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

240 Chapter Nine
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Using Tables to Design Forms
Use the information in Tables 9-7 through 9-12 to design the forms for
a concrete wall 12 in. thick and 11 ft high. The forms will be filled at a
rate of 4 ft per hr at a temperature of 80°F. The 150 lb per cu ft concrete
with Type I cement and no retarders will be consolidated with an
internal vibrator at a depth not to exceed 4 ft.
Table 3-3 indicates a maximum pressure of 600 lb per sq ft. There
are several possible designs. Generally, the most satisfactory design is
the one that will furnish the necessary services at the lowest cost. The
forms will be built at the job using Plyform Class I sheathing, No. 2
grade Hem-Fir for 2 × 4 studs and two 2 × 4 members for wales, all
S4S lumber.
The Plyform will be placed with face grain across supports.
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Table 9-7 indicates two possible choices for thicknesses of Plyform
Class I sheathing: ¾-in.-thick Plyform with spacing of supports at 12 in.
on centers, or 1¹⁄8-in.-thick Plyform with spacing of supports at 16 in.
on centers. For this design, choose the ¾-in.-thick Plyform Class I.
For 12-in. spacing of support studs, the uniform load on a stud
will be (600 lb per sq ft)(1 ft) = 600 lb per lin ft. Table 9-12 indicates the
maximum span of 28 in. for 2 × 4 No. 2 grade Hem-Fir lumber. Therefore,
the spacing of wales must not exceed 28 in.
For this spacing of wales, the load per foot on a wale will be
(28/12 ft)(600 lb per sq ft) = 1,400 lb per ft. Consider using standard
3,000-lb load capacity form ties. The spacing of the ties must not
exceed 3,000 lb/1,400 lb per ft = 2.14 ft, or 12(2.14) = 25.7 in. For constructability,
the form ties will be placed at 24 in. center to center
along the wales.
The bottom wale should be placed not more than 8 in. above the
bottom of the form, and other wales at vertical intervals of 28 in.
thereafter, with the top wale at 12 in. below the top of the form.
As illustrated in Figure 9-5, the pressure on the form will increase
at a uniform rate from zero at the top of the wall to a maximum value
of 600 lb per sq ft at a depth of 4 ft below the top of the wall. The 4 ft is
calculated as (300 lb per sq ft)/(150 lb per cu ft) = 4.0 ft. The spacing of
the wales in this top 4 ft of wall may be increased because of the reduced
pressure, but the permissible increase will not be enough to eliminate a
row of wales. Therefore, five rows of wales will be required.
Forms for Walls with Batters
Figure 9-6 illustrates a set of forms for a wall whose thickness varies.
Variable-length ties are available for use with forms of this type. The
procedures presented in earlier sections can be used to design the
forms for this type of wall.
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