slab formworks design
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Temporary StructuresCM 420
Slab Form Design
Lecture 5
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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 andspacing of its supports (joist spacing)
Step 3: Joist size and spacingof supports (stringer spacing)
Step 4: Stringer size and span (shorespacing)
Step 5: Shore design to support stringers
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 forming members and steel
shoring. Ceiling height is 8 ft. and baysare 15x15 ft. Since forms will havecontinuing reuse, do not adjust basedesign 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 forms
50 psf(refer to lecture #1)
Weight of forms, estimated
8 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 plywoodsheathing, from Tables 4-2 and 4-3:
Fb
= 1545 psi
FS
= 57 psi
E= 1,500,000 psi
S= 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-widestrip of plywood. Then:
lb/lf158ft.1psf158ofloaddesign
w
in.0.22
158
412.0154595.10
l
w
fSl 95.10
Substituting in the equation:
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Slab form Design Example
CHECK DEFLECTION
ForD = l/360:
ForD= 1/16:
in.8.20187069.1
158
197.0150000069.169.1 333
w
EIl
in.2.21187023.3158
197.0150000023.323.3 444 wEIl
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Slab form Design Example
CHECK ROLLING SHEAR For design purposes, consider a 1-foot-wide
strip of plywood. Then:
since Vmax
=0.6wL ,so: Ib
VQFS
inches48orft.0.4762.6
1586.0
57
6.0
Q
Ib
w
FL S
Ib
QwL
Ib
VQFS 6.0
Q
Ib
w
FL S
6.0 Substituting in above equation:
or:
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Slab form Design Example
From the above calculations, l= 20.8 in.governs.
Meaning that joist supports CANNOTbe more than 20.8 inches apart.
HOWEVER, in order to select the span,we must consider the size of theplywood sheets and equal spacing of
supports. In this case, 5 equal spaces of 19.2
inches on an 8-ft. wide plywood sheetwill 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, sothere is no short-term load adjustment).
From Table 4-2: Fb
= 1000 psi and FV
= 95 psiand should be adjusted for horizontal shearby a factor of 2. E= 1,500,000 psi.
psi190950.2 VF
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Slab form Design Example
From Table 4-1B, forS4S2x4s: bd= 5.25 in.2,I= 5.36 in.4, and S= 3.06 in.3
psfload,designft.in.12
in.spacing,Joist
w
lflb253psf158ft.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
ForD = 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 formulafor a uniformly loaded continuous beam(similar to calculations on page 19):
12
29.0 dL
bd
wfV
12
5.32
25.5
2539.0190 LfV
ft.69.43.2537.43190 LL
Or L = 4.69x12 in./ft. = 59.5 inches
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Slab Form Design Example
Comparing the three spans
calculated above, l= 38.1 inchesgoverns.
Considering 15x15 ft. bays and
desire for uniform spacing, 36 inchspacing is a reasonable number.
This means that the spacing of
stringers will be at 5 equal spacesper bay.
feet15inches180635
C 20
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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 forS4S4x4s: bd= 12.25 in.2, I= 12.50 in.4, S= 7.15in.3; and d= 3.5 in.
lflb744psf158ft.in.12
in.36psfform,onload
ft.in.12
in.spacing,Stingerw
CHECK BENDING
in.5.42474
15.7100095.1095.10
w
SFl V
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Slab Form Design Example
CHECK DEFLECTION
ForD = l/360in.6.5707.3469.13955769.1
474
50.12150000069.169.1 333
w
EIl
CHECK SHEAR Use the horizontal shear stress formula for
a uniformly loaded continuous beam:
122
9.0
1229.0 d
wbdFLdL
bdwF VV
in.4.72ft03.658.045.512
5.32
4749.0
25.12190
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 thedimensions of the bay.
The 15-ft. bay could be divided into 5equal spaces of 36 inches (180/5 = 36)which is less than the maximum
allowable span of 42.5 inches.
CM 420
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Slab Form Design Example
Alternatively, we can check the possibility
of using a deeper stringer, i.e. 3x6, inorder to increase the shore spacing.
Since bending is dominant here, we will
check bending for a 3x6 member. ForS4S3x6s from Table 4-2: F
b= 1000 psf,
and from Table 4-1B, S= 12.60 in.3
in.4.5616.595.10474
60.12100095.1095.10
wSFl b
Now we can use 45-in. support spacing forthe 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 inchesapart. The area of support for each shore
is:
2ft.25.1112/4512/36Area
lb.1778psf158ft.25.11 2
Then the total load per shore is:
T St tCM 420
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Slab form Design Example
Schematic design:
T St tCM 420
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Slab form Design Example
Refer to Table 7-11 for wood shoringmaterial. Both 3x4 and 4x4 are morethan adequate to carry 1778 lbs for aneffective length of 8 ft.
T St tCM 420
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Slab form Design Example
Step 6: Check Bearing Stresses:
Bearing should be checked wherestringers bear on shores and where
joists bear on stringers.
Stringers bearing on shore: Assume the head piece of the adjustable
steel shore is 11x3 5/8". The 3x6 stringeris actually 2 in. thick.
T St tCM 420
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Slab form Design Example
If the headpiece is placed parallel tothe stringer, bearing area is 2x11 0r28.75 in.2. Bearing stress will be:
This is well below the base Fc, whichis obtained from Table 4-2 (the value ofcompression to grain, Fc, for No. 224 Douglas Fir-Larch is 625 psi).
psi6275.28
1778
areabearing
loadshoretotal
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Joist bearing on Stringers:
The two members are 1 and 2 in. wide.
Contact bearing area = 2x1 = 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
psi202in.75.3
lb7582
Bearing at this point is also low relative
to the 625 psi base value for Fc
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