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

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Pressure of Concrete on Formwork 33
Checks on limitations on pressures calculated from Eq. (3-2):
Limited to greater than 600 C w
= 600(1.0) = 600 lb per sq ft
Limited to less than P m
= wh = 150(12) = 1,800 lb per sq ft
The calculated value from Eq. (3-4) is 2,340, which is above the
maximum of wh = 150(12 ft) = 1,800. Therefore, use 1,800 lb per sq ft
lateral pressure on the forms.
Example 3-6
Concrete with a density of 150 lb per cu yd will be pumped from the
base of a column form 10 ft high. Because the concrete is pumped
from the base of the form, the maximum lateral concrete pressure on
the form is the full hydrostatic pressure of wh plus a minimum allowance
of 25% for pump surge pressure. The pressure on the column
form can be calculated as follows:
P m
= 1.25 wh
= (1.25)(150 lb per cu ft)(10 ft)
= 1,875 lb per sq ft
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Relationship between Rate of Fill, Temperature,
and Pressure for Column Forms
Table 3-5 gives the relationship between the rate of filling forms, lateral
pressure, and the temperature of the concrete for column forms,
using Eq. (3-3) with C w
and C c
= 1. As previously presented, values
calculated using Eq. (3-4) should not exceed wh. For example, for concrete
with a density of 150 lb per cu ft the pressure should not exceed
150h, where 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 high will be 150(6 ft) = 900 lb per sq ft, regardless of the rate of
filling the form. Using Eq. (3-4) for a rate of placement of 6 ft per hr at
a temperature of 90 ° F, the calculated value is 750 lb per sq ft. However,
for a rate of placement of 6 ft per hr and a temperature of 60 ° F,
using Eq. (3-4), the calculated value is 1,050 lb per sq ft, which is
greater than the limiting pressure of wh = 900 lb per sq ft. As previously
stated, the American Concrete Institute recommends Eq. (3-1)
for the design of concrete formwork.
Graphical Illustration of Pressure Equations for Walls
and Columns
Figure 3-6 provides a graphical illustration of the relationship between
the rate of filling wall and column forms, maximum pressure, and the
temperature of concrete. The graphs are based on concrete made with
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