Download - Design Traffic Load
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DESIGN TRAFFIC LOADTRAFFIC COUNT DATA, AXLE WEIGHT DATA, GROWTH OF TRAFFIC
passage of people or vehicles moving on a road
or public highway.
TRAFFIC
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AXLE
A supporting shaft
or member on or
with which a
wheel or a set of
wheels revolves.
AXLE LOAD
The axle load of a wheeled vehicle is the
total weight felt by the roadway for all
wheels connected to a given axle. Viewed
another way, it is the fraction of total vehicle
weight resting on a given axle.
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GENERAL
For the purposes of structural pavement design, it is
necessary to determine the traffic load to be carried
by the pavement during the design life.
The traffic which will pass over the pavement
will be composed of a number of different vehicle
types, each of which will apply a different load.
In order to combine these loads, the concept
of an equivalent standard axle (ESA) based on an
8 tonne axle load has been developed.
GENERAL
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The total number of equivalent standard axles
which will pass over the pavement during the selected
design life is the design traffic load.
For pavement design purposes, axle loads less than
3 tonnes can be ignored. This eliminates private saloon
cars and most jeepneys from the calculations of design
traffic load in equivalent standard axles. As a guide a
vehicle having at least one axle with dual wheels and/or
having more than two axles should be considered.
GENERAL
3 axles
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MAIN PARAMETERS
There are four fundamental factors to be
determined in order to be able to compute the
cumulative equivalent standard axles for
pavement design purposes.These are as follows:
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MAIN PARAMETERS
(a) Existing traffic flows on the length of road under
consideration, subdivided by vehicle class.
(b) A sample of axle weights for each class of heavy
goods vehicle.
(c) Growth rates for each class of vehicles, or an over
all growth rate applicable to all classes of vehicle.
(d) Design life for the pavement.
Conduct classified
t raffic count
Process data to
determine average
annual daily t raffic
(AADT) by vehicles
type
Obtain grow th
factors and apply to
AADT to determine
t raffic by vehicle
class at t ime of
opening road.
Apply grow th factors
to opening traffic by
vehicle class to
determine cumulat ive
t raffic over design life
by vehicles class
Apply equivalent factor by
vehicles class to determine
cumulat ive ESA by vehicle
class over design life
Sum cumulat ive ESA by
vehicle class to determine
total ESA for design life.
Conduct ax le
w eight survey by
vehicles class for
vehicles > 3 tonne
gross w eight
Apply equivalence
factors to ax le
w eights to
determine average
equivalence factor
per ax le
Determine average
equivalence factor
by vehicle type.
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TRAFFIC COUNT DATA
Whether collected by a traffic survey conducted specifically for the project
under consideration, or based on earlier surveys on the same length of road,
the minimum requirements from the traffic survey data are as follows:
(i) Average annual daily traffic (AADT) i.e. average total flow of traffic on the
road in a 24-hour period. This is normally a 2 way flow i.e. the total of the
vehicular flow in each direction.
(ii) The make-up or composition of the AADT in terms of the number of
vehicles in each vehicles classification. This may be presented as a
percentage of flow for each vehicles classification.
AXLE WEIGHT DATA
At the same time as the traffic count, it is desirable to
conduct an axle weight survey.An axle load survey should
be carried out by weighing a sample of vehicles at the
roadside. The sample should be chosen such that a
maximum of about 60 vehicles per hour are weighed.
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Equivalence Factor
Equivalence Factor = Axle Load in kg
8160
4.5
Wheel load (single and dual) Axle load Equivalence Factor
(1000 kg) (1000 kg)
1.50 3 0.01
2.00 4 0.04
2.50 5 0.11
3.00 6 0.25
3.50 7 0.50
4.00 8 0.91
4.50 9 1.55
5.00 10 2.50
5.50 11 3.83
6.00 12 5.67
6.50 13 8.13
7.00 14 11.35
7.50 15 15.48
8.00 16 20.70
8.50 17 27.19
9.00 18 35.17
9.50 19 44.85
10.00 20 56.50
Equivalence Factors For
Different Axle Loads
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TYPE OF VEHICLE
TRUCK W/ 2 AXLES
(6 WHEELS)TRUCK W/ TANDEM REAR WHEELS
(3 AXLES/10 WHEELS)
TYPE OF VEHICLE
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GROWTH OF TRAFFICThe determination of the cumulative total of equivalent
standard axles (ESA) that will pass over the pavement during the
design life must take into consideration the growth or increase in
traffic over the selected designlife.
The determination of appropriate growth factors can be a
complex procedure. Unless accurate information is available,
from which predictions can be made with a high degree of
confidence, it is recommended that average growth figures should
be obtained from Planning Services Department of DPWH.
Formula for Traffic Growth
Factor at time of opening year
Where:
i = growth rate, %
n = no.of years from initial study to the opening year
GF = (1 + i)n
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Formula for Traffic Growth
Factor over design lifeUsing summationnotation:
(1 + i)x
n−1
x=0
Where:
i = growth rate, %
n = analysis period, years
GF = (1 + i)n
Analysis Period Years (n) No Growth 2.00 4.00 5.00 6.00 7.00 8.00 10.00
1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
2 2.00 2.02 2.04 2.05 2.06 2.07 2.08 2.10
3 3.00 3.06 3.12 3.15 3.18 3.21 3.25 3.31
4 4.00 4.12 4.25 4.31 4.37 4.44 4.51 4.64
5 5.00 5.20 5.42 5.53 5.64 5.75 5.87 6.11
6 6.00 6.31 6.63 6.80 6.98 7.15 7.34 7.72
7 7.00 7.43 7.90 8.14 8.39 8.65 8.92 9.49
8 8.00 8.58 9.21 9.55 9.90 10.26 10.64 11.44
9 9.00 9.75 10.58 11.03 11.49 11.98 12.49 13.58
10 10.00 10.95 12.01 12.58 13.18 13.82 14.49 15.94
11 11.00 12.17 13.49 14.21 14.97 15.78 16.65 18.53
12 12.00 13.41 15.03 15.92 16.87 17.89 18.98 21.38
13 13.00 14.68 16.63 17.71 18.88 20.14 21.50 24.52
14 14.00 15.97 18.29 19.16 21.01 22.55 24.21 27.97
15 15.00 17.29 20.02 21.58 23.28 25.13 27.15 31.77
16 16.00 18.64 21.82 23.66 25.67 27.89 30.32 35.95
17 17.00 20.01 23.70 25.84 28.21 30.84 33.75 40.55
18 18.00 21.41 25.65 28.13 30.91 34.00 37.45 45.60
19 19.00 22.84 27.67 30.54 33.76 37.38 41.45 51.16
20 20.00 24.30 29.78 33.06 36.79 41.00 45.76 57.28
25 25.00 32.03 41.65 47.73 54.86 63.25 73.11 98.35
30 30.00 40.57 56.08 66.44 79.06 94.46 113.28 164.49
35 35.00 49.99 73.65 90.32 111.43 138.24 172.32 271.02
Note: The above growth factors multiplied by the first year traffic estimate will give the total volume
of traffic expected during the analysis period
Annual Growth Rate, Percent
Traffic Growth Factors
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DESIGN LIFE
Since pavements, new or rehabilitated, are usually designed for
period ranging from 10 years to 20 years or more, it is necessary to
predict the ESA for this period of time i.e. the performance period.
The performance period often referred to as the design period or
design life of the pavement is defined as the period of time that an
initial (or overlaid) pavement will last before reaching its terminal
serviceability.
For roads in the Philippines, it is normal to use a design life of
10 years for asphalt surfaced roads and 20 years for concrete
surfaced roads.
UNEQUAL TRAFFIC LOADS
In some circumstances, it may be determined that the
traffic loading in one direction along a road is much
greater than that in the opposite direction. In this case,
design traffic loads computations should be carried out
for each direction and the highest loading used for
pavement design purposes.
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EXAMPLE CALCULATION of DESIGN
TRAFFIC LOAD DETERMINATIONThe following data are results of an earlier traffic counts and axle weight
surveys:
(a) Initial AADT (Year of Study: 1995) = 3,000 vehicles per day
(b) Buses make up 5% of the AADT, 2 axle trucks 8% and others 4%
(c) Equivalence factors have been established as follows:
Buses = 1.2 per bus
2 axle trucks = 1.85 per truck
Other trucks = 1.37 per truck
(d) Growth rates to determine AADT by vehicle class at opening year:
Buses = 3%
2 axle trucks = 5%
Other trucks = 5%
The opening year is 1998 and the selected design life is 20 years (rigid
pavement). Determine the design traffic load.
Conduct classified
t raffic count
Process data to
determine average
annual daily t raffic
(AADT) by vehicles
type
Obtain grow th
factors and apply to
AADT to determine
t raffic by vehicle
class at t ime of
opening road.
Apply grow th factors
to opening traffic by
vehicle class to
determine cumulat ive
t raffic over design life
by vehicles class
Apply equivalent factor by
vehicles class to determine
cumulat ive ESA by vehicle
class over design life
Sum cumulat ive ESA by
vehicle class to determine
total ESA for design life.
Conduct ax le
w eight survey by
vehicles class for
vehicles > 3 tonne
gross w eight
Apply equivalence
factors to ax le
w eights to
determine average
equivalence factor
per ax le
Determine average
equivalence factor
by vehicle type.
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Solution
Initial AADT = 3,000 vehicle per day
Buses = 0.05 x 3000 = 150 per day
2 Axle Trucks = 0.08 x 3000 = 240 per day
Other Trucks = 0.04 x 3000 = 120 per day
2. Apply growth factors to determine AADT by vehicle class at opening year.
1. Determine average annual daily traffic (AADT) by vehicles type.
From 1995 (Year of Study) to 1998 (Opening year) = 3 years
Therefore growth by class is as follows:
Buses = 150 x 1.033 = 164 per day
2 Axle Trucks = 240 x 1.053 = 278 per day
Other Trucks = 120 x 1.053 = 139 per day
3. Apply growth factors to each class to determine total number of axles that will pass over the
pavement in the design life.
Analysis Period Years (n) No Growth 2.00 4.00 5.00 6.00 7.00 8.00 10.00
1 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
2 2.00 2.02 2.04 2.05 2.06 2.07 2.08 2.10
3 3.00 3.06 3.12 3.15 3.18 3.21 3.25 3.31
4 4.00 4.12 4.25 4.31 4.37 4.44 4.51 4.64
5 5.00 5.20 5.42 5.53 5.64 5.75 5.87 6.11
6 6.00 6.31 6.63 6.80 6.98 7.15 7.34 7.72
7 7.00 7.43 7.90 8.14 8.39 8.65 8.92 9.49
8 8.00 8.58 9.21 9.55 9.90 10.26 10.64 11.44
9 9.00 9.75 10.58 11.03 11.49 11.98 12.49 13.58
10 10.00 10.95 12.01 12.58 13.18 13.82 14.49 15.94
11 11.00 12.17 13.49 14.21 14.97 15.78 16.65 18.53
12 12.00 13.41 15.03 15.92 16.87 17.89 18.98 21.38
13 13.00 14.68 16.63 17.71 18.88 20.14 21.50 24.52
14 14.00 15.97 18.29 19.16 21.01 22.55 24.21 27.97
15 15.00 17.29 20.02 21.58 23.28 25.13 27.15 31.77
16 16.00 18.64 21.82 23.66 25.67 27.89 30.32 35.95
17 17.00 20.01 23.70 25.84 28.21 30.84 33.75 40.55
18 18.00 21.41 25.65 28.13 30.91 34.00 37.45 45.60
19 19.00 22.84 27.67 30.54 33.76 37.38 41.45 51.16
20 20.00 24.30 29.78 33.06 36.79 41.00 45.76 57.28
25 25.00 32.03 41.65 47.73 54.86 63.25 73.11 98.35
30 30.00 40.57 56.08 66.44 79.06 94.46 113.28 164.49
35 35.00 49.99 73.65 90.32 111.43 138.24 172.32 271.02
Annual Growth Rate, Percent
Traffic Growth Factors
For 5% growth rate, use the table below.
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Buses = 164 x 26.87 x 365 = 1,608,461
2 Axle Trucks = 278 x 33.06 x 365 = 3,354,598
Other Trucks = 139 x 33.06 x 365= 1,677,299
For 3%, use the following formula:
GF = (1 + i)x
n−1
x=0
GF = (1 + 0.03)x
20−1
x=0
GF = 26.87
Therefore,
Buses = 1,608,461 x 1.2 = 1,930,153
2 Axle Trucks = 3,354,598 x 1.85 = 6,206,006
Other Trucks = 1,677,299 x 1.37 = 2,297,900
5.Total ESAL.
4. Multiply each class by respective equivalence factors to determine ESAL.
Total ESAL = 1,930,153 + 6,206,006 + 2,297,900 = 10,434,059
Thus, the total ESA is 10,434,059. Therefore, the design traffic
load is per lane of carriageway which is 50% of ESA equal to 5.217 x 106
ESA.