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Slab Form Design
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Slab formwork
Parts of typical slab formwork
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Slab formwork Design Steps:
Step 1: Estimate design loads Step 2: Sheathing thickness and spacing of
its supports (joist spacing) Step 3: Joist size and spacing of
supports (stringer spacing) Step 4: Stringer size and span (shore
spacing) Step 5: Shore design to support load Step 6: Check bearing stresses Step 7: Design lateral bracing
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Slab form Example Design forms to support a flat slab floor
8 in. thick of normal weight concrete, using construction grade Douglas Fir-Larch framing members and steel shoring. Ceiling height is 8 ft. and bays are 15x15 ft. Since forms will have continuing reuse, do not adjust base design values for short term load.
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Slab form Design Example STEP 1: ESTIMATE LOADS:
Dead load, concrete and rebar,[8 in. / (12 in./ft.)]x 150 pcf = 100 psf
Minimum construction live load on forms50 psf (refer to lecture #1)
Weight of forms, estimated8 psf
Total form design load
100 + 50 + 8 = 158 psf
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Slab form Design Example STEP 2: SHEATHING DESIGN:
Assuming 3/4-in. form grade plywood sheathing, from Tables 4-2 and 4-3:
Fb = 1545 psiFS = 57 psiE = 1,500,000 psiS = 0.412 in.3
I = 0.197 in.4
Ib/Q = 6.762 in.2
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Slab form Design Example STEP 2: SHEATHING DESIGN:
Tables 4-2 and 4-3, for plywood:
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Slab form Design Example CHECK BENDING
For design purposes, consider a 1-foot-wide strip of plywood. Then:
lb/lf 158ft. 1psf 158 of loaddesign w
in. 0.22158
412.0154595.10
l
w
fSl 95.10
Substituting in the equation:
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Slab form Design Example CHECK DEFLECTION
For D = l/360:
For D = 1/16”:
in. 8.20187069.1158
197.0150000069.169.1 333
w
EIl
in. 2.21187023.3158
197.0150000023.323.3 444
w
EIl
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Slab form Design Example
CHECK ROLLING SHEAR For design purposes, consider a 1-foot-wide
strip of plywood and use rolling shear equation from chapter 7:
inches 50.285.1762.6158
57205.1
'20
Q
Ib
w
Fl S
Substituting in above equation:
5.1'20
Q
Ib
w
Fl S
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Slab form Design Example From the above calculations, l = 20.8 in.
governs. Meaning that joist supports CANNOT
be more than 20.8 inches apart. HOWEVER, in order to select the span,
we must consider the size of the plywood sheets and equal spacing of supports.
In this case, 5 equal spaces of 19.2 inches on an 8-ft. wide plywood sheet will be appropriate.
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Slab form Design Example STEP 3: JOIST SIZE AND SPACING OF
STRINGERS TO SUPPORT THE JOISTS: Check 2x4 construction grade Douglas-Fir-
Larch as joist (forms are used repeatedly, so there is no short-term load adjustment).
From Table 4-2: Fb = 1000 psi and FV = 180 psi and E = 1,500,000 psi.
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Slab form Design Example
From Table 4-1B, for S4S 2x4s: bd = 5.25 in.2, I = 5.36 in.4, and S = 3.06 in.3
psf load,design ft.in. 12
in.spacing,Joist w
lflb 253psf 158ft.in. 12
in. 19.2w
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Slab Form Design Example CHECK BENDING
in. 1.38253
06.3100095.1095.10
w
SFl b
CHECK DEFLECTION For D = l/360
in. 5.5367.3169.13177869.1253
36.5150000069.169.1 333
w
EIl
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Slab Form Design Example CHECK SHEAR
Using the horizontal shear stress formula for a uniformly loaded continuous beam (similar to calculations on page 19):
dw
vbdFl 2
3
'40
56.8 3.522533
5.35.118040
l
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Slab Form Design Example Comparing the three spans
calculated above, l = 38.1 inches governs.
Considering 15x15 ft. bays and desire for uniform spacing, 36 inch spacing is a reasonable number.
This means that the spacing of stringers will be at 5 equal spaces per bay.
feet 15inches 180635
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Slab form Design Example STEP 4: STRINGER SIZE AND SPAN:
Use 4x4 Construction grade Douglas-Fir-Larch stringers. From Table 4-1B for S4S 4x4s: bd = 12.25 in.2, I = 12.50 in.4, S = 7.15 in.3; and d = 3.5 in.
lflb 744psf 158ft.in. 12
in. 36psf form,on load
ft.in. 12
in.spacing,Stinger w
CHECK BENDING
in. 5.42474
15.7100095.1095.10
w
SFl V
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Slab Form Design Example CHECK DEFLECTION
For D = l/360
in. 6.5707.3469.13955769.1474
50.12150000069.169.1 333
w
EIl
CHECK SHEAR Use the horizontal shear stress formula for
a uniformly loaded continuous beam:
dw
vbdFl 2
3
'40
inches 69.02 3.524743
5.35.318040
l
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Slab Form Design Example From the above calculations, l = 42.5 in.
governs. Meaning that stringers CANNOT be
more than 42.5 inches apart (span of stringers).
HOWEVER, in order to select an appropriate span, we must consider the dimensions of the bay.
The 15-ft. bay could be divided into 5 equal spaces of 36 inches (180”/5 = 36”) which is less than the maximum allowable span of 42.5 inches.
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Slab Form Design Example Alternatively, we can check the possibility
of using a deeper stringer, i.e. 3x6, in order to increase the shore spacing.
Since bending is dominant here, we will check bending for a 3x6 member.
For S4S 3x6s from Table 4-2: Fb = 1000 psf, and from Table 4-1B, S = 12.60 in.3
in. 4.5616.595.10474
60.12100095.1095.10
w
SFl b
Now we can use 45-in. support spacing for the 3x6 stringers, which will divide the bay into 5 equal spaces.
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Slab form Design Example
STEP 5: SHORE DESIGN: Stringers are placed 36-inches apart,
supported by shores spaced 45 inches apart. The area of support for each shore is:
2ft. 25.1112/4512/36Area
lb. 1778 psf 158ft. 25.11 2
Then the total load per shore is:
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Slab form Design Example Schematic design:
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Slab form Design Example Refer to Table 7-11 page 7-29 for
Construction Grade Douglas-Fir-Larch. Use 8-foot effective length and pick 3 x 4 S4S (capacity 2560lbs) or 4x4 (capacity 6670lbs). Both are more than adequate to carry 1778 lbs.
Alternately 2 x __ members can also be used if these are adequately laced and braced
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Slab form Design Example Step 6: Check Bearing Stresses: Bearing should be checked where
stringers bear on shores and where joists bear on stringers. Stringers bearing on shore:
Assume the head piece of the adjustable steel shore is 11½x3 5/8". The 3x6 stringer is actually 2½ in. thick.
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Slab form Design Example If the headpiece is placed parallel to
the stringer, bearing area is 2½x11½ 0r 28.75 in.2. Bearing stress will be:
This is well below the base Fc, which is obtained from Table 4-2 (the value of compression to grain, Fc, for No. 2 24 Douglas Fir-Larch is 625 psi).
psi 6275.28
1778
area bearing
load shore total
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Joist bearing on Stringers: The two members are 1½ and 2½ in. wide. Contact bearing area = 2½x1½ = 3.75 in.2
Average load transmitted by joist to stringer is:
Joist spacing x joist span x form load
Slab form Design Example
lb. 75815812
36
12
2.19
psi 202in. 75.3
lb 7582
Bearing at this point is also low relative to the 625 psi base value for Fc.
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Credits Prof. Nemati originally made this
PowerPoint presentation for the Slab Form Design Example in the ACI Formwork for Concrete Sixth Edition.
Prof. Shanker modified the Presentation to reflect data in Slab Form Design Example in the ACI Formwork for Concrete Seventh Edition.