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

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Forms for Beams and Floor Slabs 347
The unsupported length of Plyform will be the distance between the
plywood and the runners. This distance is calculated as follows:
Total width across the concrete beam bottom = 16.0 in.
Subtracting the width of 3 runners, 3(1.5 in.) = 4.5 in.
Total clearance of Plyform between runners = 11.5 in.
Thus, the net unsupported length of Plyform is 11.5/2 = 5.75 in.
An examination of Tables 11-2 and 11-3 for Class I Plyform indicates
the ¾-in. Plyform can safely support the 354 lb per sq ft load
over an unsupported length that is greater than 5.75 in. Therefore, the
¾-in. 4 × 4¹⁄16 thickness is satisfactory. The load transferred from the
runners must now be transferred between the shores.
Discarding the vertical support provided by the side members of
the beam sides, the three 2 × 4 runners that are upright must transfer
the dead load and the live load of the beam bottom between shores.
However, the center runner is the most heavily loaded. The center
runner must sustain the pressure from its center to the midpoint
between the adjacent runners on each side as follows:
ww.EasyEngineering.n
Total width of Plyform beam bottom = 16.0 in.
Width of three runners, 3(1.5 in.) = 4.5 in.
Net distance between runners = 11.5 in.
The width of the effective pressure carried by the center runner is
one-half of this distance plus the width of the center runner, or 11.5/2 +
1.5 = 7.25 in. The center runner must sustain the pressure over this
portion of the beam bottom. The uniform load on the center runner in
the direction of the beam may be calculated as follows:
Concrete dead load,
150 lb per cu ft (24/12)(7.25/12) = 181.3 lb per ft
Assumed dead load of forms,
4 lb per sq ft (7.25/12) = 2.4 lb per ft
Live load on beam bottom,
50 lb per sq ft (7.25/12) = 30.2 lb per ft
Total load = 213.9 lb per ft
For calculations, use w = 214 lb per lin ft
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