ESAL and Structural Number

LEF = (\frac{L _{x}}{18})^{4}, SN = a_{1} D_{1} + a_{2} D_{2} m_{2} + a_{3} D_{3} m_{3}

Flexible-pavement damage is often taken as the fourth power of the axle-load ratio to an 18-kip equivalent single-axle load (ESAL). A 22-kip axle is (22/18)^4 ≈ 2.23 ESALs. Structural number SN sums layer coefficients a_i times thicknesses D_i (inches) times drainage coefficients m_i (m1 = 1 for the surface). AASHTO empirical design equates required SN to traffic (ESALs), reliability, and subgrade Mr.

$LEF$: Load equivalency factor (ESALs per axle)
$L_{x}$ (kip): Axle load
$SN$: Structural number
$a_{i}, D_{i}$ (1/in , in): Layer coefficient and thickness
$m_{i}$: Drainage coefficient (base/subbase)

Step-by-step solved example

Surface a1 = 0.44, D1 = 4 in; base a2 = 0.14, D2 = 8 in, m2 = 1.0. A 12-kip single axle. Find SN and LEF.

Axles on the pavement — damage relative to an 18-kip axle.

1. Structural number
SN = 0.44×4 + 0.14×8×1.0 = 1.76 + 1.12 = 2.88.
$SN = 2.88$
2. Load equivalency
LEF = (12/18)^4 = (0.667)^4 = 0.198 ESAL per axle.
$LEF = 0.198$

Answer: SN = 2.88, LEF = 0.20 ESAL/axle

10 practice questions

0/10 correct

1.An 18-kip single axle is defined as

2.A 9-kip single axle has LEF = (9/18)^4 =

3.Doubling the axle load multiplies flexible-pavement damage by about

4.SN = 0.44×5 + 0.14×10×1.0 + 0.11×12×0.90 is nearest

5.Layer coefficient a1 for asphalt concrete is typically about

6.Drainage coefficient m2 < 1 means the base

7.Rigid (PCC) pavements use

8.Truck traffic dominates ESALs because

9.A 22-kip single axle LEF is nearest

10.Required SN increases when