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

222 Chapter Nine
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3. Determine the spacing of wales based on the strength and
deflection of studs.
4. Check the bearing strength between the studs and wales.
5. Determine the span length of wales between ties based on the
strength and deflection of wales.
6. Determine the required strength of form ties.
Lateral Pressure of Concrete on Forms
The lateral concrete pressure against the wall forms can be obtained
from Table 3-3 as 713 lb per sq ft. Or the pressure can be calculated
using Eq. (3-2), which applies to wall placement heights up to 14 ft
and rates of placement less than 7 ft per hr. Using Eq. (3-2) to calculate
ww.EasyEngineering.n
the pressure:
P m
= C w
C c
[150 + 9,000R/T]
From Table 3-1, for 150 lb per cu yd concrete, C w
= 1.0
From Table 3-2, for Type I cement without retarders, C c
= 1.0
P m
= 1.0 × 1.0 × [150 + 9,000(5/80)]
= 712.5 lb per sq ft, round up to 713 lb per sq ft
Checks on limitations on pressures calculated from Eq. (3-2):
Limited to greater than 600 Cw = 600 × (1.0) = 600 lb per sq ft
Limited to less than P m
= wh = 150 × (10) = 1500 lb per sq ft
The calculated value from Eq. (3-2) is 713, which is above the minimum
of 600 and below the maximum of 1,500. Therefore, use 713 lb
per sq ft lateral pressure on the forms. The 713 lb per sq ft maximum
pressure will occur at a depth of 713/150 = 4.75 ft below the top of the
form as shown in Figure 9-3.
Plyform Sheathing to Resist Pressure from Concrete
Because the type and thickness of Plyform has already been chosen,
it is necessary to determine the allowable span length of the plywood
(the distance between studs) such that the bending stress, shear stress,
and deflection of the plywood will be within allowable limits. Table 9-3
provides the section properties for the ¾-in.-thick Plyform Class I
selected for the design. The plywood will be placed with the face
grain across the supporting studs; therefore, the properties for stress
applied parallel to face grain will apply. Below are the physical properties
for a one foot wide strip of the Plyform.
Approximate weight = 2.2 lb per sq ft
Moment of inertia, I = 0.199 in. 4
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