Bridge Formula

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Bridge Formula Definition

The bridge formula is a mathematical expression used to find the gross weight of a vehicle. The calculations are generally based on the axle number as well as their spacing. The bride formula's design was initially intended to reduce any typical commercial vehicle's weight-to-length ratio crossing a bridge.

The appropriate weight distribution for your cargo and vehicle can be achieved in one of two ways. You can increase the length between the existing axles to attain compliance or spread the shipment's weight over several additional axles.

How do I Achieve the Correct Bridge Weight?

The bridge formula takes into account three variables:

  • Tandem axel weight
  • Single axle weight
  • Gross vehicle weight

  • Your vehicle's gross weight (GWV) is the total weight of the cargo, trailer, and truck. The Federal Gross Vehicle Weight means that interstate travel is restricted to 80 000 lbs. There are, however, cases when the bridge formula might dictate otherwise.

    In the event when your axles are spaced more than 40' apart, the weight limit of a single axle is 20 000 lbs. If the shafts are less than 97' apart, the interstate weight limit is set at 34 000 lbs. Some interstates may have lower or higher weight limits. This disparity comes as a result of some weight limits being "grandfathered." This means that the original weight restrictions have been maintained despite changes to the formula.


    Who Checks Bridge Formula Compliance?

    Officials at weigh stations are the ones tasked with checking compliance with federal weight regulations. Weigh stations are typically located on the outskirts of any major cities or at state borders. Federal officials will measure and weigh the vehicles at the weigh stations to determine whether your shipment falls within the legal limits.

    Tankers are the only exception to the bridge formula table. This exception was made following a direct request to the Federal government from the American Trucking Association. This request was duly granted, and as such standard five-axle semi-truck configurations are allowed to make use of the maximum gross weight limit. This is the case despite their configuration falling outside the accepted limit.

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    Why is the Bridge Formula Important?

    The bridge formula was created to prevent heavy trucks from damaging both bridges and roadways. Because of this formula, officials can determine which bridges are safe to travel without the risk of the bridge collapsing. It is also applied to safeguard dirt and gravel roads from damage by huge, sold wheels on heavy vehicles.


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