This document discusses methods for determining equivalent single wheel loads (ESWL) and equivalent single axle loads (ESAL) for pavement design. ESWL is defined as the load from a single tire that causes the same stresses/strains as a multi-wheel load. Methods include equal stress, LCN, and FAA approaches. ESAL quantifies the effect of varying axle loads as a number of standard single axle loads. Factors like thickness and subgrade reaction are considered. Cars have minimal impact compared to trucks and buses.

1. CT-407
Pavements &
Foundations

2. DETERMINING EQUIVALENT
SINGLE-WHEEL LOADS (ESWL)
• An equivalent single wheel (ESWL) is defined as
the load on a single tire that will cause an equal
magnitude of a preselected parameter (stress,
strain, deflection) at a given location within a
specific pavement system to that resulting from
a multiple-wheel load at the same location within
the pavement structure.
• Depending upon the procedure selected, either
the tire pressure or contact area of the ESWL
may be equal to that of one tire of the multiple-
gear assembly.

3. DETERMINING EQUIVALENT
SINGLE-AXLE LOAD (ESAL)
• Equivalent Single Axle Load (ESAL) is the effect
on pavement performance of any combination of
axle loads of varying magnitude equated to the
number of standard single-axle loads that are
required to produce an equivalent effect.

4. ESWL & ESAL
• Normally, in ESWL design approach, only a
“critical” or “design” vehicle is used (e.g.
heaviest or most damaging aircraft) and the
damage effects of the other vehicle types are
ignored. This procedure is largely applied to
airports.
• For highway ESAL design approach is used as
the damage is related to the number of load
repetition.

5. DETERMINING EQUIVALENT
SINGLE-WHEEL LOADS (ESWL)
• Several parameters have been used in
pavement analysis to define the method of
evaluating the ESWL. In general, these
parameters fall into two categories:
• (a) theoretically calculated or experimentally
derived stress, strain, or deflection.
• (b) pavement distress parameters such as
cracking, serviceability level, or equal failure
conditions.

6. FLEXIBLE PAVEMENT, EQUAL
STRESS ESWL

7. FLEXIBLE PAVEMENT, EQUAL
STRESS ESWL
Figure illustrates the concept behind this approach.
The total load of the dual tire assembly is Pd, with
Sd being the center to center spacing and d being
the clear distance between tire edges (d = Sd –
2ac).
Because of the approximate zone of the stress
bulb, it is assumed that for a pavement thickness
t, less than or equal to d/2, no stress overlap
occurs. Thus, the stress at these depths is due to
that of only one wheel of the dual.

8. FLEXIBLE PAVEMENT, EQUAL
STRESS ESWL
Likewise, at a depth of approximately 2Sd, the effect of
stress overlap is such that it is equivalent to the stress
caused by a single-wheel load of Pd.
It is further assumed that the wheel load acting on the
intermediate depths between d/2 and Sd is linear when
plotted on a log load versus log thickness diagram.
Using this relationship, the ESWL, as a function of
thickness, can be found directly from the diagram.
For tandem gears, after the ESWL for the dual portion
of the gear is found at a given depth, the procedure is
repeated between the tandem distance.

9. RIGID PAVEMENT ESWL
The use of maximum tensile concrete stress is
universally recognized as the salient distress
parameter by all design methods, even though
differences between methods are present as to
where (edge, center etc.) the stress is computed.

10. RIGID PAVEMENT ESWL
LCN (LOAD CLASSIFICATION NUMBER) SYSTEM:
One such design method using the ESWL is the
Load Classification Number (LCN) system used in
the United Kingdom.

11. RIGID PAVEMENT ESWL
LCN (LOAD CLASSIFICATION NUMBER) SYSTEM:

12. RIGID PAVEMENT ESWL
LCN (LOAD CLASSIFICATION NUMBER) SYSTEM:

13. RIGID PAVEMENT ESWL
FAA METHOD:
In contrast to LCN procedure, which assumes no
dependence of the ESWL upon the pavement
thickness or subgrade reaction, the FAA has used
an ESWL analysis that takes these factors into
account.

14. RIGID PAVEMENT ESWL
FAA METHOD:

15. RIGID PAVEMENT ESWL
FAA METHOD:

16. RIGID PAVEMENT ESWL

17. DETERMINING EQUIVALENT
SINGLE-AXLE LOAD (ESAL)
• Equivalent Single Axle Load (ESAL) is the effect
on pavement performance of any combination of
axle loads of varying magnitude equated to the
number of standard single-axle loads that are
required to produce an equivalent effect.

18. DETERMINING EQUIVALENT
SINGLE-AXLE LOAD (ESAL)
Notice that cars are insignificant and thus usually ignored
in pavement design.
1.35
1.85
5.11
0.100.0007
0
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Car Delivery Truck Loaded 18-Wheeler Loaded 40' Bus Loaded 60'
Articulated Bus
ESALsperVehicle