Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

284 Chapter Ten
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For the limited placement condition of concrete with a slump 7 in.
or less and normal internal vibration to a depth of 4 ft or less,
formwork for columns may be designed for the following lateral
pressure:
P m
= C w
C c
[150 + 9,000R/T] (10-2)
where P m
= calculated lateral pressure, lb per sq ft
C w
= unit weight coefficient
C c
= chemistry coefficient
R = rate of fill of concrete in form, ft per hr
T = temperature of concrete in form, degrees Fahrenheit
Minimum value of P m
is 600C w
, but in no case greater
than wh
Applies to concrete with a slump of 7 in or less
Applies to normal internal vibration to a depth of 4 ft
or less
ww.EasyEngineering.n
Values for the unit weight coefficient C w
in Eq. (10-2) are shown in
Table 3-1 and the values for the chemistry coefficient C c
are shown in
Table 3-2. The minimum pressure in Eq. (10-2) is 600C w
lb per sq ft,
but in no case greater than wh.
This equation should be used with discretion. For example, the
pressure should not exceed wh, where w is the unit weight of concrete
and h is the depth in feet below the upper surface of freshly
placed concrete. Thus, the maximum pressure at the bottom of a form
6 ft tall with 150 lb per cu ft concrete will be (150 lb per cu ft)(6 ft) =
900 lb per sq ft, regardless of the rate of filling the form. However,
using Eq. (10-2) for a rate of placement of 6 ft per hour at a temperature
of 90°F, the calculated value is 750 lb per sq ft. For a rate of placement
of 6 ft per hour and a temperature of 60°F, using Eq. (10-2), the
calculated value is 1,050 lb per sq ft. As stated previously, the American
Concrete Institute recommends Eq. (10-1) for the design of concrete
formwork.
Designing Forms for Square or Rectangular Columns
Figure 10-1 illustrates a form for a representative square column,
using sheathing and patented column clamps. If the thickness of the
sheathing is selected, the design consists of determining the maximum
safe spacing of the column clamps, considering the pressure
from the concrete and the strength of the sheathing. The strength of
the sheathing must be adequate to resist bending and shear stresses,
and it must be sufficiently rigid so that the deflection will be within
an acceptable amount, usually less than l/360 or ¹⁄16 in.
As illustrated in Figure 10-1(b), it is assumed that the magnitude
of the pressure on a given area of sheathing will vary directly
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