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

352 Chapter Eleven
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considerable time to analyze each beam element of formwork to
determine the applied and allowable stresses for a given load condition,
and to compare the calculated deflection with the permissible
deflection. For each beam element, a check must be made for bending,
shear, and deflection.
For simplicity in the design of 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 member size,
allowable stress, and loads on the member. Table 5-3 summarizes the
equations for calculating the allowable span lengths of wood members
and Table 5-4 provides a summary of the equations for calculating
the allowable span lengths of plywood members. These two
tables enable the designer to design formwork in an organized manner.
Usually the steps in designing concrete slab forms follow the
sequence listed below:
1. Determine the total load on the floor decking, including the
effect of impact, if any. See Chapter 3 for static loads on formwork
and Chapter 7 for impact loads.
2. Select the kind, grade, and thickness of the material to be used
for the decking, 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 joists, considering
the allowable unit fiber stress in bending, the horizontal
shear stress, and the permissible deflection in the decking. Table 5-4
lists the equations for calculating the allowable span lengths of
plywood based on bending and shear stresses, and the deflection
criteria.
3. Select the grade and size of joists to be used. More than one size
may be considered. See Table 4-1 for the physical properties of lumber
and Tables 4-2 and 4-3 for reference design values of wood members,
noting that adjustment factors are given in Tables 4-4 through
4-8. Determine the maximum safe spacing of stringers, based on the
allowable unit stresses in bending and shear and the permissible
deflection whichever limits the span length for the joists. Table 5-3
provides the equations for calculating the allowable span lengths of
lumber based on stresses and deflection.
4. Select the grade and size of lumber to be used for stringers, see
Tables 4-2 and 4-5. Determine the maximum safe span lengths of
stringers, based on allowable unit stresses in bending and shear and
the permissible deflection, whichever limits the span length for the
stringers. Table 5-3 lists the equations to calculate the allowable span
lengths. A check must also be made for the bearing stress between the
stringers and joists.
5. Select the size and spacing of shores that will safely withstand
the forces transmitted to them by the stringers. An alternate method
is to select the shoring system, and then determine the maximum safe
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