What is the Equivalent Single Axle Load (ESAL)?
The equivalent single axle load (ESAL) is a unit that expresses any axle load as passes of an 18,000 lb (80 kN) single axle. It is the most commonly used equivalent load in the U.S. It comes from highway design, and the data behind it are from the AASHO Road Test of the early 1960s.
Load equivalency factors
A load equivalency factor (LEF) compares the damage of one pass of an axle with the damage of one pass of the standard axle. Damage is roughly related to the load by a power of four, so a small increase in load gives a large increase in damage. The AASHTO 1993 tables give a LEF of 1.4 for a 20,000 lb single axle on a flexible pavement and 1.57 on a rigid pavement. For a 20,000 lb tandem axle they give 0.162 and 0.206. The factors depend on the axle type and on the pavement.
Design ESALs
The truck factor is the average number of ESALs per truck. A designer counts the trucks and multiplies by the truck factor. The count must be projected over the design life with a growth estimate. Multiplying one year of traffic by the design life underestimates the total. The 1993 AASHTO Guide recommends a multiplier of 1.5 to convert flexible ESALs to rigid ESALs.
Airport pavements
Airport pavements do not use the ESAL. FAA AC 150/5320-6G designs them with FAARFIELD, which sums the cumulative damage factor of each aircraft type in the traffic mix. The AC states that designing with the departures of one design aircraft is not equivalent to designing with the full mix.
Frequently Asked Questions
- How much more damage does a heavier axle do?
- Much more, because damage is roughly related to the load by a power of four. The AASHTO 1993 factors show this. A 20,000 lb single axle has a load equivalency factor of 1.4 on a flexible pavement, where the 18,000 lb standard axle has 1.0. The factors also depend on the axle type and the pavement.
- Is ESAL used for airport pavements?
- No. FAA Advisory Circular 150/5320-6G designs airport pavements with FAARFIELD, which sums the cumulative damage factor of each aircraft type in the traffic mix. The ESAL is a highway unit.
References
- pavement-interactive-esal
- Equivalent Single Axle Load (ESAL) Pavement Interactive
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
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