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LAYERED ELASTIC COMPUTATIONAL PROGRAM
The layered elastic computational program used in LEDFAA 1.2, JULEA, was written in FORTRAN and
implemented as a Windows executable. The FORTRAN code was taken directly from the LEDNEW collection of
programs (see reference 8). Documentation for the program structure and mathematical basis is not available and
upgrading the program to match the new implementation of LEDFAA proved to be quite difficult. A new layered
elastic program, called LEAF, was therefore written from scratch in VB6. It is implemented as an ActiveX dynamic
link library with a defined interface and does not share any code with JULEA (7). Numerical results from LEAF and
JULEA are, for all practical purposes, identical except when the evaluation point is close to the top of the top layer.
This has no practical effect because none of the design procedures in LEDFAA require that pavement responses be
calculated close to the top of the top layer.
AIRCRAFT LIBRARY UPDATE
Several changes and additions have been made to the aircraft library in LEDFAA.
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• Added
o B-717
o B-737-900
o B-747-400ER
o B-767-400ER
o B-777-200LR
o B-777-300ER
• Corrected the dual spacing on A300-600 (from 27.0 in to 36.5 in (685.8 mm to 927.1 mm))
• Changed the name of the existing A340 to A340-200/300
• Added
o A300-600-opt
o A320-opt
o A340-500/600
o A380-800
o A380-800F
FLEXIBLE DESIGN PROCEDURE CHANGES
For flexible pavement thickness design, LEDFAA (and LEDNEW before it) uses the maximum vertical strain at the
top of the subgrade as the predictor of subgrade shear failure, which in turn is assumed to protect against rutting
failure of the complete structure. In outline, the design procedure first computes the maximum vertical strain at the
top of the subgrade for one of the aircraft, followed by computing, from the failure model, the number of departures
to subgrade failure for that aircraft. The assumed number of departures over the design life is then divided by the
predicted number of departures to failure to give the cumulative damage factor (CDF) for that aircraft. The CDFs for
all aircraft in the design mix are added together and if the sum exceeds a value of one the pavement is predicted to
fail before the end of the design life. If less than one, the pavement is predicted to last longer than the design life.
The thickness of one of the pavement layers is adjusted until the total CDF is equal to one (plus or minus a specified
tolerance).
From LEDFAA 1.2 to 1.3, significant changes have been made to two aspects of the design procedure. The
first involves changes made to the criteria for subgrade failure and the second involves changes to the computation
of vertical strain and CDF. The computation of CDF requires that the failure criteria be defined mathematically and,
consequently, changes to the failure model affect the thickness design by changing the computed values of CDF.
Failure model changes are therefore discussed first.
Failure Model for Subgrade Vertical Strain Criteria
The failure model used in LEDFAA 1.2 is expressed by the equation
C
ESG
SG
v
E
= ×
 + ×
 
 

 
×
10 000
0 000247 0 000245
0 0658 0 559
,
. . ( )
. .
log10

and is shown in figure 2 plotted on a graph of vertical strain versus coverages to failure. Also shown in the figure are
data points from full-scale tests where rutting of the subgrade is known to have occurred. Vertical strains were
computed using a layered elastic model of the structures, and coverages to failure were computed from the number
of passes to failure in the full-scale tests and the pass-to-coverage model embedded in LEDFAA.
The model expresses coverages to failure, C, as a function of vertical strain at the top of the subgrade, 
v,
and subgrade modulus, ESG. Figure 2 illustrates the model with four straight lines, one each for subgrade modulus
values of 4,500, 9,000, 15,000, and 22,500 psi (31.0, 62.0, 103.4, and 155.1 Mpa, and equivalent to 3, 6, 10, and 15
CBR). At low coverage values, allowable strain increases with decreasing modulus. This is equivalent to decreasing
the effective modulus of the subgrade when computing vertical strain to account for the more pronounced plasticity
typically exhibited by low-modulus subgrades (or converting resilient modulus to a secant modulus, as is sometimes
done). See reference 4 for the development of the model. However, the model used to represent this behavior
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reverses the trend at about 2,000 coverages and it was found to be difficult to make LEDFAA 1.2 thickness designs
　
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