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

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Forms for Thin-Shell Roof Slabs 391
This is the vertical load on the interior shores. For the outer shores,
the load will be larger because of the slope of the roof. However,
shores with capacities of 4,000 lb will be satisfactory. Shores must be
adequately braced.
Determine the Elevations of the Top of the Decking
The elevations of critical points on the top of the decking should be determined.
These points are 0, 5, 10, and 15 ft, measured horizontally, from the
centerline of the decking. Equation (13-3) gives the vertical distance from
the crown, or centerline, down to these points. R is equal to 25 ft.
2 2
y=R−
R −x
For x = 0, y = 0
For x = 5 ft, y = 25 – 625 − 25 = 0.506 ft
For x = 10 ft, y = 25 – 625 − 100 = 2.087 ft
For x = 15 ft, y = 25 – 625 − 225 = 5.000 ft
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Determine the Slope of the Decking at the Outer Edges
It is desirable to know the slope of the decking at the outer edges,
where the slope is a maximum, to determine whether it will be necessary
to use top forms for a portion of the concrete slab. Concrete with
a low slump may be placed without a top form for slopes up to
approximately 35 ° , but for steeper slopes a form may be necessary. Let
a equal the angle between a tangent to the decking at the outer edge of
the roof and a horizontal line. Reference to Figure 13-1 shows that
sin a = l/R
where l = (30 ft)/2
= 15 ft
and R = 25 ft
(a)
Substituting these values in Eq. (a) gives
sin a = 15/25 = 0.600
arcsin a = 0.60
a = 36 ° 52′
It is possible that a concrete with a low slump can be placed without
a top form.
Centering for Shell Roofs
Figure 13-3 showed wood shores used as centering to support the
forms for a shell roof. Adjustable patented shores equipped with U-
heads to receive the stringers have several advantages when compared
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