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presents the thickness design curves for new rigid and concrete overlays on flexible pavements. These curves are
based on Westergaard edge stress referenced to the flexural strength of the concrete (see, for example, references 2
and 3) and are replaced in LEDFAA by a layered elastic stress computational methodology described in references 1
and 5. The procedures for overlays on rigid pavement in paragraphs 406, 410, and 411 all use the design charts in
paragraph 332 a. to find the thickness of a single layer for the overlay design conditions and are replaced in
LEDFAA by layered elastic stress computations at the bottom of the existing and overlay layers for rigid overlays, at
the bottom of the existing layer for flexible overlays, and a corresponding prediction of the deterioration of the
existing layer over time (reference 6). With the exception of paragraphs 316 and 332 a., AC 150/5320-6D still needs
to be reviewed when using LEDFAA so that other design and construction requirements are satisfied. AC 150/5320-
16 provides additional design and construction requirements specific to the B-777 aircraft.
Change 3 to AC 150/5320-6D is in preparation and will incorporate the contents of AC 150/5320-16 in a
new chapter (Chapter 7). AC 150/5320-16 will be deleted. Thickness design by LEDFAA 1.3 will then become an
option to the use of the current thickness design charts, even when the aircraft mix falls within the current design
aircraft methodology (that is, when a design chart exists for each aircraft in the mix, or reasonable substitutions can
be made). The use of LEDFAA will be necessary when the mix includes the latest large aircraft types, such as the B2
777, A340-500/600, and A380-800. Updates will be made to the aircraft library in LEDFAA but not to the design
charts.
LEDFAA 1.2 was released in 1995 at the same time as AC 150/5320-16 was published. Since then, the
Windows® operating system has undergone two major upgrades and has progressed from the 16-bit DOS-based
versions to the current fully 32-bit versions. LEDFAA has therefore been upgraded to version 1.3 to maintain
compatibility with the Windows® operating system, to improve the user interface, and to include aircraft added to
the commercial fleet, or announced for introduction, since 1995. The main changes incorporated in Version 1.3 of
the program are:
1. The program is a full 32-bit implementation written in Visual Basic® 6.0 (VB6) instead of VB 3.0.
2. Metric units can be selected as an option to the default U.S. standard units.
3. Drag and drop has been added to the job and section file lists.
4. Additional working directories can be created.
5. The help file has been updated and displays in HTML format.
6. The layered elastic computational program has been changed from JULEA as a Windows executable to
LEAF as an ActiveX dynamic link library. LEAF is written in VB6 and is a completely new layered elastic
implementation (reference 7).
7. The aircraft library has been updated, including the addition of Boeing B-777-300ER and Airbus A380-800
and A340-500/600 aircraft.
8. For flexible design, vertical strain at the top of the subgrade is computed using the loads from all wheels of
multiple-gear aircraft such as the B-747 and A380 instead of using the loads from only one of the gears.
This makes LEDFAA compatible with the B-747 design charts in paragraph 316 of AC 150/5320-6D.
LEDFAA 1.3 replaced version 1.2 for thickness design under AC 150/5320-16, and was posted for download, in
June of 2003.
CHANGES TO THE USER INTERFACE
Figure 1 shows the two visible changes which have been made to the user interface in the Startup window. A new
button has been added which is used to switch between standard U.S. units and metric units. The setting applies to
the numerical displays in all windows of the program. Internal calculations and storage in the external data files is
always in standard units. A new text box has been added beneath the Sections list box. Clicking on the text box
displays a standard Windows dialog box which is used to set the default directory for job file storage. Individual job
files may also be stored anywhere within the resources of the computer by right clicking on the job name. Individual
jobs or sections within a job may be deleted by clicking on the name and pressing the delete key. Individual sections
can be dragged to a different job using the left mouse button.
The only other significant changes to the user interface are that the floating Status window in the Structure
window has been deleted in favor of a fixed text box and the pavement response evaluation points are now displayed
in the gear layout picture in the Aircraft Data window. (The gears are all drawn to the same scale in the picture box.
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