TRANSPORTATION
ENGINEERING II
AXLE LOAD AND ESAL
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
• 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.
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.
• 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 .0002 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.
DAMAGING EFFECT OF DIFFERENT AXLE LOADS (AASHO ROAD TEST)
AXLE LOAD
KG
LBS
AXLE LOAD
EQUIVALENCE
FACTOR
KG
LBS
EQUIVALENCE
FACTOR
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
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 multiaxle 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.
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
Traffic Loads Characterization
Pavement Thickness Design Are Developed
To Account For The Entire
Spectrum Of Traffic Loads
Cars
Pickups
Buses
Trucks
Trailers
13.6
Tons
Failure = 10,000
Repetitions
11.3 Tons
Failure = 100,000
Repetitions
4.5 Tons
Failure = 1,000,000 Repetitions
2.3 Tons
13.6 Tons
4.5
Tons
Failure = 10,000,000
Repetitions
Failure = Repetitions ?
11.3 Tons
2.3 Tons
RELATIVE DAMAGE CONCEPT
Equivalent
Standard
18000 - Ibs
ESAL
(8.2 tons)
Damage per
Pass = 1
Axle Load
• 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
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
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 = 0.0625
 This means that B-Axle causes only 0.0625 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
DAMAGE PER PASS
80
70
60
50
40
30
20
10
0
1.0
1.1
2.3
3.3
4.7
6.5
8.7
11.5
14.9
18.9
23.8
29.5
36.3
44.1
53.1
63.4
75.2
AXLE LOAD & RELATIVE DAMAGE
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SINGLE AXLE LOAD (Tons)