Instructional Objectives
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ESAL FLOW MAPS
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Basic concepts of Equivalent Single Axle Loads (ESAL)
Estimate of ESALs consider ing daily, monthly and
seasonal tr uck flows
Use in PMS and pavement design
Load Equivalency
Load Equivalency
LEF=No. of 18 kip ESAL to cause loss of serviceability
No. of X kip axle loads to cause same loss
Where:
Example:
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100,000 reps of a 18 kip (SAL)
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14,347 reps of a 30 kip load (SAL)
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Find LEF 30 kip SAL
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LEF30-kip single = 100,000/14,347 = 6.97
X = Axle load for which equivalency is calculated.
Estimate of Relative Damage
Fourth Power Law
Single Axle Load Equivalency
Relative Damage = (Ratio between axle loads)4
90
Relative damage 30 kip single axle
compared to 18 kip single axle.
LOAD EQIVELANCY
80
Example :
70
60
50
40
30
20
LEF =
(30,000/18,000)4
= 7.71
10
0
2
6
10
14
18
22
26
30
34
38
42
46
50
Truck Factor
Example of Increase In Truck Factor
Rural Interstate Flexible Pavement
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Year
1971
1975
1979
1982
1985
Aver age amount of damage done by one vehicle
in ter ms of ESALs
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May be for all tr ucks (ie. 1 ESAL / Tr uck)
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Best if expr essed for var ious vehicle classes
Truck Weight Data Collection
T F (ESAL /Tr uck)
0.595
0.691
0.766
0.929
0.992
Components of a Monitoring System
Per manent weigh stations
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Tr uck volume by classification
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Por table static scales
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Volume gr owth r ate for each tr uck class
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Weigh-in-motion (WIM)
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Tr uck factor for each tr uck class and gr owth r ate
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Lane distr ibution for the tr uck tr affic
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Var iation in aver age weight of each tr uck type
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Per cent of ESAL occur r ing each month
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ESAL Flow Maps Calculation
ESAL Flow Maps
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Development of a tr uck flow map or ESAL
tr affic load file for PMS
Flor ida and Washington data used
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Deter mine number and distr ibution of AVC and
WIM devices
Adjust data fr om shor t dur ation AVC and WIM to
estimate aver age annual conditions
Appr opr iate length of shor t dur ation counts to meet
r equir ed needs
Truck Flows and Loads
Truck Flows and Loads
Var iability in Tr uck Tr avel Patter ns
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Site Specific Var iation
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Time of Day Var iation
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Day of Week Var iation
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Season of Year
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Geogr aphic Location
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Gr oup Mean Var iation
Design of a Continuous Data Collection
Program for Vehicle Classification and Weight
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Site specific estimates of tr uck loads ar e better than
system means estimates
Number of data collection sites limited by cost and
available wor kfor ce
To Determine the Number of WIM Sites Needed
Step 1:
Step 2:
Step 3:
Deter mine number of continuously oper ating WIM
sites needed
Deter mine number of continuously oper ating AVC
sites needed
Cr eate Gr oups of Roads
Deter mine homogeneity of gr oups
Deter mine number of sites needed
Develop tr uck load (ESAL) file for r oad networ k
based on annual estimates using data measur ed on
“similar ” r oads
Determine Homogeneity
of Groups
Create Groups of Roads
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Divide networ k into r easonably homogeneous tr uck
populations and patter ns
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Subdivide to get lower var iability
Examine similar tr avel patter ns
Plot the daily damage factor s over time and compar e
plots for differ ent sites within each gr oup
Determine the Number of
AVC Sites Needed
Determine the Number of Sites Needed
n =[(Z)(So)/d]2
where:
n = number of sites required.
Z = Z-score for the desired level of confidence
So= Standard deviation of the group damage factor
d = desired precision or allowable error expressed
in damage factor units (ESALs)
Calculate Seasonal and Day
of Week Adjustment Pattern
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Calculate seasonal and day-of-week adjustment
patter ns for all sites
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Deter mine acceptability of initial r oad gr oups by
computing mean and standar d deviation of seasonal
factor s
Calculate Seasonal & Day-of-Week
Adjustment Patter ns
Step 2:
Cr eate Gr oups of Roads.
Step 3:
Deter mine Homogeneity of Gr oups.
Step 4:
Deter mine number of sites needed.
Create Groups of Roads
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Determine Homogeneity
of the Groups
Step 1:
Develop gr oups with r elatively similar tr uck volume
patter ns
Road gr oups may be differ ent fr om the damage
factor gr oups developed for WIM site development
Determine the Number of Sites Required
n =[(Z)(So)/d]2
where:
n = number of sites required
Z = Z-score for desired level of confidence
So= Standard deviation of group damage factor
d = Allowable error expressed as fraction of
mean seasonal factor for group
Develop ESAL File
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Develop ESAL file in same pr ocesses and same file
for mat as annual daily tr affic volume file
Cost Estimate
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Cost Estimate
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21 WIM sites needed for 10,000 kilometer Networ k
acr oss all functional classes
Initial Cost appr oximately $1,000,000 for installation
of 21 WIM Sites
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Annual Oper ations cost about $750,000 with staff of
12
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Appr oximately 60 per manent AVC Sites also
r equir ed to be placed on all functional class highways
Installation and oper ational costs about same for
AVCs as for WIMs; AVC cost less and ar e mor e
dur able but 3 times number r equir ed
AVCs alr eady an integr al par t of monitor ing pr ogr am
for AADT files
Following Guidelines which wer e fir st developed by a
Data Rationalization Study
Cost Estimate
Installation costs based on 19 Piezo Cable systems
and 3 Bending Plate Systems with two lanes
monitor ed at each site
Oper ational costs based on costs to calibr ate sites,
maintain sites and r eplace sites as they fail, plus
occasional pavement r epair to pr ovide smooth
appr oach to weight system
Summary
Cost Estimate
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Based on WSDOT Highway Networ k
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Module focuses on how to obtain accur ate tr affic
data
Rough costs to develop and maintain WIM System
to meet these guidelines
Input data for use in PMS affect r eliability of
pr ediction models and pavement designs
Instructional Objectives
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Basic concepts of Equivalent Single Axle Loads (ESAL)
Estimate of ESALs consider ing daily, monthly and
seasonal tr uck flows
Use in PMS and pavement design