UFC 3-260-02
30 June 2001
moisture content lower than saturation would normally consist of field moisture content measurements
under similar pavements located in the area. These measurements should be made during the most
critical period of the year when the water table is at its highest elevation. Extreme caution should be
exercised when the design is based on other than the saturated condition.
b. Traffic Data. The traffic parameters to be considered are the type of design aircraft, aircraft
loading, traffic volume, and traffic area.
(1) Traffic volume. The design traffic volume is expressed in terms of total operations of the
design aircraft expected during the life of the pavement. This traffic volume must be converted to a
number of expected strain repetitions. In converting operations to strain repetitions, the concept of
effective gear print is introduced. The effective gear print is the width of pavement that sustains an
effective strain repetition at a given depth in the pavement. The effective gear print is a function of the
number of tires in a transverse line, the transverse spacing, the width of the contact area, and the
effective thickness of pavement above the location of strain. The effective thickness of the pavement is
the sum of the thickness of unbound material plus twice the thickness of bound material where a bound
material is an asphalt concrete or stabilized layer. Thus, for a pavement having 76 millimeters (3 inches)
of asphalt and 381 millimeters (15 inches) of unbound gravel, the effective thickness with reference to
the strain at the top of the subgrade would be 381+(276) (15+(23)), or 533 millimeters (21 inches),
and with respect to the strain at the bottom of the asphalt, the effective thickness would be 276 (23),
or 152 millimeters (6 inches). With the determination of the effective thickness, the gear print is
computed as illustrated in Figures 11-2 and 11-3. If the gear is composed of tracking tires such as
tandem gear, then the number of strain repetitions may be somewhat greater than if the gear were not
tandem. When the tracking tires are located far enough apart, two distinct strain pulses will occur and
the multiplication factor for the tandem gear is 2. When the tires are sufficiently close, the strain pulses
merge into a single pulse and the multiplication factor is 1. The computation of F is shown in
Figure 11-4. In the figure, B is the spacing between tandem tires in the gear; te is the effective
pavement thickness; and Tw is the length of the ellipse that is formed by the tire imprint. When te is
less than B - Tw , F is 2. When te is greater than twice the difference between B and Tw , F is 1. For
values of te between the two conditions, F is computed based on the equation:
&
&
'
(11-1)
&
(a) The concept for conversion of aircraft operations to effective strain repetitions involves
assuming that traffic distribution on the pavement can be represented by a normal distribution. For traffic
on taxiways and runway ends (first 305 meters (1,000 feet)), the distribution has a wander width of
approximately 178 millimeters (70 inches), and traffic on runway interiors has a wander width of
approximately 355 millimeters (140 inches). (Note that wander width is defined as the width that
contains 75 percent of the applied traffic.) From the normal distribution, the fraction of traffic for which
the effective gear print will encompass a given point in the pavement can be computed. This fraction
times F gives the number or fraction of the effective strain repetitions at a point in the pavement for each
aircraft operation.
(b) The number of effective strain repetitions the pavement sustains at a point for every
aircraft operation is the pass-to-strain conversion percentage. For an effective thickness of
0.00 millimeters (0 inches), the percentage is the inverse of the pass-to-coverage ratio multiplied by 100.
11-3
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