UFC 3-260-03
15 Apr 01
types, severities, and densities associated with loads. The SCI prediction is based on a relationship
between design factor and stress repetitions as related to crack formation in the PCC slabs due to load.
An SCI of 80 corresponds to the formation of one or more cracks per slab in 50 percent of the trafficked
slabs. However, experience has indicated that an SCI of 80 is somewhat conservative, and a value of
SCI = 50 is recommended. Figure 4-9 shows the relationship of SCI in terms of design factor versus
coverages. The design factor DF is defined as the concrete flexural strength divided by the stress. The
equation for the relationship given in figure 4-9 is as follows:
DF ' A % B LOG C
(eq 4-7)
where
DF = design factor
A = 0.2967 + 0.002267 (SCI)
B = 0.3881 + 0.000039 (SCI)
C = coverage level at selected SCI
SCI = structural condition index
and
R
ALLOWABLE STRESSPCC '
(eq 4-8)
DF
where
ALLOWABLE STRESSPCC = allowable tensile stress at the bottom of the slab, pounds per
square inch
R = PCC flexural strength, pounds per square inch
c. Maximum Stresses and Strains. Stresses/strains within a pavement system are computed using
the controlling wheels of the design aircraft and the WES5 subroutine. The location of the maximum
stress/strain value is influenced by factors such as pavement structure, wheel load, and wheel spacing.
For a single-wheel aircraft, the maximum stress/strain will always occur directly underneath the wheel.
For other more complicated gear configurations, stresses/strains must be computed at several positions
to determine where the critical values occur. Gear configurations for various aircraft considered in evalu-
ation are shown in figure 4-10 with controlling wheels and the recommended minimum number of stress/
strain evaluation positions indicated. The computer program LEEP has a data file (NEWFILE.DAT) which
contains the number and location of controlling wheels and evaluation positions.
d. Evaluation of Load Transfer. The deflection ratio from joint efficiency tests defined as
DEFLECTION OF UNLOADED SLAB
DEFLECTION RATIO '
(eq 4-9)
DEFLECTION OF LOADED SLAB
should be included in the evaluation of rigid and nonrigid overlays of rigid pavements which are evaluated
as rigid. The allowable loads determined at the slab centers can be reduced for poor joint transfer using
load reduction factors. These factors are a function of the deflection ratio. The procedure was developed
by first relating the deflection ratios to the percent maximum edge stress. Finite element programs were
4-13
Previous Page
