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

214 Chapter Nine
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formwork it is common practice to rewrite the equations for stresses
and deflection in order to calculate the permissible span length of a
member in terms of the member size, allowable stress, and the loads
on the member. Table 5-3 summarizes the equations for calculating
allowable span lengths of wood members, and Table 5-4 summarizes
the equations for calculating the allowable span lengths of plywood
members. These two tables enable the designer to design formwork
in an organized manner.
There are two general approaches to the design for wall forms.
One approach is to select first the material to be used for formwork,
then use the equations in Tables 5-3 and 5-4 to determine the spacing
of the materials; including studs, wales, and form ties. The other
approach is to use tables that have been developed by a company that
supplies formwork systems and accessories. In the latter approach,
the materials and recommendations must be followed by the company
that supplies the formwork and accessories.
Usually the steps in designing forms for concrete walls follow the
sequence shown below:
1. Determine the maximum pressure exerted by the concrete on
the wall form, considering the depth of the forms, rate of placing concrete,
temperature of the concrete, type of cement and additivies,
density of concrete, and method of consolidating the concrete. See
Chapter 3 for the equations and tables for pressures on wall forms.
Note the maximum pressure of concrete on formwork is limited by
wh, where w is the density of concrete and h is the height of concrete
placement.
2. Select the kind, grade, and thickness of the material to be used
for sheathing (see Tables 4-9 and 4-10 for plywood and Tables 4-11
and 4-12 for Plyform, the plywood manufactured especially for
formwork). Then determine the maximum safe spacing of the studs,
considering the allowable unit fiber stress in bending, the horizontal
shear stress, and the permissible deflection in the sheathing. See
Table 5-4 for the equations for calculating the allowable span lengths
of plywood based on bending and shear stress and the deflection
criteria.
3. Select the grade and size of lumber to be used for studs. See
Table 4-1 for physical properties of lumber and Tables 4-2 and 4-3
give reference design values of wood members. Table 4-4 through 4-8
provides adjustment factors that are applied to the reference design
values to determine allowable stresses. Determine the maximum safe
spacing of wales, based on the permissible span length of the studs,
checking the bending and shear stresses and deflection of studs.
Table 5-3 provides the equations for calculating the allowable span
length of lumber based on stresses and deflection.
4. Select the grade and size of lumber to be used for wales (see
Tables 4-1 and 4-2). Determine the maximum safe spacing of form
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