Building Design and Construction

Cantilever Retaining Walls

Under the ACI 318 Building Code, cantilever retaining walls are designed as slabs.
Specific Code requirements are not given for cantilever walls, but when axial load becomes near zero, the Code requirements for flexure apply.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 11⁄2 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 + Dh), where Dh 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, useof 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 selected 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 (earth and weight of heel). For a footing proportioned so that the actual soil pressure approaches zero at the end of the heel, the unbalanced moment and the maximum moment in the heel caused by the top loads will be nearly equal.
The possibility of an overall sliding failure, involving the soil and the structure together, must be considered, and may require a vertical lug extending beneath the footing, tie backs, or other provisions.
The base of the stem is a point of maximum bending moment and yet also the most convenient location for splicing the vertical bars and footing dowels. The ACI 318 Building Code advises avoiding such points for the location of lap splices. But for cantilever walls, splices can be avoided entirely at the base of the stem only for low walls (8 to 10 ft high), in which L-shaped bars from the base of the toe can be extended full height of the stem. For high retaining walls (over 10 ft high), if all the bars are spliced at the base of the stem, a Class B tension lap splice is required (Art. 9.49.7). If alternate dowel bars are extended one Class A tension lapsplice length and the remaining dowel bars are extended at least twice this distance before cutoff, Class A tension lap splices may be used. This arrangement requires that dowel-bar sizes and vertical-bar sizes be selected so that the longer dowel bars provide at least 50% of the steel area required at the base of the stem and the vertical bars provide the total required steel at the cutoff point of the longer dowels (Fig. 9.40).

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