At first glance, this equation looks quite complex – it is.
| Where: | W | equals | axle applications inverse of equivalency factors (where W18 = number of 18,000 lb (80 kN) single axle loads) |
| Lx | equals | axle load being evaluated (kips) | |
| L18 | equals | 18 (standard axle load in kips) | |
| L2 | equals | code for axle configuration 1 = single axle 2 = tandem axle 3 = triple axle (added in the 1986 AASHTO Guide) x = axle load equivalency factor being evaluated s = code for standard axle = 1 (single axle) |
|
| G | equals |  a function of the ratio of loss in serviceability at time, t, to the potential loss taken at a point where pt = 1.5 |
|
| Pt | equals |  "terminal" serviceability index (point at which the pavement is considered to be at the end of its useful life) |
|
| b | equals | function which determines the relationship between serviceability and axle load applications | |
| SN | equals | structural number |
Example Calculation for a Single Axle
- Assumptions: Single axle, 30,000 lb (133 kN), SN = 3, pt = 2.5
- Answer: (Table D.4, p. D-6, 1993 AASHTO Guide) = 7.9
- Calculations
| where : | W18 | equals | predicted number of 18,000 lb (80 kN) single axle load applications |
| W30 | equals | predicted number of 30,000 lb (133 kN) single axle load applications | |
| Lx | equals | L30 = 30 | |
| L2x | equals | 1 (single axle) | |
| G | equals | serviceability loss factor | |
| equals |  |
||
| b30 | equals | curve slope factor | |
| equals |  |
||
| and | G/b30 | equals | -0.2009/4.388 = -0.04578 |
| b18 | equals |  |
|
| G/b18 | equals | -0.2009/1.2204 = -0.1646 | |
| Thus |  |
||
| and |  |
||
| Finally | LEF | equals |  (same as contained in 1993 AASHTO Guide, Appendix D) |
Example Calculation for a Tandem Axle
- Assumptions: Tandem axle, 40,000 lb (133 kN), SN = 5, pt = 2.5
- Answer: (Table D.5, p. D-7, 1993 AASHTO Guide) = 2.08
- Calculations
| where : | L40 | equals | 40 (tandem axle) |
| L18 | equals | 18 (single axle) | |
| L2x | equals | 2 (tandem axle) | |
| L2s | equals | 1 (single axle) | |
| G | equals | serviceability loss factor | |
| equals |  |
||
| b40 | equals | curve slope factor | |
| equals |  |
||
| and | G/b40 | equals | -0.2009/0.53824 = -0.37325 |
| b18 | equals |  |
|
| G/b18 | equals | -0.2009/0.50006 = -0.40175 | |
| Thus |  |
||
| Finally | LEF | equals |  (same as contained in 1993 AASHTO Guide nbsp;Appendix D) |