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2.3.7 Determination of Structural Number Figure 2.2 shows the nomograph to determine Structure Number. The nomograph can be
used if all following parameters are available: 1.
Traffic estimation in the future W
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at the end of design life 2.
Level of reliability R 3.
Standard deviation S
o
4. Effective resilient modulus of subgrade material M
R
5. Loss of serviceability ∆PSI = p
o
- p
t
The calculation of pavement thickness in this guideline is based on the relative strength of each pavement layers, using formula as shown in Equation 2.1.
2.4 Single Axle Load Equivalency Factor LEFs
Using the fourth-power relationship found at the AASHO Road Test, equations were derived to relate axle loading to pavement damage. Replicate cross sections were constructed
in different test loops to apply varying repeated axle loads on the same pavement structure. This allowed the researchers at the road test to view the damage caused by heavier axles, and
create mathematical relationships based upon that damage. The resulting pavement damage was quantified using single axle load equivalency factors LEFs, which are used to find the
number of ESALs. An LEF is used to describe the damage done by an axle per pass relative to the damage done by a standard axle per pass. This standard axle is typically an 18-kip
single axle, as defined in the road test. From the AASHO Road Test results, the LEF can be expressed in the following form according to Huang 2004. The EALF can be expressed in
the following form according to Huang 2004:
2.11
To arrive at the design ESALs , it is necessary to assume a structural number SN and then select the equivalence factors listed in nine tables provided by AASHTO 1993. These
tables vary by three types of axle and three values of terminal serviceability p
t
. The use of
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SN of 5 for the determination of 18-kip single axle equivalence factors will normally give results that are sufficiently accurate for design purposes. Even though the final design may be
somewhat different, this assumption will usually result in an over estimation of 18-kip equivalent single axle when more accurate results are desired and the computed design is
appreciably different 1 inch of asphalt concrete from the assumed value. A new value should be assumed and the design 18
–kip ESAL traffic W
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recomputed. The procedure should be continued until the assumed and computed values are sufficiently close AASHTO, 1993.
2.5 Previous of Studies
