At first glance, this equation looks quite complex – it is.
Where: | W | = | axle applications inverse of equivalency factors (where W_{18} = number of 18,000 lb (80 kN) single axle loads) | |
L_{x} | = | axle load being evaluated (kips) | ||
L_{18} | = | 18 (standard axle load in kips) | ||
L_{2} | = | 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) |
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G | = | a function of the ratio of loss in serviceability at time, t, to the potential loss taken at a point where p_{t} = 1.5 | ||
p_{t} | = | “terminal” serviceability index (point at which the pavement is considered to be at the end of its useful life) | ||
b | = | function which determines the relationship between serviceability and axle load applications | ||
SN | = | structural number |
Example Calculation for a Single Axle
- Assumptions: Single axle, 30,000 lb (133 kN), SN = 3, p_{t} = 2.5
- Answer: (Table D.4, p. D-6, 1993 AASHTO Guide) = 7.9
- Calculations
where : | W_{18 } | = | predicted number of 18,000 lb (80 kN) single axle load applications, |
W_{30} | = | predicted number of 30,000 lb (133 kN) single axle load applications, | |
L_{x} | = | L_{30} = 30 | |
L_{2x} | = | 1 (single axle) | |
G | = | serviceability loss factor | |
= | |||
b_{30} | = | curve slope factor | |
= | _{} | ||
and | G/b_{30} | = | -0.2009/4.388 = -0.04578 |
b_{18} | = | ||
G/b_{18} | = | -0.2009/1.2204 = -0.1646 | |
Thus, | |||
and | of W_{18} loads allowable with a 30,000 lb. single axle | ||
Finally, | LEF | = | (same as contained in 1993 AASHTO Guide ?nbsp;Appendix D) |
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Example Calculation for a Tandem Axle
- Assumptions: Tandem axle, 40,000 lb (133 kN), SN = 5, p_{t} = 2.5
- Answer: (Table D.5, p. D-7, 1993 AASHTO Guide) = 2.08
- Calculations
where : | L_{40} | = | 40 (tandem axle) | |
L_{18} | = | 18 (single axle) | ||
L_{2x} | = | 2 (tandem axle) | ||
L_{2s} | = | 1 (single axle) | ||
G | = | serviceability loss factor | ||
= | ||||
b_{40} | = | curve slope factor | ||
= | ||||
and | G/b_{40} | = | -0.2009/0.53824 = -0.37325 | |
b_{18} | = | |||
G/b_{18} | = | -0.2009/0.50006 = -0.40175 | ||
Thus, | ||||
Finally, | LEF | = | (same as contained in 1993 AASHTO Guide nbsp;Appendix D) |