AASHTO 1993 §3.3 traffic loading — ESALs over the design life with compound growth and lane distribution. Eq. 3.1 reliability form solved for Structural Number (SN). Eq. 3.2 layer-coefficient rollup against the proposed pavement structure.
Free preview: design ESAL, SN_required, SN_proposed, utilization %, PASS/FAIL. Pro or Civil for per-layer contribution table and export.
Design Equivalent Single-Axle Loads (W₁₈) convert mixed truck traffic into the number of 80 kN (18 000 lb) single-axle loads the pavement must withstand over its design life. ESAL_per_year = ADT × truck% × LEF × 365 × lane distribution factor (AASHTO 1993 §3.3). Compound growth summation: W₁₈ = ESAL_per_year × ((1 + g)^Y − 1) / g, falling back to ESAL_per_year × Y when g = 0.
SN is solved from the AASHTO 1993 Eq. 3.1 reliability form: log₁₀(W₁₈) = Z_R·S0 + 9.36·log₁₀(SN+1) − 0.20 + log₁₀[MR/(10⁶·σ)]/0.40 + 2.32·log₁₀(MR·10³) − 8.07. Z_R is the standard normal deviate for reliability R (e.g. −1.282 for R=85%, −1.645 for R=95%). The calculator uses the closed-form log-space inverse of Eq. 3.1, monotonic in W₁₈ and MR.
The layer coefficient a_i expresses each layer's structural contribution per inch: asphalt surface 0.40–0.44 (typically 0.42), granular base 0.10–0.14 (0.13 default), subbase 0.05–0.11 (0.09 default). AASHTO 1993 Part II Chapter 3 Table 3.1. The drainage coefficient m_i ∈ [0.6, 1.4] scales granular layers by saturation/exposure — 1.0 is "fair moisture", 0.6 is "wet", 1.4 is "dry".
What minimum asphalt depth does AASHTO recommend?
S0 is a global measure of overall design uncertainty — performance variability, traffic estimation error, subgrade variability. Common practice is S0 = 0.40–0.45 for flexible pavements with normal variability and S0 = 0.50 for high-variability urban arterials. Lowering S0 makes the design more aggressive (lower SN_required for the same R); raising S0 is more conservative.