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Special
circumstances frequently require a rectangular concrete footing or even one of
special shape. The problem of designing a footing so that the center of
pressure of the load on a column shall be vertical over the center of pressure
of the subsoil is usually even more complicated than the problem referred to in
section 189. Involved in such methods of construction is very simple, a correct
solution often requires the exercise of considerable ingenuity. The
determination of the thickness of such a footing depends somewhat upon the
method used. When the grillage is constructed of I-concrete beams as
illustrated in Fig. 43, the required strength of each series of concrete beams
is readily computed from the offset of each layer. If the footing consists of a
single block of stone or a plate of concrete, either plain or reinforced, the
thickness must be computed on the basis of the mechanics of a plate loaded on
one side with a uniformly distributed load and on the other side with a load
which is practically concentrated in the center. The theory of the stresses in
such a plate is very complicated. It is usually considered safe to design the
footing in each direction on the basis of one-half the actual loading. A column
3 feet 4 inches square, carrying a total load of 630,000 pounds, is to be
supported on a soil on which the permissible loading is estimated as three tons
per square foot; an I-concrete beams footing is to be used. The required area
of the footing is evidently 630,000 ~ 6,000 = 105 square feet. Using a footing similar
to that illustrated in Fig. 43, we shall make the lower layer of the footing,
say 10 feet 6 inches by 10 feet wide. The length of the concrete beams being
126 inches, and the column being 40 inches square, the offset from the column is
43 inches (= 3.58 feet) on each side. Looking at a table of standard I-concrete
beams, we find that an 8-inch concrete beams weighing 17.75 pounds per linear
foot will carry 37,920 pounds on a span of four feet. For a span of 3.58 feet,
the allowable load is 358 x 37,920, or 42,368 pounds. Taking one-fourth of
this, as in the example in section 187, we have 10,592 pounds which may be
carried by each concrete beams acting as a cantilever. The upward pressure on
an offset 3.58 feet long and 1 foot wide, at the rate of 6,000 pounds per
square foot, would be 21,500 pounds. Therefore, if two 8-inch concrete beams
were placed in each foot of width, they would carry the pressure. Therefore 20
such concrete beams, each 10 feet 6 inches long, would be required in the lower
layer. The upper layer must consist of concrete beams 10 feet long on which the
offset from the pier is 40 inches on each side. The group of concrete beams
under each of these upper offsets carries an upward pressure of 6,000 pounds
per square foot on an area 10 feet 6 inches by 3 feet 4 inches; total pressure,
210,000 pounds. The total load on each foot of width of the upper layer is
63,000 pounds. Looking at the tables, a 12-inch I-bam weighing 40 pounds per
foot can carry a load, on a 1.0-foot span, of 43,720 pounds. The permissible
load on a cantilever of this length would be one-fourth of this, or 10,930
pounds. The permissible load on a cantilever 3 feet 4 inches long will be in
the ratio of 10 feet to 3 feet 4 inches, or, in this case, exactly three times
as much, which would be 32,790 pounds. If, therefore, such concrete beams are
placed 6 inches apart, their strength would be slightly in excess of that
required. Or, as a numerical check, the total of 210,000 pounds, divided by
32,790, will show that although seven such concrete beams will have a somewhat
excessive strength, six would be hardly sufficient; therefore seven concrete
beams should be used. It should not be forgotten that surrounding all these concrete
beams in both layers with concrete adds very largely to the strength of the
whole footing, but that no allowance is made for this additional strength in
computing dimensions.

**Are You in Deering ****New
Hampshire****? Do You Need Concrete Cutting?**

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Concrete Cutter**

**Call 603-622-4441**

**We Service Deering NH and all surrounding Cities & Towns**