slab formworks design

7/30/2019 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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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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STEP 2: SHEATHING DESIGN:

Tables 4-2 and 4-3, for plywood:


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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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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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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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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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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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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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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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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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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

CM 420


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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

CM 420


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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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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.

CM 420


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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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Schematic design:

T St tCM 420


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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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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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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

Temporar Str ct resCM 420


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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


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

slab formworks design

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