EQUIVALENT SINGLE AXLE LOAD

Group 2
I. Background and its Purpose

What is Equivalent Single Axle Load?

ESAL is the acronym for Equivalent Single
Axle Load. ESAL is a concept developed
from data collected at the American
Association of State Highway Officials
(AASHO) Road Test to establish a damage
relationship for comparing the effects of
axles carrying different loads.

Axle Load refers to the weight limit

allowed for each axle over the highways.
 What is design ESALs? Use the following calculations to determine design ESALs:
.....................
Where:
Design ESALs is a cumulative traffic load = Truck Factor
summary statistic. The statistic represents a Use the following calculation to determine
mixed stream of traffic of different axle loads and
axle configurations predicted over the design or where:
analysis period and then converted into an = percentage of total repetitions for the ith load group
equivalent number of 18,000-lb. single axle loads = equivalent axle load factor for the ith load group
A = average number of axles per truck
summed over that period. T = percentage of trucks in average daily traffic (ADT)
G = growth factor
Use the following calculation to determine G:
G
where:
g = annual growth rate
n = analysis period in years
D = directional distribution factor
L = lane distribution factor
Y = design period in years
 ESAL INFORMATION A request for a traffic study should include the following
 Average Daily Traffic (ADT) means the volume of traffic information: 
passing a point or segment of a road. • geographic limits 
 Requests for traffic projections should be coordinated • base year and design year for the project (base year should be
with the District Director of Transportation Planning & tied to the projected opening date to traffic) 
Development (TP&D). • current land use maps for the area surrounding the proposed
project
• location and type of major traffic generators 
• past and current traffic counts for an existing facility 
How do I get the ESAL information? x • major cross streets 
• map giving general project alignment 
Google
• identification of proposed facility type 
Transportation Planning and Programming Division (TP&P), • one or two way operation 
Traffic Analysis Section provides beginning and ending two • number of lanes (for facilities with frontage roads, the designer
direction average daily traffic (ADT) statistics for the beginning should also specify requirements for mainlanes and frontage
and ending year of the analysis period, directional roads separately) 
distribution, percentage trucks in the ADT, the average ten
• preliminary schematic or straight-line map 
heaviest wheel loads daily (ATHWLD) statistic, the percent
tandem axles in the ATHWLD, and one-direction cumulative • length (in ft.) for each link of the proposed facility (new location
ESALs for both flexible and rigid pavement design. Further projects only).
summary statistics are provided for the percent trucks in
terms of medium duty and heavy duty.
Estimating ESALs
A typical ESAL estimate consists of:

A basic element in pavement 1 Traffic count. A traffic count is used as a starting point for
ESAL estimation.
design is estimating the ESALs a
specific pavement will encounter
over its design life. This helps
determine the pavement 2 A count or estimate of the number of heavy vehicles. Requires
vehicle classification within the traffic count.
structural design (as well as the
HMA mix design in the case of
Superoave). This is done by
forecasting the traffic the 3 An estimated traffic (and heavy vehicle) growth rate over the
design life of the pavement.
pavement will be subjected to
over its design life then

4
converting the traffic to a specific Select appropriate LEFs to convert truck traffic to ESALs. Different
number of ESALs based on its regions may experience different types of loads
makeup.

5 An ESAL estimate. An ESAL estimate can be made based on the

preceding steps.
Figure 1. Resulting damage from Figure 2. Likely cause of increased
a marked increase in ESALs ESALs: increased bus traffic.
II. Calculating Equivalent Single Axial Load
and the Load Equivalency Factors

Equivalent Single Axial Load

The most common historical approach is to convert
damage from wheel loads of various magnitudes and
repetitions (“mixed traffic”) to damage from an equivalent
number of “standard” or “equivalent” loads. The most
commonly used equivalent load in the U.S. is the 18,000 lb
(80 kN) equivalent single axle load (normally designated
ESAL).
Two Standard US ESAL Equation

There are two standard U.S. ESAL

equations one each for flexible and rigid
pavements that are derived from
AASHTO Road Test results. Both these
equations involve the same basic format,
however the exponents are slightly
different.

Flexible Pavement ESAL Equation:

𝑊 𝑥
=¿
𝑊 18
Sample Calculation

Assumptions: Solution:
Single Axle, 30,000 lbs (133kN), SN=3, pt=2.5 G/b30 = -0.2009/4.388 = -0.04578
 
Using the formula above: b18= = 1.2204
W18= predicted number of 18000 lb (80kN) single
 
axle load applications
G/b18= -0.2009/1.2204 = -0.1646
W30= predicted number of 30000 lb (133kN) Thus:
single load axle load applications =
Lx=L30=30
L2x= 1 (single axle)
Serviceability loss factor,
G = log() = -0.2009
 
Curve slope factor,
b30= = 4.388
  Finally:
LEF = 1/0.1260 = 7.9365 ≃7.9 (same as contained in AASHTO
Guide)
 Where: W axle applications inverse of equivalency factors (where W18 = number of 18,000 lb
•Rigid Pavement ESAL Equation (80 kN) single axle loads)

 
Lx axle load being evaluated (kips)

 
L18 18 (standard axle load in kips)

 
L2 code for axle configuration 3 = triple axle (added in the 1986 AASHTO Guide)
1 = single axle x = axle load equivalency factor being evaluated
2 = tandem axle s = code for standard axle = 1 (single axle)

  G
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 "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
Sample Calculation

Assumptions: Solution:
Single Axle, 30,000 lb (133kN), D=7 in., pt=2.5 b18= = 1.3709
  G/b18= -0.1761/1.3709 = -0.12845
Using the formula above:  
W18= predicted number of 18000 lb (80kN) single Thus:
axle load applications
W30= predicted number of 30000 lb (133kN)
single load axle load applications
Finally:
Lx=L30=30
L2x= 1 (single axle) LEF = 1/0.1305 = 7.67 7.7
Serviceability loss factor, (same as contained in AASHTO Guide)
G = log() = -0.1761
Curve slope factor,
b30= = 5.7298
G/b30= -0.1761/5.7298 = -0.03073
 
II. How to calculate ESAL using Average Daily Traffic?

Pavement designers use estimates of the

volume and mix of vehicles anticipated to
use a road in designing a road pavement.
Calculations convert current and forecast
average daily traffic, or ADT, on a road
segment to an equivalent single axle load,
or ESAL, that equals the load of a single
pass of an 18,000-pound truck axle load.

ADT (Average Daily Traffic): The total traffic

volume during a given time period, ranging
from 2 to 364 consecutive days, divided by the
number of days in that time period, and
expressed in vpd (vehicles per day).
 Follow these steps to estimate the ESAL for two-lane, two-way asphalt roads:

1
Assemble current and 20-year forecast of ADT data and the percentage of
trucks. Forecast traffic volumes will come from a formal traffic forecasting
process, or from growth factoring current traffic volumes.

2
Select a design year ADT. The Iowa DOT procedures suggest the
design year ADT should be the current ADT plus 70 to 80 percent of the
20-year forecast increase in ADT.

Calculate the number of trucks in the design year ADT by multiplying the

3
design year ADT by the percentage of trucks. For example, if the design
year ADT is 2,000 vehicles per day and the percent of trucks is estimated
at 10 percent, the number of trucks in the design year is 2,000 x 10
percent, which equals 200 trucks per day.
 4
Apply an ESAL factor to the number of trucks. The Iowa DOT
provides a range for this factor--0.40 for low-volume roads to 0.50
for heavy-truck routes.

Compute the number of ESALs per day by multiplying the number of trucks in the

5 design year by the ESAL factor. For our example, if the appropriate ESAL factor is
0.50, the number of ESALs per day is: 200 trucks per day x 0.50, which equals 100
ESALs per day.

6 Annual ESALs are computed by multiplying the daily ESAL by 300

for low-volume roads or 365 for high-volume roads.

7
Compute the 20-year ESAL by multiplying the annual ESALs by
20. This number is then input into pavement design calculations.
 ESAL = ADT x TKS x DD x LD x TF x 365 x
G or
ESAL = W80 kN x DD x LD x 365 x G

where ;
ADT = Average daily traffic;
TKS = Percentage of truck traffic;
DD = Directional distribution of truck traffic;
LD = Lane distribution of truck traffic;
TF = Average truck factor;
G = Annual traffic growth factor;
W80 kN = Total ESAL of traffic
In the above example, assume that the traffic count was made for a two-way, two lane highway section,
and DD = 0.80, LD = 0.70

TF = 850.53/(120+90) = 4.05,
G = ; where g = Annual traffic growth rate (%),

For the above example if 15 years after traffic (n = 15) is estimated with an annual traffic growth rate of
4%;
G = = 20.02
Note that If n = 1 is used, then G = 1, so that the current year traffic will be estimated.
 
ESAL = 1220 x (120+90)/1220 x 0.80 x 0.70 x 850.53/(120+90) x 365 x 20.02 = 3,480,443.6 or ESAL =
850.53 x 0.80 x 0.70 x 365 x 20.02 = 3,480,443.6
 

`
REFERENCES

References for ESAL: x

  Google
https://www.google.com/url?sa=t&source=web&rct=j&url=https://ftp.dot.state.tx.us/pub/txdot-info/cst/tips/flex_pave_des_fa
q.pdf&ved=2ahUKEwiWg-i71tjzAhUsyIsBHUFgAVAQFnoECAsQBg&usg=AOvVaw0kwF-DwI8Pv6W1bxeW0JnC
https://www.google.com/amp/s/pdf4pro.com/amp/view/what-is-esal-5a0550.html
 
https://pavementinteractive.org/reference-desk/design/design-parameters/equivalent-single-axle-load/
 
https://careertrend.com/how-7652268-calculate-esal-adt.html
 
https://www.google.com/url?sa=t&source=web&rct=j&url=https://
akademik.adu.edu.tr/fakulte/muhendislik/personel/uploads/aoyucel/exampletrafficanalysis-1583219042.pdf&ved=2ahUKEwi
Wg-i71tjzAhUsyIsBHUFgAVAQFnoECAQQAQ&usg=AOvVaw0rc7YZeD6k_dWhTue9bLEf
https://careertrend.com/how-7652268-calculate-esal-adt.html
 
https://www.google.com/url?sa=t&source=web&rct=j&url=https://akademik.adu.edu.tr/fakulte/muhendislik/
personel/uploads/aoyucel/exampletrafficanalysis-1583219042.pdf&ved=2ahUKEwiWg-i71tjzAhUsyIsBHUFgAV
AQFnoECAQQAQ&usg=AOvVaw0rc7YZeD6k_dWhTue9bLEf