30 June 2001
From plots in Appendix E showing the conversion from passes to strain repetitions for the taxiway and
runway, the conversion percentages are determined. For the taxiway and depth to the top of subgrade
of 760 millimeters (30 inches), the maximum conversion percentage for converting passes to effective
strain repetitions from Figure E-10 is approximately 100. This maximum occurs at a distance of
26 meters (86 inches) from the centerline of the taxiway. Thus, the effective number of subgrade
repetitions would be 200,000. For consideration of the asphalt strain at a depth of 130 millimeters
(5 inches), the conversion percentage is approximately 50, resulting in 100,000 strain repetitions. From
Figure E-9, the conversion percentages for the runway are 60 and 30 for consideration of subgrade
strain and asphalt strain, respectively.
(3) The effective number of strain repetitions for a traffic group then is determined by
multiplying the total strain repetitions by the factor of traffic occurring in a group.
e. Step 5 - Computation of Damage Factors.
(1) The damage factor for one traffic group is defined as n/N where n represents the
effective strain repetitions for that group and N equals the allowable numbers of strain repetitions as
computed from Equations 11-3 and 11-4. The damage factors for the different periods are summed to
obtain the cumulative damage factor.
(2) The computations were performed by use of the computer programs SUBGRADE for the
subgrade damage and ASPHALT for the asphalt damage. The data file, SDATA1, required by the
program SUBGRADE for computing the subgrade damage factor for 840-, 760-, and 685-millimeter (33-,
30-, and 27-inch) pavements is given in Table 11-8 and the output is given in Table 11-9. The data file
ADATA1 required by the program ASPHALT for computing the asphalt damage factor for the asphalt is
given in Table 11-10 and for the output in Table 11-11.
(3) To speed the design procedure, the subgrade damage factor was computed for several
pavement thicknesses, and the results for the taxiway pavement were plotted as shown in Figure 11-15.
For pavements having an asphalt concrete thickness of 130 millimeters (5 inches), the subgrade
damage factor was computed for thicknesses of 685, 760, and 840 millimeters (27, 30, and 33 inches).
From the plot of damage factor versus pavement thickness, it is determined that the required thickness
for the taxiway pavement would be 735 millimeters (29 inches). Using the 735-millimeter (29-inch)
overall thickness as a constant thickness, the subgrade damage factor can be computed for varying
thickness of asphalt concrete. Lines can then be constructed that will provide the total thickness for
each asphalt concrete thickness. Alternate designs, rounded to the nearest inch, might be
152-millimeter (6-inch) asphalt concrete with 710 millimeters (28 inches) in total thickness or
102-millimeter (4-inch) asphalt concrete with 762 millimeters (30 inches) in total thickness. The
relationship between pavement thickness and subgrade damage factors for the runway is given in
(4) For these designs, the asphalt damage must be computed. Plots of asphalt damage
versus asphalt thickness for both the taxiway and runway are given in Figure 11-17. The asphalt
damage factor would not control the asphalt thickness since the damage factor for this case is always
less than 1.0. The minimum thickness of the asphalt layer would be dictated by the minimum thickness
criterion in the basic manual.