It should be noted that ESALs as calculated by the ESAL equations are dependent upon the pavement type (flexible or rigid) and the pavement structure ( structural number for flexible and slab depth for rigid). This factor relates various axle load combinations to the standard 80 kN (18,000 lbs) single axle load. The equation outputs are load equivalency factors (LEFs) or ESAL factors. Both these equations involve the same basic format, however the exponents are slightly different. ESAL equations (one each for flexible and rigid pavements) that are derived from AASHO Road Test results. ![]() At the time of its development (early 1960s at the AASHO Road Test) it was much easier to use a single number to represent all traffic loading in the somewhat complicated empirical equations used for predicting pavement life. is the 18,000 lb (80 kN) equivalent single axle load (normally designated ESAL). The most commonly used equivalent load in the U.S. The most common historical approach is to convert damage from wheel loads of various magnitudes and repetitions (“mixed traffic”) to damage from an equivalent number of “standard” or “equivalent” loads. ![]() Furthermore, it is not the wheel load but rather the damage to the pavement caused by the wheel load that is of primary concern. Although it is not too difficult to determine a wheel or an axle load for an individual vehicle, it becomes quite complicated to determine the number and types of wheel/axle loads that a particular pavement will be subject to over its design life.
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