: Overall standard deviation (typically 0.45 for flexible pavements). cap delta cap P cap S cap I : Serviceability loss, calculated as cap P sub o is initial (usually 4.2) and cap P sub t is terminal serviceability (typically 2.0–2.5). cap M sub cap R : Resilient modulus of the subgrade in psi. Federal Highway Administration (.gov) 2. Set Up the Iterative Calculation To solve for cap S cap N
That formula calculates the for your trial SN. aashto flexible pavement design excel spreadsheet
The equation is non-explicit; that is, the Structural Number ($SN$) cannot be easily isolated on one side of the equation. Solving for $SN$ requires iterative trial-and-error or complex logarithmic manipulation. Furthermore, because the Structural Number is a composite value derived from the thickness and material coefficients of the surface, base, and sub-base layers, engineers must balance these variables to achieve a cost-effective design. Performing these iterations by hand is time-consuming and prone to arithmetic errors, making computerized solutions a necessity. : Overall standard deviation (typically 0