What is the Structural Number (SN)?
The structural number (SN) is an index of the total pavement thickness that a flexible pavement needs in the AASHTO 1993 design method. It is a highway design value. It is not a measured property of a pavement surface.
How SN is calculated
SN is the sum of the contribution of each pavement layer:
SN = a1D1 + a2D2m2 + a3D3m3 + …
- ai is the layer coefficient. It shows the relative strength of the material in layer i.
- Di is the thickness of layer i in inches.
- mi is the drainage coefficient of layer i. It reflects the strength that the layer loses because of drainage and time near saturation.
Layer 1 is the asphalt surface. The formula has no drainage coefficient for it.
Where SN is used
The AASHTO design equations come from the AASHO Road Test. They relate the loss in serviceability, the traffic and the pavement thickness. The inputs are:
- the predicted number of 80 kN (18,000 lb) equivalent single axle loads (ESALs), W18
- the standard normal deviate, ZR, and the combined standard error, So, which set the reliability
- the difference between the initial and terminal serviceability indexes, delta PSI
- the subgrade resilient modulus, MR, in psi
For an overlay, the designer compares the SN of the existing pavement (SNeff) with the SN that the future traffic needs. The overlay supplies the difference. The New Jersey DOT example shows the steps.
SN and airport pavement
AC 150/5320-6G does not use SN for airport pavement thickness. It requires the FAARFIELD program for all pavement thickness designs. The FAA reports pavement strength with the ACR-PCR method (AC 150/5335-5D).
Frequently Asked Questions
- What is the difference between the required SN and the effective SN?
- The effective SN (SNeff) is the structural number of the existing pavement. The required SN is the structural number that the pavement must reach to carry the future traffic. In the New Jersey DOT overlay example, the future SN equals SNeff. Subtract the milled depth times the coefficient of the existing asphalt. Add the overlay coefficient times the overlay thickness.
- What does the drainage coefficient mean?
- The drainage coefficient (m) shows how much strength a layer loses because of poor drainage and time spent near saturation. Pavement Interactive gives values from as low as 0.40 for slow-draining, often saturated layers to as high as 1.4 for quick-draining layers. The value is often set to 1.0.
- Do FAA airport pavement designs use the structural number?
- No. FAA AC 150/5320-6G does not use the structural number. It requires the FAARFIELD program for all airfield pavement thickness designs. The guidance for flexible pavement rests on layered elastic theory.
References
- pavement-interactive-aashto-1993
- 1993 AASHTO Flexible Pavement Structural Design Pavement Interactive
- njdot-flexible-overlay-example
- Flexible Overlay Pavement Design Example, 1993 AASHTO Pavement Design New Jersey Department of Transportation
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- faa-ac-150-5335-5d
- AC 150/5335-5D, Standardized Method of Reporting Airport Pavement Strength - PCR FAA
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