The Equivalent Standard Axle (ESA) and the Standard Axle Repetition (SAR) is the Austroads method in determining a standardised wheel load and a correlation for different wheel load configurations using the material damage exponent theorem.

Equivalent Standard Axle (ESA):

  • Is based on a Single Axle Dual Tire (SADT) exhibiting a force of 80kN (or 8T)
  • Standardises various axle configurations and loads
  • Adopts a load magnitude to the 4th power
  • Is the empirical process used for the design thickness of granular pavements with thin bituminous surfacings

Standard Axle Repetition (SAR):

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  • Follows on from the ESA calculation and is used for pavement profiles with more than one layer of bound material
  • Provides indicative performance factors (SARASPHALT, SARSUBGRADE and SARCEMENT) for various materials and pavement configurations including fatigue in bound materials and rutting in the subgrade
  • Adopts various load magnitudes for the material types (SARASP = 5, SARSUB = 7 & SARCEM =12)
  • Provides a value based upon the Traffic Load Distribution (TLD) for the subject section
  • Is used for the Austroads mechanistic design process (CIRCLY)

Understanding and adopting this wheel load calculation is focused on the ratio of a nominated wheel loading to the base load for a ‘standard axle’ as outlined in Austroads Guide to Pavement Technology – Part 2 Structural Design.

Generally speaking, the theory for calculating the load induced wheel loading is detailed using the load damage equation:

Where:

SAR = Standard Axle Repetition (Asphalt, Cement, Subgrade)

Lij = Load Magnitude (nominated axle load)

SLi = Standard Axle Configuration Base Load

m = Damage Exponent (ESA = 4, SARASPHALT = 5, SARSUBGRADE = 7, SARCEMENT = 12)

This procedure is the first principles method of determining ESAs and SARs for each Heavy Vehicle Axle Group (HVAG) for a heavy vehicle and ultimately a nominated ESA/Heavy Vehicle. Conventional HVAGs include:

  • Single Axle Single Tires (SAST)
  • Single Axle Dual Tires (SADT)
  • Tandem Axle Single Tires (TAST)
  • Tandem Axle Dual Tires (TADT)
  • Tri-axle Single Tires (TRST)
  • Tri-axle Dual Tires (TRDT)
  • Quad-axle Single Tires (QAST)
  • Quad-axle Dual Tires (QADT)

This first principles method is particularly relevant when the road section is characterised by a consistent traffic profile or where the traffic load distribution is known.

For more information on The Equivalent Standard Axle (ESA) and the Standard Axle Repetition (SAR) or Global Road Technology products please contact GRT.