|lb in Maine to a high of convert|28000|lb in Massachusetts. These laws were enacted to protect earth and gravel-surfaced roads from damage caused by the iron and solid rubber wheels of early heavy trucks. By 1933, all states had some form of varying truck weight regulation. The Federal-Aid Highway Act of 1956 instituted the first federal truck weight regulation (set at convert|73280|lb|disp=/) and authorized the construction of the Interstate Highway System.cite web |url=http://www.fhwa.dot.gov/reports/tswstudy/Vol2-Chapter2.pdf |title=Chapter 2 - Truck Size and Weight Limits |work= [http://www.fhwa.dot.gov/reports/tswstudy/TSWfinal.htm Comprehensive Truck Size and Weight Study] - Volume 2: Issues and Background |format=PDF |accessdate=2008-05-03 |publisher=Federal Highway Administration ]

In the late 1950s, the American Association of State Highway and Transportation Officials (AASHTO) 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 gross weight limits still in use today.


The formula was enacted as law to limit the weight-to-length ratio of a CMV.cite web |url=http://ops.fhwa.dot.gov/freight/sw/brdgcalc/calc_page.htm |title=Bridge Formula Weights Calculator |accessdate=2008-02-03 |publisher=U.S. Department of Transportation - Federal Highway Administration ] The formula is necessary in order to prevent the concentrated weight on a truck's axle from producing stress on bridge members (possibly producing a bridge collapse). 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 lying down that might otherwise collapse under their body weight while standing up. In order for an overweight truck to comply with the formula, more axles must be added, or the length between axles must be increased.

A division of the DOT, the Federal Motor Carrier Safety Administration (FMCSA) regulates all safety-related aspects of the trucking industry. [cite web |url=http://www.fmcsa.dot.gov/about/what-we-do/strategy/strategy.htm |title=FMCSA's Strategy |accessdate=2008-02-03 |publisher=Federal Motor Carrier Safety Administration ] The FMCSA regulates the length, width, and weight limits of CMVs 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 (NN). Provided the truck remains on the NN, they are not subject to the state limits. State limits (which can be lower or higher than federal limits) come into effect for "intrastate" commercial traffic, provided the vehicle is "not" on the NN.)cite web |url=http://ops.fhwa.dot.gov/freight/publications/size_regs_final_rpt/size_regs_final_rpt.pdf |title=Federal Size Regulations for Commercial Motor Vehicles |accessdate=2008-03-27 |publisher=Federal Highway Administration |format=PDF ]

CMVs are defined by the FMCSA as vehicles engaged in interstate commerce, used to transport passengers or property; with a gross vehicle weight of convert|10001|lb or more; designed or used to transport more than 8 passengers (including the driver) for compensation; designed or used to transport more than 15 passengers (including the driver) without compensation; or is used to transport hazardous materials in quantities requiring the vehicle to be marked or placarded under hazardous materials regulations.cite web |url=http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?rule_toc=778&section=658.5&section_toc=2129 |title=§658.5 Definitions |accessdate=2008-03-27 |publisher=Federal Motor Carrier Safety Administration ]

The weight and size of CMVs are restricted for practical and safety reasons. CMVs are restricted by gross weight (total weight of vehicle and cargo), and by axle weight (i.e., the weight carried by each tire). The federal weight limits for CMVs are convert|80000|lb for gross weight (unless the bridge formula dictates a lower limit), convert|34000|lb for a tandem axle, and convert|20000|lb for a single axle. A tandem axle is defined as two or more consecutive axles whose centers are spaced more than convert|40|in|cm but not more than convert|96|in|cm apart. Axles spaced less than convert|40|in|cm apart are considered a single axle.

In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles (see table below). For example, a convert|25|ft|adj=on three-axle dump truck would have a gross weight limit of convert|54500|lb, instead of convert|80000|lb, which is the normal weight limit for a five-axle semi-truck.

Bridge collapse

The August 2007 catastrophic collapse of the Interstate 35W Mississippi River bridge in Minneapolis has brought renewed attention to the issue of truck weights and their relation to bridge stress.cite web |url=http://minnesota.publicradio.org/display/web/2007/08/10/bridgetruckweight/ |title=Heavy trucks strain roads and bridges |accessdate=2008-03-28 |publisher=Minnesota Public Radio ] As of January 2008, the National Transportation Safety Board has not yet determined the official reason for the bridge collapse, although design failures are blamed in initial assessments. [cite web |url=http://www.ntsb.gov/Pressrel/2008/080115.html |title=NTSB urges bridge owners to perform load capacity calculations before modifications; I-35W investigation continues|accessdate=2008-03-28 |publisher=National Transportation Safety Board ] Reports suggest that as early as 1998, the Federal Highway Administration (FHWA) expressed concern over bridges on the I-35 corridor due to an expected increase of international truck traffic from Canada and Mexico. [cite web |url=http://www.wnd.com/news/article.asp?ARTICLE_ID=57017 |title=NAFTA Superhighway traffic tied to bridge collapse |accessdate=2008-03-31 |publisher=WorldNetDaily.com ]

Federal estimates suggest truck traffic has increased over 200% since 1970, shortly before the federal gross weight limit for trucks was increased by convert|30000|lb. This is also the period from when many of today's interstate bridges were built. Research clearly shows that increased truck traffic (and therefore, increased stress) shortens the life of bridges. Estimates indicate that a single convert|80000|lb|adj=on truck does as much damage to roads as 750 convert|3800|lb|adj=on cars. [cite web |url=http://www.oregon.gov/ODOT/CS/EA/policy_notes/03_policy_notes/0603_New_Resrch_on_Pavement_Damage.pdf |title=New Research on Pavement Damage Factors |accessdate=2008-03-31 |publisher=Oregon Department of Transportation ]

Some smaller bridges have a weight limit (or gross weight load rating) usually indicated by a sign posted in front of the bridge and visible to anyone driving over the bridge. These are necessary when the weight limit of the bridge is lower than the federal or state gross weight limit for trucks. [cite web |url=http://www2.dot.state.fl.us/proceduraldocuments/procedures/bin/850010035.pdf |format=PDF |title=Bridge load rating, permitting and posting|accessdate=2008-03-28 |publisher=Florida Department of Transportation ] 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 caught during mandated inspections of bridges. Most bridge collapses occur in rural areas, result in few injuries or deaths, and receive relatively little media attention. As many as 150 bridges collapse each year, and most of those are the result of soil erosion around bridge supports. [cite web |url=http://query.nytimes.com/gst/fullpage.html?res=9B0DE6DA1F3EF931A25757C0A961948260&sec=&spon=&pagewanted=all |title=Bridge failures |accessdate=2008-03-28 |publisher=New York Times ]

In special cases involving unusually overweight trucks (which require special permits), not observing a bridge weight limit can lead to disastrous consequences. Just fifteen days after the collapse of the Minneapolis bridge, a heavy truck collapsed a small bridge in Oakville, Washington. It was determined that the truck was convert|145000|lb over the weight limit of the bridge. However, there was no indication that the truck was in violation of the bridge formula. [cite web |url=http://www.komotv.com/news/9187062.html |title=Truck collapses bridge in Grays Harbor County |accessdate=2008-03-31 |publisher=Fisher Communications ]

The formula

CMVs such as semi trucks are required to pass through weigh stations at the borders of most states and some large cities. These weigh stations are run by their respective state's DOT, 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 is normally where a truck's compliance with the formula is checked. [cite web |url=http://ops.fhwa.dot.gov/freight/sw/overview/index.htm |title=Commercial Vehicle Size and Weight Program |accessdate=2008-03-27 |publisher=Federal Highway Administration ] FMCSA regulation §658.17 states:cite web |url=http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?rule_toc=778&section=658.17&section_toc=2135 |title=§658.17 Weight |accessdate=2008-02-03 |publisher=Federal Motor Carrier Safety Administration ]

*No vehicle or combination of vehicles shall be moved or operated on any interstate highway when the gross weight on two or more consecutive axles exceeds the limitations prescribed by the following formula:::w = 500 left ( frac{l cdot n}{n-1} + 12n + 36 ight )
*"w" = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest convert|500|lb.
*"l" = spacing in feet between the outer axles of any two or more consecutive axles.
*"n" = number of axles being considered.

Two or more consecutive axles may not exceed the weight computed by the bridge formula, even if the gross weight of the truck (or the weight on one axle) 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, known as the "outer group"; "and" the interior axle groups, known as the "tractor group" and "trailer group" 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 normally will be satisfactory.


There is one exception to the formula.cite web |url=http://ops.fhwa.dot.gov/freight/publications/brdg_frm_wghts/index.htm |title=Bridge Formula Weights |accessdate=2008-02-03 |publisher=U.S. Department of Transportation - Federal Highway Administration ] Two consecutive sets of tandem axles may carry convert|34000|lb each if the overall distance between the first and last axles of these tandems is convert|36|ft|m or more. For example, a five-axle truck may carry 34,000 pounds both on the tractor tandem axles (2 and 3) and the trailer tandem axles (4 and 5), provided axles 2 and 5 are spaced at least convert|36|ft|m apart.

This exception allows for the standard 5-axle semi-truck configuration to weigh up to convert|80000|lb (the legal limit) without being in violation of the bridge formula law. Without this exception, the bridge formula would allow an actual weight of only convert|66000|lb to convert|67500|lb on tandems spaced convert|36|ft to convert|38|ft apart. This exception was sought by the American Trucking Associations specifically so trucking companies could use convert|40|ft|adj=on trailers and gross convert|80000|lb. It was the only way tank truck operators could reach 80,000 pounds without adding axles. [cite web |url=http://www.clermontengineer.org/PlanBridgeFormula.htm |title=Living With The "Bridge" |accessdate=2008-03-27 |publisher=Clermont County Engineers Office ]

Bridge formula table

*1 Calculated values reflect FHWA policy of rounding down when distances fall exactly between convert|6|in|cm|sing=on increments.cite web |url=http://vsw.fhwa.dot.gov/qa/qa.jsp?category=23%20CFR%20658.17 |title=Questions and Answers about Vehicle Size and Weight |accessdate=2008-03-27 |publisher=Federal Highway Administration ]
*2 Calculated values reflect FHWA policy of rounding down when weights fall exactly between 500 pound increments.
*3 Tandem axle by definition.
*4 Distances between convert|8|ft|m to convert|8|ft|11|in|m may not be rounded down.
*5 __ Maximum legal weight limit based on number of axles. Increased axle lengths beyond these do not increase maximum legal weight.
*6 __ Exception to the formula: when the four axles under consideration are two tandem axles spaced at least convert|36|ft apart, a gross weight of convert|68000|lb is allowed.
*__ Upper blank areas represent unrealistic configurations.

ee also

*Trucking industry in the United States



External links

* [http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?rule_toc=778&section=658.17&section_toc=2135 Bridge Formula Law]
* [http://ops.fhwa.dot.gov/freight/sw/brdgcalc/calc_page.htm Bridge formula weights calculator]

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