11-04-2013, 03:48 PM
Rectangular Concrete Tanks
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INTRODUCTION
While cylindrical shapes may be structurally best for
tank construction, rectangular tanks frequently are preferred
for specific purposes. Special processes or operations
may make circular tanks inconvenient to use.
When several separate cells are required, rectangular
tanks can be arranged in less space than circular tanks
of the same capacity. Tanks or vats needed inside a
building are therefore often made in rectangular or
square shapes. For these and other reasons, breweries,
tanneries, and paper mills generally use rectangular
tanks.
Data presented here are for design of rectangular
tanks where the walls are subject to hydrostatic pressure
of zero at the top and maximum at the bottom.
Some of the data can be used for design of counterforted
retaining walls subject to earth pressure for which
a hydrostatic type of loading may be substituted in the
design calculations. Data also can be applied to design
of circular reservoirs of large diameter where lateral
stability depends on the action of counterforts built integrally
with the wall.
Another article on tank construction, Circular Concrete
Tanks Withouf Prestressing, has been published
by the Portland Cement Association.
Moment Coefficients
Moment coefficients were calculated for individual
panels considered fixed along vertical edges, and coefficients
were subsequently adjusted to allow for a certain
rotation about the vertical edges. First, three sets of
edge conditions were investigated, in all of which vertical
edges were assumed fixed while the other edges
were as follows:
1. Top hinged-bottom hinged
2. Top free-bottom hinged
3. Top free-bottom fixed*
Moment coefficients for these edge conditions are
given in Tables 1, 2, and 3, respectively. In all tables, a
denotes height and b width of the wall. In Tables 1, 2,
and 3, coefficients are given for nine ratios of b/a, the
limits being b/a = 3.0 and 0.5. The origin of the coordinate
system is at midpoint of the top edge; the Y axis is
horizontal; the X axis is vertical and its positive direction
downward. The sign convention for bending moments is
based on the coordinate fiber that is being stressed. For
example, A$ stresses fibers parallel to the X axis, The
sign convention used here is not compatible with two
other conventions-namely, that (1) the subscript is the
axis of the moment,
Shear Coefficients
Shear values along the edges of a tank wall are needed
for investigation of shear and development stresses.
Along vertical edges, shear in one wall is also used as
axial tension in the adjacent wall and must be combined
with bending moment to determine tensile reinforcement.
Various data for shear were computed and are given
in Table 7. The wall is considered fixed at the two vertical
edges while top and bottom edges are assumed to
be hinged. The wall panel with width b and height a is
subject to hydrostatic pressure due to a liquid weighing
w lb per cubic foot.
The first five lines in Table 7 are shears per linear foot
in terms of wa*. The remaining four lines are total shears
in kips or pounds depending on how w is given. Shears
per linear foot are for ratios of b/a = %, 1,2, and infinity.
The difference between the shear for b/a = 2 and infinity
is so small that there is no necessity for computing coefficients
for intermediate values.
Closed Single-Cell Tank
The tank in this section differs from the preceding one
only in that the tops of the walls are considered hinged
rather than free. This condition exists when the tank is
covered by a concrete slab with dowels extending from
the wall into the slab without moment reinforcement
across the bearing surface.