Structural Number

The Structural Number (SN) represents the overall structural requirement needed to sustain the design’s traffic loadings. It is an abstract number that expresses the structural strength of a pavement required for given combinations of soil support (MR), total traffic expressed in ESALs, terminal serviceability and environment.

The following equation can be used to relate individual material types and thicknesses to the structural number:

SN = a1D1 + a2D2M2 + a3D3M3


  • a1, a2, a3 = structural-layer coefficients of the wearing surface, base, and subbase layers, respectively,
  • D1, D2, D3 = thickness of the wearing surface, base, and subbase layers in inches, respectively, and
  • M2, M3 = drainage coefficients for the base and subbase, respectively.

The Structural Number is converted to actual layer thicknesses (e.g., 150 mm (6 inches) of HMA) using a layer coefficient (a) that represents the relative strength of the construction materials in that layer. Additionally, all layers below the surface layer are assigned a drainage coefficient (m) that represents the relative loss of strength in a layer due to its drainage characteristics and the total time it is exposed to near-saturation moisture conditions. The drainage coefficient is often assumed to be 1, although quick-draining layers that almost never become saturated can have coefficients as high as 1.4 while slow-draining layers that are often saturated can have drainage coefficients as low as 0.40. Keep in mind that a drainage coefficient is basically a way of making a specific layer thicker, but not necessarily more dense. If a fundamental drainage problem is suspected, a better solution is to address the actual drainage problem by using very dense layers (to minimize water infiltration) or designing a drainage system.