Understanding the Federal Bridge Formula -or- Federal Bridge Rob Peterson Western Peterbilt Dan Condon Titan Truck Equipment

History 1933  All states have some form of regulation 1956  GVW: 73,280 lbs  Construction of Interstate Highway System 1974  GVW: 80,000 lbs  Federal-Aid Highway Act establishes bridge formula as law

Why Federal Bridge?

The Formula FMCSA regulation § states: 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:

The Formula w = the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds (230 kg) ℓ = spacing in feet between the outer axles of any two or more consecutive axles n = number of axles being considered

The Chart

The Limiting Factor  Axle Beam  Brakes  Suspension  Tire  Rating  Width Even before the Federal Bridge Formula and Chart, these requirements must be met:

Axles Steering Axle Single Axle Tandem Axle Tridem Axle

Steering Axle 600 lbs. per inch of tire  10R = 12,000  11R = 13,200  12R = 14,400  315 = 14,880  385 = 18,180  425 = 20,078  445 = 21,023 Millimeter to Inch Conversion: Inches = mm/25.4

Steering Axle

Single Axle Maximum: 20,000 lbs Minimum: (4) 10-inch tires

Single Axle “Wide-base” singles are not legal for maximum single axle weight due to lack of tread width.

Tandem Axle Maximum: 34,000 lbs Minimum: (4) 8.5-inch tires 49” spacing

Tandem Axle 445/2.54= X 500 = 8,760 8,760 X 2 = 17,520 17,520 X 2 = 35,040

Tridem Axle To qualify for greater than Tandem Axle weight, spacing must exceed 96” If axle spacing measures 6” or greater, round to next larger foot >8’ : 42,000 lbs 9’ : 42,500 lbs 10’ : 43,500 lbs 11’ : 44,000 lbs 12’ : 45,000 lbs

The Chart

Tridem Axle Minimum tread width is dependent on spacing: Singles – 14” (355 42,000 lbs Duals – 7” (178 42,000 lbs Singles – 15” (381 45,000 lbs Duals – 7.5” (191 45,000 lbs

Real Life Examples

Caution: GVWR MAY NOT EQUAL Federal Bridge!!!!

The Bucket Truck Class 7 GVW Rating:33,000 lbs FAX: 12,000 lbs 11R22.5 Tires RAX: 21,000 lbs 11R22.5 Tires FBC: 20,000 lbs *Even though the truck is rated at 33,000 lbs, it can only carry 32,000 lbs!

The Flatbed Warehouse Truck Class 8 GVW Rating:52,000 lbs FAX: 12,000 lbs 11R22.5 Tires RAX: 40,000 lbs 11R22.5 Tires FBC: 34,000 lbs WB: 232” FBC: 52,500 lbs *Even though the truck bridges at 52,500 lbs, it can only carry 46,000 lbs. Additionally, this unit “axles out” before “bridging out”.

The Bigger Bucket Truck Class 8 GVW Rating:60,000 lbs FAX: 20,000 lbs 385/80R22.5 Tires FBC: 18,180 lbs RAX: 40,000 lbs 11R22.5 Tires FBC: 34,000 lbs WB: 226” FBC: 51,500 lbs *Even though the truck axles out at 52,180 lbs, it can only carry 51,500 lbs! This unit “bridges out” before it “axles out”.

Combination Vehicles

Review

The Chart Review

Combination Vehicles 105, , , ,500 No limit 129, ,500 85,000 95,000 Maximum: GCVW 80,000 lbs

5 Axle Tractor & Semi-trailer Minimum axle spacing from axle 1 to axle 5 is 607” 1 1: 12,000 lbs 2-3: 34,000 lbs 4-5: 34,000 lbs

7 Axle Super Dump Minimum axle spacing from axle 1 to axle 7 is 403” 1 7

Dump Truck & Tag Trailer Minimum axle spacing from axle 1 to axle 4 is 427” 1 4

Digger Derrick & Tag Trailer Minimum axle spacing from axle 1 to axle 5 is 607” 15

Bucket Truck & Tag Trailer Minimum axle spacing from axle 1 to axle 6 is 511” 1 6

Thank you! Rob Peterson Western Peterbilt Phone: (509) Dan Condon Titan Truck Equipment Phone: (509)

References Ken Goudy picture collection; Dana Eaton Corp; Wikipedia; Bridgestone Corp;