I recently met with Ralph Gillmann at the WIM conference in Portugal, and
we had a discussion about the merits of storing ESALs (Equivalent Standard
Axle Loads) in traffic databases.
Ralph works with traffic databases at FHWA. These generally store ESALs as
the main measure of traffic loading over a particular site. I am
personally against this practice, and believe that traffic databases should
store probability distributions (histograms) of the raw axle load data for
each axle group type (single axle, tandem, triaxle, etc).
We have subsequently continued our discussion by email. I thought it would
be interesting to widen the discussion to include others on the
road-transport-technology email list. The following is the text of a
recent message. (Please note that Ralph stresses that his views are his
own and do not represent any official FHWA position.)
Comments by other list members would be most welcome...
Gillmann:
------------
>I wanted to ask about your objection to ESALs. I wasn't sure if it's about
>the particular functional form of ESALs or what.
>
>Consider pavement design as a function of a load vector L (axle group
>spectrum) and other (O) factors: f(L,O). You seem to object to reducing L to
>a scalar ESAL. Fine. But L must be implicitly reduced to determine the
>design scalars (depth, etc.). So the dispute seems to be whether you reduce L
>to a scalar as an input to the final pavement design or the reduction to
>scalars occurs with the final design.
Cebon:
---------
I disagree entirely that L has to be reduced to a scalar. Consider
structural design of an engineering structure such as an aircraft wing.
The designer develops a design concept and then simulates (eg) the fatigue
life of the structure using a realistic spectrum of loading - with variable
amplitude and frequency. There is no attempt to reduce the service loading
history to an equivalent number of 'standard wing bends'. This would
severly reduce the ability to make accurate life predictions.
So it should be with pavement design. Long gone are the days when it was
necessary to design pavements using a simple chart or a slide rule - when
it was necessary to have a single number of ESALs representing all traffic.
There are now an increasing number of very powerful pavement modelling
programs which are capable of making realistic predictions of pavement life
as a function of both traffic and environmental loads. These programs
cannot use traffic data measured in ESALS - because the fourth power law
used in calculating the number ESALs implicitly assumes a mode of pavement
failure.
The fourth power is a reasonable exponent in a fatigue damage model. It is
completely unsuited to (for example) permanent deformation of asphalt. The
new generation of realistic pavement modelling programs need, as input,
probability distributions of the axle loads (or perhaps axle group loads) -
not weighted by arbitrary powers. (I am not the only person that believes
the fourth power law to be inaccurate. There is a wide body of evidence to
suggest that the power should be anything from 1 to 12, depending on the
application.)
By coverting everything to ESALS, the traffic data continues to be affected
by the 4th power law assumption - or whatever power is used - because there
is no way of getting back to the raw data. The possibility of using the
data in the pavement design programs of the future is therefore being
thrown away. However, storing the axle load distributions gets you the
best of both worlds - because they can easily be converted to ESALs if
necessary.
Now is the time to recognise that ESALs are not the future, and to make
sure that all traffic data is stored and reported in a way that will be
useful to future generations. The legacy of the AASHO road test has lasted
for 40 years. We should be aiming for a similar lifespan for LTPP data and
other traffic data being collected today.
- David Cebon
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Dr David Cebon
Cambridge University Engineering Department
Trumpington Street, Cambridge CB2 1PZ, UK
Tel +44 1223 332665 Email: [log in to unmask]
Fax +44 1223 332662 URL: http://club.eng.cam.ac.uk/~dc
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