Federal Bridge Gross Weight Formula Explained

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 (DOT) officials to determine the appropriate maximum gross weight for a commercial motor vehicle (CMV) based on axle number and spacing. The formula is part of federal weight and size regulations regarding interstate commercial traffic (intrastate traffic is subject to state limits). 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 (which had no prescribed limits on length). 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 (set at 73280lb) and authorized the construction of the Interstate Highway System.[1]

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 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.[1]

Usage

The formula was enacted as law to limit the weight-to-length ratio of a commercial motor vehicle (CMV).[2] The formula is necessary to prevent the concentrated truck's axles from overstressing pavements and bridge members (possibly causing a bridge collapse).[3] 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.[4]

While the Federal Motor Carrier Safety Administration (FMCSA), regulates safety for the U.S. trucking industry.,[5] the Federal Highway Administration (FHWA) oversees the State enforcement of truck the size and weight Federal limits set by Congress for the Federal Aid System as described in 23 CFR 658. The Federal size limits apply in all States to the National Network (NN) which is a network of Interstate Highways, U.S. Highways, and state highways. Provided the truck remains on the NN, in all States and a truck is not subject to State size limits.[6] In a similar fashion, the Federal weight limits and the Federal Bridge Formula apply to the Interstate System in all States. The State truck size and weight regulations apply to the Federal Aid System routes that do not have Federal limits.

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 for gross weight (unless the bridge formula dictates a lower limit), 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 40inches but not more than 96inches apart.[7] Axles spaced less than 40inches apart are considered a single axle.[8]

In effect, the formula reduces the legal weight limit for shorter trucks with fewer axles (see table below). For example, a 25feet three-axle dump truck would have a gross weight limit of, instead of, which is the standard weight limit for 63feet 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.[9] In November 2008, the National Transportation Safety Board determined there had been several reasons for the bridge's collapse, including (but not limited to): faulty gusset plates, inadequate inspections, and the extra weight of heavy construction equipment combined with the weight of rush hour traffic.[10] The I-35 Trade Corridor Study reported that 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, with the FHWA listing it as "high-priority" in 2005.[11]

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 (and therefore, increased stress) shortens the life of bridges.[9] National Pavement Cost Model (NAPCOM) estimates indicate that one 80000lb truck does as much damage to roads as 750 3800lb cars.[12]

Some smaller bridges have a weight limit (or gross weight load rating) indicated by a posted sign (hence the reference to a "posted bridge"). These are necessary when the weight limit of the bridge is lower than the federal or state gross weight limit for trucks.[13] [14] 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.[15] [16] 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.[17]

In special cases involving unusually overweight trucks (which require special permits), 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.[18]

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.[19] FMCSA regulation §658.17 states:[20]

W=500\left(

LN
N-1

+12N+36\right)

Two or more consecutive axles may not exceed the weight computed by the bridge formula, even the gross weight of the truck.[2] This means that the "outer group" or axles 1-5 which comprises the entire Gross Vehicle Weight (GVW) of truck and all interior combination of axles must also comply with the bridge formula. State may not issue less than four citations when a truck violate each of the Federal weight limits on the Interstate System which are: 1) Single axle 2) Tandem axle, 3) Gross Vehicle Weight (GVW), 4) Inner Group.[21]

Penalties for violating weight limits vary between states (bridge formula weight violations are treated as gross weight violations), as the states are responsible for enforcement and collection of fines. Some states, such as Connecticut, issue fines on a percentage basis (e.g. 20% overweight at $10 per 100lb), 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 (e.g., 5,000 pounds overweight equals a $300 fine). 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 (e.g. 5000lb overweight equals a $200 fine).[22]

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 (e.g., moved from the front towards the rear of the vehicle) for the vehicle to comply with axle weight limits, without penalty.[23]

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 36feet 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 36feet apart.[24]

This exception allows for the standard 5-axle semi-truck configuration to gross up to (the legal limit)[20] 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 38feet apart; compared to with the exception. This exception was sought by the American Trucking Associations so trucking companies could use 40feet 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.[25]

A CMV may exceed the bridge formula limits (or gross weight and its axle weight limits) by up to 550lb if the vehicle is equipped with an auxiliary power unit (APU) or idle reduction technology. 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.[26]

Issues

The bridge formula (also referred to as Formula B) 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.[27] Shorter wheelbase vehicles (usually specialized trucks such as garbage trucks and water trucks) 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 (TRB) 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 (later known as the TTI HS-20 Bridge Formula) 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 .[28] TTI HS-20 also failed to address the problem of multiple-span bridges.[27]

Distance in feet between any
group of two or more axles 1
Gross weight in pounds (kilograms) 2
2 axles3 axles4 axles5 axles6 axles7 axles
Less than 8 3340000NaN0340000NaN0
More than 8 4380000NaN0420000NaN0
9390000NaN0425000NaN0
10 400000NaN05435000NaN0
11400000NaN0440000NaN0
12400000NaN0450000NaN0500000NaN0
13400000NaN0450000NaN0505000NaN0
14400000NaN0465000NaN0515000NaN0
15400000NaN0470000NaN0520000NaN0
16400000NaN0480000NaN0525000NaN0580000NaN0
17400000NaN0485000NaN0535000NaN0585000NaN0
18400000NaN0495000NaN0540000NaN0590000NaN0
19400000NaN0505000NaN0545000NaN0600000NaN0
20400000NaN0510000NaN0555000NaN0605000NaN0660000NaN0
21400000NaN0515000NaN0560000NaN0610000NaN0665000NaN0
22400000NaN0525000NaN0565000NaN0615000NaN0670000NaN0
23400000NaN0530000NaN0575000NaN0625000NaN0680000NaN0
24400000NaN0540000NaN0580000NaN0630000NaN0685000NaN0740000NaN0
25400000NaN0545000NaN0585000NaN0635000NaN0690000NaN0745000NaN0
26400000NaN0555000NaN0595000NaN0640000NaN0695000NaN0750000NaN0
27400000NaN0560000NaN0600000NaN0650000NaN0700000NaN0755000NaN0
28400000NaN0570000NaN0605000NaN0655000NaN0710000NaN0765000NaN0
29400000NaN0575000NaN0615000NaN0660000NaN0715000NaN0770000NaN0
30400000NaN0585000NaN0620000NaN0665000NaN0720000NaN0775000NaN0
31400000NaN0590000NaN0625000NaN0675000NaN0725000NaN0780000NaN0
32400000NaN0600000NaN05635000NaN0680000NaN0730000NaN0785000NaN0
33400000NaN0600000NaN0640000NaN0685000NaN0740000NaN0790000NaN0
34400000NaN0600000NaN0645000NaN0690000NaN0745000NaN0800000NaN05
35400000NaN0600000NaN0655000NaN0700000NaN0750000NaN0800000NaN0
36400000NaN0600000NaN0660000NaN06705000NaN0755000NaN0800000NaN0
37400000NaN0600000NaN0665000NaN06710000NaN0760000NaN0800000NaN0
38400000NaN0600000NaN0675000NaN06715000NaN0770000NaN0800000NaN0
39400000NaN0600000NaN0680000NaN0725000NaN0775000NaN0800000NaN0
40400000NaN0600000NaN0685000NaN0730000NaN0780000NaN0800000NaN0
41400000NaN0600000NaN0695000NaN0735000NaN0785000NaN0800000NaN0
42400000NaN0600000NaN0700000NaN0740000NaN0790000NaN0800000NaN0
43400000NaN0600000NaN0705000NaN0750000NaN0800000NaN05800000NaN0
44400000NaN0600000NaN0715000NaN0755000NaN0800000NaN0800000NaN0
45400000NaN0600000NaN0720000NaN0760000NaN0800000NaN0800000NaN0
46400000NaN0600000NaN0725000NaN0765000NaN0800000NaN0800000NaN0
47400000NaN0600000NaN0735000NaN0775000NaN0800000NaN0800000NaN0
48400000NaN0600000NaN0740000NaN0780000NaN0800000NaN0800000NaN0
49400000NaN0600000NaN0745000NaN0785000NaN0800000NaN0800000NaN0
50400000NaN0600000NaN0755000NaN0790000NaN0800000NaN0800000NaN0
51400000NaN0600000NaN0760000NaN0800000NaN05800000NaN0800000NaN0
52400000NaN0600000NaN0765000NaN0800000NaN0800000NaN0800000NaN0
53400000NaN0600000NaN0775000NaN0800000NaN0800000NaN0800000NaN0
54400000NaN0600000NaN0780000NaN0800000NaN0800000NaN0800000NaN0
55400000NaN0600000NaN0785000NaN0800000NaN0800000NaN0800000NaN0
56400000NaN0600000NaN0795000NaN0800000NaN0800000NaN0800000NaN0
57400000NaN0600000NaN0800000NaN05800000NaN0800000NaN0800000NaN0
__ Maximum legal weight limit based on number of axles. Increased axle lengths beyond these do not increase maximum legal weight.[8] __ Exception to the formula: when the four axles under consideration are two tandem axles spaced at least 36feet apart, a gross weight of 68000lb is allowed.[8] __ Upper blank areas represent unrealistic configurations.[2]

See also

External links

Notes and References

  1. Web site: Chapter 2: Truck Size and Weight Limits . Comprehensive Truck Size and Weight Study . Volume 2: Issues and Background . May 3, 2008 . Federal Highway Administration .
  2. Web site: Bridge Formula Weights Calculator . February 3, 2008 . Federal Highway Administration .
  3. Web site: Federal Bridge Formula: How It Influences Vehicle Dynamic Behavior . John . Woodrooffe . University of Michigan Transportation Research Institute . Ann Arbor . March 8, 2009 . dead . https://web.archive.org/web/20090324233721/http://www.mne.psu.edu/ifrtt/ConferenceProceedings/ISHVWD_9_2006/docs/pdfs/session%2011/s11-2%20152.pdf . March 24, 2009 . mdy-all .
  4. Web site: 2006 Montana Commercial Vehicle Size and Weight and Safety Trucker's Handbook . March 9, 2009 . .
  5. Web site: FMCSA's Strategy . February 3, 2008 . . dead . https://web.archive.org/web/20080124194531/http://www.fmcsa.dot.gov/about/what-we-do/strategy/strategy.htm . January 24, 2008 .
  6. Web site: Federal Size Regulations for Commercial Motor Vehicles . March 27, 2008 . Federal Highway Administration .
  7. Web site: §658.5 Definitions . dead . https://web.archive.org/web/20080305123637/http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?rule_toc=778&section=658.5&section_toc=2129 . March 5, 2008 . March 27, 2008 . Federal Motor Carrier Safety Administration . mdy-all.
  8. Web site: Federal Bridge Gross Weight Formula . https://web.archive.org/web/20030308203728/http://www.randmcnally.com/pdf/tdm/Federal_Bridge.pdf . March 8, 2003 . March 8, 2009 . .
  9. Web site: Heavy Trucks Strain Roads and Bridges . March 28, 2008 . Minnesota Public Radio . Dan . Gunderson . August 10, 2007 .
  10. Web site: Highway Accident Report: Collapse of I-35W Highway Bridge . December 31, 2009 . .
  11. Web site: I-35 Trade Corridor Study . March 8, 2013 . I-35 Trade Corridor Study . I-35 Trade Corridor Study . Winter 1998.
  12. Web site: New Research on Pavement Damage Factors . March 31, 2008 . . June 2003 . John Merriss . dead . https://web.archive.org/web/20080625110812/http://www.oregon.gov/ODOT/CS/EA/policy_notes/03_policy_notes/0603_New_Resrch_on_Pavement_Damage.pdf . June 25, 2008 . mdy-all .
  13. Web site: Bridge Load Rating, Permitting and Posting . March 28, 2008 . . dead . https://web.archive.org/web/20080625110814/http://www2.dot.state.fl.us/proceduraldocuments/procedures/bin/850010035.pdf . June 25, 2008 . mdy-all .
  14. Web site: Chapter 191: Authorization to Use Bridges Due to Condition of Bridge . March 21, 2009 . .
  15. News: Bridge Failures . March 28, 2008 . . April 12, 1987 . Martin . Tolchin .
  16. News: Many Clues to Collapse . March 3, 2009 . The Daily Herald . Everett, WA . August 3, 2007 .
  17. Web site: Fault Tree Analysis of Schoharie Creek Bridge Collapse . . Keary H. . LeBeau . Sara J. . Wadia-Fascetti . July–August 2007. March 8, 2009.
  18. Web site: Truck Collapses Bridge in Grays Harbor County . March 31, 2008 . Seattle . . . August 15, 2007 . dead . https://web.archive.org/web/20070818022735/http://www.komotv.com/news/9187062.html . August 18, 2007 . mdy-all .
  19. Web site: Commercial Vehicle Size and Weight Program . March 27, 2008 . Federal Highway Administration .
  20. Web site: §658.17 Weight . February 3, 2008 . Federal Motor Carrier Safety Administration . dead . https://web.archive.org/web/20080104024439/http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.asp?rule_toc=778&section=658.17&section_toc=2135 . January 4, 2008 . mdy-all .
  21. Web site: Questions and Answers About Vehicle Size and Weight - FHWA Freight Management and Operations, Question 10 . 2022-06-08 . Federal Highway Administration (FHWA).
  22. Web site: Criminal Penalties for Overweight Trucks . Zachary. Schurin . April 23, 2009 . March 20, 2007 . .
  23. Web site: 316.545 Weight and Load Unlawful; Special Fuel and Motor Fuel Tax Enforcement; Inspection; Penalty; Review . April 23, 2009 . .
  24. Web site: Bridge Formula Weights . February 3, 2008 . .
  25. Web site: Living With The 'Bridge' . March 27, 2008 . Clermont County Engineers Office . January 2, 2002 . https://web.archive.org/web/20071010175926/http://www.clermontengineer.org/PlanBridgeFormula.htm . October 10, 2007 .
  26. Web site: 23 USC 127: Vehicle weight limitations-Interstate System.
  27. Web site: Chapter 6: Bridge . Comprehensive Truck Size and Weight Study . Volume 2: Issues and Background . March 17, 2009 . Federal Highway Administration .
  28. Book: Truck Weight Limits: Issues and Options . March 9, 2009 . 1990 . . 978-0-309-04955-9 .
  29. Web site: Questions and Answers about Vehicle Size and Weight . March 27, 2008 . Federal Highway Administration . US . https://web.archive.org/web/20080315090620/http://vsw.fhwa.dot.gov/qa/qa.jsp?category=23%20CFR%20658.17 . March 15, 2008 . dead .