Civil Engineering Reference
In-Depth Information
After considering the possible arrangements of the steel in Figure 13.21 and the re-
quired areas of steel at different elevations in Table 13.1, the author decided to use dowels
for load transfer at the stem base.
Use #8 dowels at 6 in. extending 33 in. down into footing and key
If these dowels are spliced to the vertical stem reinforcing with no more than one-half
the bars being spliced within the required lap length, the splices will fall into the class B
category (ACI Code 12.15) and their lap length should at least equal 1.3
(1.3)(33) 43
in. Therefore, two dowel lengths are used—half 3 ft 7 in. up into the stem and the other
half 7 ft 2 in.—and the #7 bars are lapped over them, half running to the top of the wall
and the other half to middepth. Actually, a much more refined design can be made that in-
volves more cutting of bars. For such a design, a diagram comparing the theoretical steel
area required at various elevations in the stem and the actual steel furnished is very useful.
It is to be remembered (Code 12.10.3) that the bars cut off must run at least a distance
d
or
12 diameters beyond their theoretical cutoff points and must also meet the necessary de-
velopment length requirements.
d
13.11
CRACKS AND WALL JOINTS
Objectionable horizontal cracks are rare in retaining walls because the compression
faces are the ones that are visible. When they do occur it is usually a sign of an unsat-
isfactory structural design and not shrinkage. In Chapter 6 of this topic the ACI proce-
dure (Section 10.6) for limiting crack sizes in tensile zones of one-way beams and
slabs was presented. These provisions may be applied to vertical retaining wall steel.
However, they are usually thought unnecessary because the vertical steel is on the
earth side of the wall.
On the other hand, vertical cracks in walls are quite common unless sufficient con-
struction joints are used. Vertical cracks are related to the relief of tension stresses due to
shrinkage, with the resulting tensile forces exceeding the longitudinal steel capacity.
Construction joints may be used both horizontally and vertically between successive
pours of concrete. The surface of the hardened concrete can be cleaned and roughened, or
keys may be used as shown in Figure 13.28(a) to form horizontal construction joints.
If concrete is restrained from free movement when shrinking, as by being attached to
more rigid parts of the structure, it will crack at points of weakness. Contraction joints are
weakened places constructed so that shrinkage failures will occur at prepared locations.
When the shrinkage tensile stresses become too large, they will pull these contraction
joints apart and form neat cracks rather than the crooked unsightly ones that might other-
wise occur. In addition to handling shrinkage problems, contraction joints are useful in
handling differential settlements. They need to be spaced at intervals about 25 ft on center
(the AASHTO says not greater than 30 ft). The joints are usually constructed with rubber
strips that are left in place or with wood strips that are later removed and replaced with
caulking.
Expansion joints are vertical joints that completely separate the different parts of a
wall. They are placed approximately 50 to 100 ft on centers (the AASHTO says maxi-
mum spacing should not be greater than 90 ft). Reinforcing bars are generally run through
all joints so that vertical and horizontal alignment is maintained. When the bars do run
