Axle Load Equivalency Factor
Computes how many passes of the standard axle are equivalent to one pass of the real axle, using the power law of pavement design: factor = (axle load ÷ standard axle load) raised to the damage exponent. This factor is what converts a traffic count into the number N of standard axle repetitions, which in Brazil is the 8.2 tf, or 80 kN, single axle with dual wheels. The exponent amplifies overload brutally: an axle 20% heavier than the standard does not consume 20% more pavement but 2.07 times as much, which is why a single overloaded truck weighs more on the life of the road than thousands of cars, whose factor is practically zero. The exponent is an input rather than fixed at 4, the AASHTO value known as the fourth power law, because rigid pavement and fatigue cracking models work with exponents between 3 and 5 and the result shifts by a whole level depending on the choice. Enter the axle load, the standard axle load and the damage exponent.
Result
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Axle Load Equivalency and the Fourth Power Law
A traffic count on a highway brings in axles of every weight, from the empty front axle of a pickup to the tandem of an overloaded truck. Pavement design, though, needs a single number: N, the total repetitions of the standard axle the section will take over its design life. The load equivalency factor performs that translation, one axle at a time, and it is what turns a weigh-station spreadsheet into an input the design method can actually use.
The factor comes from a power law: LEF = (P ÷ P standard) raised to n. P is the real axle load and P standard the reference axle — in Brazil the 8.2 tf single axle with dual wheels, the 80 kN axle of AASHTO. The exponent n measures how fast damage accelerates with load: 4 is the value enshrined as the fourth power law and comes prefilled, but the field takes others, since rigid pavement and fatigue cracking models work anywhere from 3 to 5. The scale is brutally nonlinear. The 10 tf in the example give 2.212; a 12 tf axle jumps to 4.586; a car axle near 1 tf is worth 0.0002, under the three decimals the screen prints.
The power law describes fatigue, and fatigue is one failure mode among several: it sees nothing of subgrade rutting, binder ageing or thermal cracking. The factor applies per axle, never per vehicle, and a single exponent is a simplification — the Brazilian DNIT publishes separate curves for single axles with single wheels, tandems and tridems, while the original AASHTO work makes the factor depend on the structural number and on the terminal serviceability chosen. Mind the units: both load fields must share one. Mixing 100 kN in the real axle with 8.2 tf in the standard returns more than 22,000 instead of 2.212.
Frequently asked questions
What does the default result of 2.212 actually mean?
Can I enter the gross weight of the truck in the axle load field?
Does switching the exponent from 4 to 3 or 5 change much?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.