Building Design and Construction Handbook - Merritt - Ventech!

9.104 SECTION NINE
FIGURE 9.38 Factored loads and critical sections for design of cantilever retaining
walls.
Minimum clear cover for bars in walls cast against and permanently exposed to
earth is 3 in. Otherwise, minimum cover is 2 in. for bar sizes No. 6 and larger, and
1 1 ⁄2 for No. 5 bars or 5 ⁄8-in wire and smaller.
Two points requiring special consideration are analysis for load factors of 1.7
times lateral earth pressure and 1.4 times dead loads and fluid pressures, and provision
of splices at the base of the stem, which is a point of maximum moment.
The footing and stem are usually cast separately, and dowels left projecting from
the footing are spliced to the stem reinforcement.
A straightforward way of applying Code requirements for strength design is
illustrated in Fig. 9.38. Soil reaction pressure p and stability against overturning
are determined for actual weights of concrete D and soil W and assumed lateral
pressure of the soil H. The total cantilever bending moment for design of stem
reinforcement is then based upon 1.7H. The toe pressure used to determine the
footing bottom bars is 1.7p. And the top load for design of the top bars in the
footing heel is 1.4(W � D h), where D h is the weight of the heel. The Code requires
application of a factor of 0.9 to vertical loading that reduces the moment caused
by H.
Where the horizontal component of backfill pressure includes groundwater above
the top of the heel, use of two factors, 1.7 for the transverse soil pressure and 1.4
for the transverse liquid pressure, would not be appropriate. Because the probability
of overload is about the same for soil pressure and water pressure, use
FIGURE 9.39 Loads for simplified strength
design for toe of wall.
of a single factor, 1.7, is logical, as recommended
in the Commentary to the
ACI 318 Building Code. For environmental
engineering structures where
these conditions are common, ACI
Committee 350 had recommended use
of 1.7 for both soil and liquid pressure
(see ‘‘Environmental Engineering Concrete
Structures,’’ ACI 350R). Committee
350 also favored a more conservative
approach for design of the toe. It is more
convenient and conservative to consider
1.7 times the entire vertical reaction uniformly
distributed across the toe as well
as more nearly representing the actual
end-point condition (Fig. 9.39).
The top bars in the heel can be se-
lected for the unbalanced moment between the factored forces on the toe and the
stem, but need not be larger than for the moment of the top loads on the footing