Roads and transportation
Pavement Structural Number Calculator (AASHTO 93)
In the AASHTO 1993 flexible pavement method, the structural number (SN) expresses pavement strength. Compare the SN of a proposed section with the SN required for the design traffic and subgrade. For rough sizing only.
Formula
AASHTO 1993: SN = a1·D1 + a2·D2·m2 + a3·D3·m3, with D in inches. Typical coefficients from the AASHTO Road Test: 0.44 for dense-graded hot mix asphalt, 0.14 for crushed stone base, 0.11 for granular subbase. Drainage coefficients m range from about 0.4 (poor drainage, frequently saturated) to 1.4 (excellent).
Worked example
4 in of asphalt, 8 in of crushed stone base and 12 in of subbase with m = 1.0: SN = 0.44 × 4 + 0.14 × 8 × 1.0 + 0.11 × 12 × 1.0 = 1.76 + 1.12 + 1.32 = 4.20.
Assumptions and limits
- The required SN comes from the AASHTO 93 design equation using ESALs, reliability, standard deviation, serviceability loss and subgrade resilient modulus.
- Many state DOTs now use AASHTOWare Pavement ME (mechanistic-empirical design) rather than AASHTO 93.
- Minimum practical thicknesses apply regardless of SN.
Frequently asked questions
What SN does a residential street need?
It depends on traffic and subgrade strength. Light residential streets commonly need an SN on the order of 2 to 3, while arterials can exceed 5. Local agencies often set standard sections.
Where do layer coefficients come from?
From the AASHTO Guide for Design of Pavement Structures (1993) correlations with material stiffness, or state DOT tables.
Is AASHTO 93 still used?
Yes, widely for local roads and parking lots, though many state DOTs have adopted the Mechanistic-Empirical Pavement Design Guide for highways.