1. TRANSPORTATION ENGINEERING II
AXLE LOAD AND ESAL 1

2. AXLE LOADS
One of the primary functions of a pavement is its load distribution. Therefore, in order to adequately design a pavement something must be known about the expected loads it will carry during its design life. Loads, the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes), can be characterized by the following parameters:  
Tire loads
Axle and tire configurations
Repetition of loads
Distribution of traffic across the pavement
Vehicle speed 2

3. Damage caused to the pavements increases sharply with the axle loading
Damage caused to the pavements increases sharply with the axle loading. Vehicle loads of private cars and vans contributes very little to structural deterioration. For the purpose of pavement thickness design, only commercial traffic is considered. 3

4. STANDARD AXLE LOAD
Different tests were performed to calculate the thickness of pavement, in relation to the no. of vehicles passing over various portions of roads. Statistical analysis of the data collected showed that relative damaging effect of an axle was approximately proportional to the fourth power of the load which it carries, irrespective of the type or thickness of the pavement. 4

5. An axle carrying a load of 18,000 lbs (8160 kg) was arbitrarily defined in the AASHO road test as a Standard Axle, with a damaging effect of unity.
The damaging effects of lighter and heavier axles were expressed as equivalence factors as shown in Table. The equivalence factor of for the 910-kg axle load means that 5000 a passes of such an axle would do the same damaging effect as one pass of standard axle. 5

6. DAMAGING EFFECT OF DIFFERENT AXLE LOADS (AASHO ROAD TEST)
EQUIVALENCE FACTOR KG
LBS
910
2000
0.0002
9980
22000
2.3
1810
4000
0.0025
10890
24000
3.2
2720
6000
0.01
11790
26000
4.4
3630
8000
0.03
12700
28000
5.8
4540
10000
0.09
13610
30000
7.6
5440
12000
0.19
14520
32000
9.7
6350
14000
0.35
15420
34000
12.1
7260
16000
0.61
16320
36000 15
8160
18000 1*
17230
38000
18.6
9070
20000
1.5
18140
40000
22.8
*Standard axle 6

7. EQUIVALENT STANDARD AXLE LOAD
This approach converts wheel loads of various magnitudes and repetitions ("mixed traffic") to an equivalent number of "standard" or "equivalent" loads based on the amount of damage they do to the pavement. The commonly used standard load is the 18,000 lb. Equivalent Single Axle load. Using the ESAL method, all loads (including multi-axle loads) are converted to an equivalent number of 18,000 lb. single axle loads, which is then used for design. A "load equivalency factor" represents the equivalent number of ESAL’s for the given weight-axle combination. 7

8. Generalized Fourth Power Law
A rule-of-thumb, the damage caused by a particular load is roughly related to the load by a power of four (for reasonably strong pavement surfaces). For example,
A 18,000 lb (80 kN) single axle, LEF =1.0
A 30,000 lb (133 kN) single axle, LEF = 7.6
Comparing the two, the ratio is: 7.6/1.0 = 7.6
Using the fourth power rule-of-thumb:
Thus, the two estimates are approximately equal 8

9. 9

10. 10

11. 11

12. Traffic Loads Characterization
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars
Pickups
Buses
Trucks
Trailers 12

13. Failure = 10,000 Repetitions Failure = 100,000 Repetitions
13.6 Tons
Failure = 100,000 Repetitions
11.3 Tons
Failure = 1,000,000 Repetitions
4.5 Tons
Failure = 10,000,000 Repetitions
2.3 Tons
11.3 Tons
Failure = Repetitions ?
13.6 Tons
4.5 Tons
2.3 Tons 13

14. RELATIVE DAMAGE CONCEPT
Equivalent
Standard ESAL
Axle Load
Ibs
(8.2 tons)
Damage per Pass = 1
Axle loads bigger than 8.2 tons cause damage greater than one per pass
Axle loads smaller than 8.2 tons cause damage less than one per pass
Load Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4 14

15. Consider two single axles A and B where:
EXAMPLE
Consider two single axles A and B where:
A-Axle = 16.4 tons
Damage caused per pass by A -Axle = (16.4/8.2)4 = 16
This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e,
16.4 Tons Axle =
8.2 Tons Axle 15

16. Consider two single axles A and B where:
EXAMPLE
Consider two single axles A and B where:
B-Axle = 4.1 tons
Damage caused per pass by B-Axle = (4.1/8.2)4 =
This means that B-Axle causes only times damage per pass as caused by 1 pass of standard 8.2 tons axle.
In other works, 16 passes (1/0.625) of B-Axle cause same amount of damage as caused by 1 pass of standard 8.2 tons axle i.e., =
4.1 Tons Axle
8.2 Tons Axle 16

17. AXLE LOAD & RELATIVE DAMAGE17