C = C
v
when C 12,100
0.002428
14.21
>
=
v
C
It should be noted that, because of the use of the LEDNEW Modulus procedure (4) for aggregate layers in
LEDFAA, increased sensitivity of design thickness to reduced subgrade modulus (or strength) has not been
completely abandoned by eliminating dependence on subgrade modulus in the failure model. The Modulus
procedure adjusts the modulus values of unbound aggregate layers to be compatible with modulus values of the
bounding layers. This is intended to represent the assumed inability of aggregate layers to support tensile stresses.
But also has the effect of reducing the effective stiffness of the complete structure when subgrade modulus is
reduced and, consequently, the tendency increases for the layered elastic model to predict significant tensile stresses
at the bottom of the aggregate layers above the subgrade. The effect is demonstrated by running LEDFAA with two
different subgrade modulus values and noting the modulus values of the aggregate layers above the subgrade.
Computation of Vertical Strain and Cumulative Damage Factor (CDF)
When computing the CDF of B-747 aircraft in LEDFAA 1.2, subgrade strain is computed from the loads applied by
the four wheels in a single dual-tandem landing gear truck. Pass-to-coverage is then computed for all 16 wheels and
the CDF computed as described below with four-wheel strain and sixteen-wheel pass-to-coverage. This
methodology is reasonable in the sense that each truck in the main gear has the same geometry and the same number
of wheels. But when trying to add the A380 to the program, difficulties in implementation by the same methodology
are immediately apparent because the aircraft has different numbers of wheels in the wing and body trucks (four and
six, respectively). After completing an analytic study of the effects on thickness design of including different
numbers of wheels in the strain computation, it was decided to change the methodology for computing CDF for
multiple-gear aircraft such as the B-747 and the A380 so that all of the wheels in the main landing gear are assumed
to contribute to the maximum strain to be used for design. This assumption is also compatible with the B-747 design
charts in AC 150/5320-6D, which were generated with equivalent single wheel load (ESWL) found using all sixteen
wheels in the main landing gear and pass-to-coverage computed for one four-wheel truck. The pass-to-coverage
computation is reasonable because it is based on the width of the tires at the surface and adding more gears has little
effect on the computed number. The methodology used to generate the design charts also complies with the CBR
design procedure as originally defined by the U.S. Army Corps of Engineers, which requires that the number of
wheels used to compute ESWL for design be that which gives the thickest pavement over all possible combinations
of wheels.
The computation of vertical strain and CDF is straightforward in principle but can become quite
complicated in the programming details. The procedure used in LEDFAA 1.2 is therefore described first so that the
significance of the changes is clearer. The LEDFAA 1.2 procedure closely follows the programmed implementation
in LEDNEW:
1. Select a “representative” gear for response computation. For two-gear aircraft the selected gear is simply
one of the main landing gears. Belly gear aircraft such as the MD-11 and A340 all have duals at the belly
position. The spacing of the gears and the amplitude of the responses with these aircraft mean that
5
separating into two equivalent aircraft (wing gears on one and belly gear on the other) does not
significantly affect the computed responses. The two equivalent aircraft appear explicitly in the design
aircraft list and are treated as being separate aircraft at all times in the computational procedures. There are
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