Pavement
AASHTO Pavement Design
AASHTO 1993 flexible pavement design calculator — structural number from ESALs, reliability, DPSI, and subgrade resilient modulus.
AASHTO Pavement Design (SN)
The AASHTO 1993 method determines the required structural number (SN) for flexible pavement. SN is a composite index reflecting the combined strength of all pavement layers above the subgrade.
Key inputs: 18-kip ESAL loading (W₁₂) for the design period, reliability level (R), overall standard deviation (S₀ = 0.45 typical), change in serviceability index (ΔPSI), and subgrade resilient modulus (MR). SN is solved iteratively from the AASHTO design equation. SN is then converted to layer thicknesses using structural coefficients (a₁D₁ + a₂m₂D₂ + a₃m₃D₃).
log₁₀(W₁₂) = Z𝑹·S₀ + 9.36·log₁₀(SN+1) − 0.20
+ log₁₀(ΔPSI/2.7) / [0.40 + 1094/(SN+1)^5.19]
+ 2.32·log₁₀(M𝑹) − 8.07
For 5 million design ESALs at 80% reliability (Z_R = −0.841), a standard deviation of 0.45, ΔPSI of 2.0, and a subgrade M_R of 10,000 psi, the AASHTO 1993 equation solves iteratively to a required structural number of SN = 3.63. Using a typical HMA structural coefficient of a₁ = 0.44, that translates to a minimum surface thickness of 8.5 inches if the full SN were carried by asphalt alone — in practice, split across HMA, base, and subbase layers.
AASHTO Pavement Design Calculator
Flexible pavement structural number (SN)