Pavement Structural Number (AASHTO)
Pavement Structural Number (AASHTO)
The AASHTO structural number (SN) is a single index that combines every layer of a flexible pavement — asphalt surface, base, and sometimes subbase — into one number representing the pavement’s overall load-carrying capacity. It is the central design output of the AASHTO 1993 pavement design procedure.A pavement built with too low an SN for its traffic loading will rut and crack years ahead of schedule, while an unnecessarily high SN wastes asphalt and base material, so getting the layer thicknesses right against a target SN has direct cost and durability consequences.
The structural number is SN = a₁·D₁ + a₂·D₂·m₂, the sum of each layer’s coefficient times its thickness (and drainage factor for base layers). where a_1 and a_2 are the layer coefficients (per inch of thickness), D_1 and D_2 are the asphalt and base thicknesses, and m_2 is the base layer’s drainage coefficient.
Inputs
a_1 := 0.44 in^-1=17.32 1/m
D_1 := 4 in=101.6 mm
a_2 := 0.14 in^-1=5.51 1/m
D_2 := 8 in=203.2 mm
m_2 := 1.0=1
Each layer contributes its thickness weighted by how structurally effective that material is, so the layers add directly to a single composite index.
SN := a_1·D_1 + a_2·D_2·m_2=2.88
Results
A structural number near 2.5–3.5 is typical for a moderately trafficked collector road; arterial roads carrying heavy trucks need SN well above that, achieved with thicker or higher-quality layers. If the computed SN falls short of the SN required for the design traffic (ESALs) and subgrade strength, the fix is to add asphalt or base thickness, not to change the formula.
Assumptions & limitations
- Flexible (asphalt) pavement only — not applicable to rigid concrete pavements.
- Layer coefficients are empirical values tied to material quality.
- Subgrade strength and traffic loading (ESALs) are evaluated separately from this SN check.