UFC 3-260-03
15 Apr 01
(3) Determine the design passes in terms of the critical aircraft by multiplying the projected passes
of the critical aircraft by the ratio of projected passes for each individual aircraft to allowable passes of
each individual aircraft at the maximum thickness. The program outputs a traffic mix analysis showing
how each individual aircraft contributes to the total design pass level and will identify the critical aircraft
and design pass levels.
b. For Navy and Marine Corps evaluations, and to determine equivalent traffic, it is recommended
to use the maximum peace-time take-off weight, and the maximum design landing weight from the Army
ETL 1110-3-394 and the Navy Aircraft Characteristics supplement (see Table 4-2). If desired, the analyst
may use maximum war-time take-off weights, but these weights are unlikely, even during war-time. If
desired, the analyst may also use more realistic, measured weights (in this case it is advised that the
measured average weight plus one standard deviation be used).
c. As indicated, TRAFFIC will express the total traffic in terms of one critical aircraft and a cor-
responding design pass level. It is possible also to express the total traffic in terms of any other aircraft, in
particular the aircraft representative of the existing (typically five or less) Navy categories. This is done by
(1) dividing each aircraft equivalent passes by the total equivalent passes (design pass level) to obtain
each aircraft participation, and (2) by dividing the each actual aircraft passes by its participation. If this is
done for each aircraft category, a tentative PCN can be found for each one.
8. STEP 5 (COMPUTE ALLOWABLE AIRCRAFT LOADS, ALLOWABLE PASSES, REQUIRED
OVERLAY THICKNESS, AND PCN). Allowable load-carrying capacities and required overlay thicknesses
are evaluated using the computer program WESPAVE. For a particular aircraft (gear configuration, load,
pass intensity level, etc.), WESPAVE uses modulus values from WESDEF and computes stresses (rigid
and nonrigid overlay on rigid pavement) and strains (flexible pavement) that will occur in the pavement
system. WESPAVE then calculates the limiting stress or strain values from empirically developed layered-
elastic values. Allowable load for the aircraft at the design pass level and allowable passes of the design
aircraft at maximum load are determined by comparing the predicted stress or strain to the limiting value.
Criteria and methodology incorporated in WESPAVE are presented in the remainder of this section.
a. Passes/Coverages. Regarding the evaluation criteria, an important point that should be empha-
sized is that the surface criteria (AC and PCC) are based on coverages to failure, while the subgrade
criteria are based on repetitions to failure. The lateral distribution of traffic has a greater effect on the
number of maximum stress applications that occur at a point near the surface than for a point deep within
the pavement structure (Miscellaneous Paper S-73-56). The incremental detriment to a pavement caused
by a wheel of an aircraft at a particular location on the pavement is influenced by many factors such as
number of tires on the aircraft, tire spacing, load on each tire, tire contact pressure, location of aircraft on
the pavement, and previous loading history. As a result of different assumptions and development
procedures used in analyzing results of traffic tests, the term coverage has different meanings for rigid
and flexible pavements. For rigid pavements, coverage is a measure of the number of maximum stress
applications that occur within the pavement due to the applied traffic. A coverage occurs when each point
in the pavement within the limits of the traffic lane has been subjected to a maximum stress. For flexible
pavements, coverage is a measure of the number of maximum stress applications that occur on the
surface of the pavement due to the applied traffic. A coverage occurs when all points on the pavement
surface within the traffic lane have been subjected to one application of maximum stress. Thus, a
twin-tandem gear would produce two applications of stress on the surface of a flexible pavement, but it
would produce only one maximum stress application within a rigid pavement if the tandem spacing was
small and would produce two maximum stresses if the tandem spacing was large. The influence of the
lateral distribution of aircraft traffic is expressed in terms of pass-to-coverage ratios derived for each
aircraft.
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