ce522a 02 traffic loading and volume [compatibility mode]

Chapter 6

Traffic Loading and

VolumeVolume

Yang H. Huang, Pavement Analysis and Design (2nd Edition), Prentice Hall, Inc., 2004

Traffic Loading and Volume

Design Procedures

Traffic is the most important factor in pavement design. The

consideration of traffic should include both the loading

magnitude and configuration and the number of load magnitude and configuration and the number of load

repetitions.


Design Procedures

There are three different procedures for considering vehicular

and traffic effects in pavement design:

• fixed traffic• fixed traffic

• fixed vehicle

• variable traffic and vehicle


Fixed Traffic

In fixed traffic, the thickness of pavement is governed by a

single-wheel load, and the number of load repetitions is not

considered as a variable. considered as a variable.

If the pavement is subjected to multiple wheels, they must be

converted to an equivalent single-wheel load (ESWL ) so that the

design method based on a single wheel can be applied .


Fixed Vehicle

In the fixed vehicle procedure, the thickness of pavement is

governed by the number of repetitions of a standard vehicle or

axle load, usually the 18-kip (80-kN) single-axle load. axle load, usually the 18-kip (80-kN) single-axle load.

If the axle load is not 18 kip (80 kN) or consists of tandem or

tridem axles, it must be converted to an 18-kip single-axle load

by an equivalent axle load factor (EALF) .


Fixed Vehicle

The number of repetitions under each single- or multiple-axle

load must be multiplied by its EALF to obtain the equivalent

effect based on an 18-kip (80-kN) single-axle load. effect based on an 18-kip (80-kN) single-axle load.

A summation of the equivalent effects of all axle loads during the

design period results in an equivalent single-axle load (ESAL),

which is the single traffic parameter for design purposes.


Variable Traffic and Vehicle

For variable traffic and vehicles, both traffic and vehicle are

considered individually, so there is no need to assign an

equivalent factor for each axle load. equivalent factor for each axle load.

The loads can be divided into a number of groups, and the

stresses, strains, and deflections under each load group can be

determined separately and used for design purposes .


Variable Traffic and Vehicle

This procedure is best suited for the mechanistic methods of

design, wherein the responses of pavement under different

loads can be evaluated by using a computer .loads can be evaluated by using a computer .


EQUIVALENT SINGLE-WHEEL LOAD

Equal Vertical Stress Criterion

Equal Vertical Deflection Criterion

Equal Tensile Strain CriterionEqual Tensile Strain Criterion

Criterion Based on Equal Contact Pressure



The ESWL can be determined from theoretically calculated or

experimentally

measured stress, strain, or deflection . It can also be determined measured stress, strain, or deflection . It can also be determined

from pavement distress and performance such as the large-scale

WASHO and AASHO road tests (HRB, 1955, 1962).

Any theoretical method can be used only as a guide and should

be verified by performance. This is particularly true for the

empirical methods of design in which the ESWL analysis is an

integral part of the overall design procedure .



Erroneous results may be obtained if different ESWL methods

are transposed for a given set of design curves.




The method assumes that

the ESWL varies with the the ESWL varies with the

pavement thickness, as shown

in Figure 6.1 .




For thicknesses smaller than half the

clearance between dual tires:clearance between dual tires:

ESWL is equal to one-half the total

load

indicating that the subgrade

vertical stresses caused by the

two wheels do not overlap .




For thicknesses greater than twice

the center to center spacing of the center to center spacing of

tires:

ESWL is equal to the total load

indicating that the subgrade

stresses due to the two wheels

overlap completely.




By assuming a straight -line relationship between pavement

thickness and wheel load on logarithmic scales, the ESWL for any thickness and wheel load on logarithmic scales, the ESWL for any

intermediate thicknesses can be easily determined .

After the ESWL for dual wheels is found, the procedure can be

applied to tandem wheels.




Instead of plotting, it is more convenient to compute the ESWL

by by


EQUIVALENT SINGLE-WHEEL LOAD: Equal Vertical Stress Criterion


EQUIVALENT SINGLE-WHEEL LOAD: Equal Vertical Stress Criterio n




In this method, the pavement system is considered as a homogeneous half-

space and the vertical deflections at a depth equal to the thickness of the

pavement can be obtained from Boussinesq solutions. pavement can be obtained from Boussinesq solutions.

A single-wheel load that has the same contact radius as one of the dual

wheels and results in a maximum deflection equal to that caused by the dual

wheels is the ESWL.




Since then ESWL for layered systems is greater than that for a homogeneous

half-space, Huan g (1968b) suggested the use of layered theory and

presented a simple chart for determining ESWL based on the interface presented a simple chart for determining ESWL based on the interface

deflection of two layered systems, as shown in Figure 6.4.

Given contact radius a, pavement thickness h1, modulus ratio E1/E2 , and

dual spacing Sd , the chart gives a load factor

EQUIVALENT

SINGLE-WHEEL

LOAD

Figure 6.4

Chart for Chart for

determining

equivalent single

wheel load.

(1in = 25.4mm)



The procedure can be summarized as follows:

Traffic Loading and VolumeEQUIVALENT SINGLE-WHEEL LOAD: Equal Tensile Strain Criterion


EQUIVALENT SINGLE-WHEEL LOAD: Equal Tensile Strain Criterion



Equal Tensile Strain Criterion



Criterion Based on Equal Contact Pressure

All of the above analyses of ESWL are based on the assumption that the single

wheel has the same contact radius as each of the dual wheels.

Another assumption, which has been frequently made, is that the single

wheel has a different contact radius but the same contact pressure as the

dual wheels .


EQUIVALENT AXLE LOAD FACTOR

An axle is a central shaft for a rotating wheel or gear. ..



An equivalent axle load factor (EALF) defines the damage per pass to a

pavement by the axle in question relative to the damage per pass of a

standard axle load, usually the 18-kip (80-kN) single-axle load.

The design is based on the total number of passes of the standard axle load

during the design period, defined as the equivalent single-axle load (ESAL)

and computed by



The EALF depends on the type of pavements, thickness or structural capacity

, and the terminal conditions at which the pavement is considered failed .

Most of the EALFs in use today are based on experience.

One of the most widely used methods is based on the empirical equations

developed from the AASHO Road Test (AASHTO, 1972).

The EALF can also be determined theoretically based on the critical stresses

and strains in the pavement and the failure criteria.



Flexible Pavements



Rigid Pavements

AASHTO Equivalent Factors The AASHTO equations for determining the EALF

of rigid pavements are as follows :



Rigid Pavements


Traffic Analysis


Traffic Analysis

To design a highway pavement, it is necessary to predict the number of

repetitions of each axle load group during the design period.

Information on initial traffic can be obtained from field measurements or Information on initial traffic can be obtained from field measurements or

from the W-4 form of a loadometer station that has traffic characteristics

similar to those of the project in question .

The initial daily traffic is in two directions over all traffic lanes and must be

multiplied by the directional and lane distribution factors to obtain the initial

traffic on the design lane .


Traffic Analysis

The traffic to be used for design is the average traffic during the design

period, so the initial traffic must be multiplied by a growth factor . If n, is the

total number of load repetitions to be used in design for the ith load group,

thenthen


Traffic Analysis

If the design is based on the equivalent 18-kip (80-kN) single-axle load, then

the initial number of repetitions per day for the ith load group can be

computed from


Traffic Analysis


Average Daily Truck Traffic


Average Daily Truck Traffi c

The minimum traffic information required for a pavement design is the

average daily truck traffic (ADTT) at the start of the design period . The ADTT

may be expressed as a percentage of ADT or as an actual value. This

information can be obtained from the actual traffic counts on the existing information can be obtained from the actual traffic counts on the existing

roadway where the pavement is to be constructed or on nearby highways

with similar travel patterns .


Truck Factor

A single truck factor can be applied to all trucks, or separate truck factors can

be used for different classes of trucks .

The latter case should be considered if the growth factors for different types The latter case should be considered if the growth factors for different types

of trucks are not the same .



Rigid Pavements

Traffic Loading

and VolumeTruck Factor


Growth Factor


Lane Distribution Factor

For two-lane highways, the lane in each direction is the design lane, so the

lane distribution factor is 100%.

For multilane highways, the design lane is the outside lane. Table 6 .14 showsFor multilane highways, the design lane is the outside lane. Table 6 .14 shows

the truck distribution on a multiple-lane highway based on 129 counts from

1982 to 1983 in six states (Darter et al., 1985).

For four-lane highways with two lanes in each direction, the lane distribution

factors range from 66 to 94% .

For multiple-lane highways with three or more lanes in each direction, the

lane distribution factors range from 49 to 82% .


Lane Distribution

Factor


Lane Distribution Factor



Seatwork

6.6 Estimate the equivalent 18-kip single-axle load applications (ESAL) for a

four-lane pavement (two lanes in each direction) of a rural interstate highway

with a truck count of 1000 per day (including 2-axle, 4-tire panel, and pickup with a truck count of 1000 per day (including 2-axle, 4-tire panel, and pickup

trucks), an annual growth rate of 5%, and a design life of 20 years. [Answer: 2

.82 X 106 ]

Important points in Chapter 6

The concept of load equivalency has been used most frequently in the

empirical methods of pavement design.

In the mechanistic methods, it is not necessary to apply the load equivalency

concept because different loads can be considered separately in the design

process . This is why the concept has been used more frequently on flexible process . This is why the concept has been used more frequently on flexible

pavements than on rigid pavements .


There are two types of load equivalency:

• equivalent single-wheel load (ESWL) and

• equivalent axle load factor (EALF)

The ESWL is based on the fixed traffic procedure of converting the most

critical wheel loads, usually in multiple wheels, into an equivalent single-critical wheel loads, usually in multiple wheels, into an equivalent single-

wheel load and using it for design purposes.

The EALF is based on the fixed vehicle procedure of converting the number of

repetitions of a given axle load, either single, tandem, or tridem, into an

equivalent number of repetitions of an 18-kip (80-kN) single-axle load.


Various criteria have been used for determining the ESWL for a two-layer

system, e.g., equal vertical interface stress, equal interface deflection, and

equal tensile strain.

The equivalency based on one criterion may be quite different from that

based on other criteria . Also, the equivalency based on equal contact radius based on other criteria . Also, the equivalency based on equal contact radius

is different from that based on equal contact pressure.


The values of EALF depend on the failure criterion employed . The EALF based

on fatigue cracking is different from that based on permanent deformation .

The use of a single value for both modes of failure is approximate at best. The

most widely used method for determining the EALF is that which uses the

empirical equations developed from the AASHO Road Test . However, damage empirical equations developed from the AASHO Road Test . However, damage

analyses by KENLAYER and KENSLABS indicate that the values of EALF vary a

great deal and are difficult to predict.

The complex interactions among a large number of variables make it

impossible to select an appropriate EALF that can be applied to all situations .

For a truly mechanistic design method, each load group should be considered

individually, instead of via equivalent single-axle loads .


The traffic to be used in design is not the average traffic during the first year,

but the average during the design period.

Three different methods based on the compound rate are presented to

compute the traffic growth factor.

The use of average traffic at the start and the end of the design period gives

the highest growth factor, while the use of the traffic at the middle of the

design period results in the lowest growth factor .


Traffic is the most important factor in pavement design.

Every effort should be made to collect actual traffic data. If actual data are

not available, Tables 6 .9 and 6.10, which give average values for different

classes of highways in the United States, can be used as a guide.


In addition to the initial traffic and the growth factor, the directional and lane

distribution factors must also be considered.

Unless the traffic loading or volume is heavier in one direction than in the

other due to some special reason, a directional distribution of 0.5 is assumed.

The lane distribution factor varies with the number of lanes and the ADT and

can be obtained from Figure 6 .8 or Tables 6 .15 and 6.16, although these

tables do not consider ADT as a factor .


When the design is based on the equivalent 18-kip (80-kN) single-axle load,

the use of a truck factor is very convenient .

The method for computing truck factors based on the number of axles and

trucks weighed, number of trucks counted, and the AASHTO equivalent

factors is illustrated.factors is illustrated.

****PRE-MIDTERM EXAM IS SCHEDULED THIS THURSDAY*****

ce522a 02 traffic loading and volume [compatibility mode]

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