Axle spacing is another consideration that must be taken into account when looking at Federal weight compliance. To protect bridges, the number and spacing of axles carrying the vehicle load must be calculated. Thus, a bridge weight formula is also applied to commercial vehicles in determining their compliance with Federal weight limits. The Federal bridge formula applies when the gross weight on two or more consecutive axles exceeds the limitations of the formula, except that two consecutive sets of tandem axles may carry a gross load of 34,000 lbs. pounds each if the overall distance between the first and last axle is 36’ or more.

W = 500(LN/N-1 + 12N + 36)

W = Overall gross weight on any group of 2 or more consecutive axles to the nearest 500 pounds.
L = Distance in feet between the extreme of any group of 2 or more consecutive axles.
N = Number of axles in the group under consideration.

Exception to the Bridge Weight Formula

Federal law (23 U.S.C. 127) includes an exception to the Bridge Formula and the Bridge Table—two consecutive sets of tandem axles may carry 34,000 lbs. pounds each if the overall distance between the first and last axles of these tandems is 36’ or more. For example, a five-axle tractor-semitrailer combination may carry 34,000 lbs. 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 36’ apart. Without this exception, the Bridge Formula would allow an actual weight of only 66,000 lbs. to 67,500 pounds on tandems spaced 36 to 38’ apart.

The Federal Bridge Weight Chart

2
axles
3
axles
4
axles
5
axles
6
axles
7
axles
8
axles
9
axles
Less than 8’3″34,00034,000
More than 8’4″38,00042,000
939,00042,500
1040,00043,500
1144,000
1245,00050,000
1345,00050,500
1446,50051,500
1547,00052,000
1648,00052,50058,000
1748,50053,50058,500
1849,50054,00059,000
1950,50054,50060,000
2051,00055,50060,50066,000
2151,50056,00061,00066,500
2252,50056,50061,50067,000
2353,00057,50062,50068,000
2454,00058,00063,00068,50074,000
2554,50058,50063,50069,00074,500
2655,50059,50064,00069,50075,000
2756,00060,00065,00070,00075,500
2857,00060,50065,50071,00076,50082,000
2957,50061,50066,00071,50077,00082,500
3058,50062,00066,50072,00077,50083,000
3159,00062,50067,50072,50078,00083,500
3260,00063,50068,00073,00078,50084,50090,000
3364,00068,50074,00079,00085,00090,500
3464,50069,00074,50080,00085,50091,000
3565,50070,00075,00080,50086,00091,500
3666,00070,50075,50081,00086,50092,000
3766,50071,00076,00081,50087,00093,000
3867,50071,50077,00082,00087,50093,500
3968,00072,50077,50083,50088,50094,000
4068,50073,00078,00084,00089,00094,500
4169,50073,50078,50084,50089,50095,000
4270,00074,00079,00084,50090,00095,500
4370,50075,00080,00085,00090,50096,000
4471,50075,50080,50085,50091,00096,500
4572,00076,00081,00086,00091,50097,500
4672,50076,50081,50087,00092,50098,000
4773,50077,50082,00087,50093,00098,500
4874,00078,00083,00088,00093,50099,000
4974,50078,50083,50088,50094,00099,500
5075,50079,00084,00089,00094,500100,000
5176,00080,00084,50089,50095,000100,500
5276,50080,50085,00090,50095,500101,000
5377,50081,00086,00091,00096,500101,500
5478,00081,50086,50091,50097,000102,000
5578,50082,50087,00092,00097,500102,500
5679,50083,00087,50092,50098,000103,000
5780,00083,50088,00093,00098,500104,000
5884,00089,00094,00099,000104,500
5985,00089,50094,50099,500105,000
6085,50090,00095,000100,500105,500

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