Calculation Procedure:- Evaluate P when e = IO in (254 mm)
As the preceding calculations show, the eccentricity corresponding to point E in the inter-
action diagram is 8.03 in (203.962 mm). Consequently, an eccentricity of 10 in (254 mm)
corresponds to a point on EF, and an eccentricity of 6 in (152.4 mm) corresponds to a
point on ED.
By Eq. 456, P = 203(1140)7(1140 -1630 + 203 x 10) = 150 kips (667.2 kN). - Evaluate P when e = 6 in (152.4 mm)
By Eq. 44Z>, P = 506(2720)7(2720 + 506 x 6) = 239 kips (1063.1 kN).
Design of Column FootingsA reinforced-concrete footing supporting a single column differs from the usual type of
flexural member in the following respects: It is subjected to bending in all directions, the
ratio of maximum vertical shear to maximum bending moment is very high, and it carries
a heavy load concentrated within a small area. The consequences are as follows: The foot-
ing requires two-way reinforcement, its depth is determined by shearing rather than bend-
ing stress, the punching-shear stress below the column is usually more critical than the
shearing stress that results from ordinary beam action, and the design of the reinforcement
is controlled by the bond stress as well as the bending stress.
Since the footing weight and soil pressure are collinear, the former does not contribute
to the vertical shear or bending mo-
ment. It is convenient to visualize the
footing as being subjected to an upward
load transmitted by the underlying soil
and a downward reaction supplied by
the column, this being, of course, an in-
version of the true form of loading. The
footing thus functions as an overhang-
ing beam. The effective depth of foot-
ing is taken as the distance from the top
surface to the center of the upper row of
bars, the two rows being made identical
to avoid confusion.
Refer to Fig. 24, which shows a
square footing supporting a square,
symmetrically located concrete column.
Let P = column load, kips (kN); p = net
soil pressure (that caused by the column (°) Plon
load alone), lb/ft^2 (kPa); A = area of
footing, ft^2 (m^2 ); L = side of footing, ft
(m); h = side of column, in (mm); d =
effective depth of footing, ft (m); t =
thickness of footing, ft (m);fb = bearing
stress at interface of column, lb/in^2
(kPa); vi = nominal shearing stress un-
der column, lb/in
2
(kPa); V 2 = nominal
(b) Elevotlon
shearing stress caused by beam action, FIGURE 24
lb/in^2 (kPa); b 0 = width of critical sec-