lec-23- analysis and design of slabs

8/13/2019 Lec-23- Analysis and Design of Slabs

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Plain & Reinforced

Concrete-1CE-313

Lecture # 239th May 2006

Analysis and Designof Slabs

By Engr. Azhar

Sixth TermCivil Engineering


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Plain & Reinforced Concrete-1

Slabs In reinforced concrete construction, slabs are used

to flat, useful surfaces..

A reinforced slab is a broad, flat plate, usually

horizontal, with top and bottom surfaces parallelor nearly so.

It may be supported by reinforced concrete beams(and is usually cast monolithically with such

beams), by masonry or by reinforced concretewalls, by steel structural members, directly bycolumns, or continuously by ground.


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One-Way SlabThe slab which resists the entire/major part of appliedload by bending only in one direction

If slab is supported on all four sides and

5.0SideLonger

SideShortedR

..

..

it behaves as one-way slab.

Slabs having supports on less than four sides can bedesigned as one-way.

Two edge supported slab is always one-way.

Cantilever slab is always one-way.


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One-Way Slab (contd)

free

supported

free

supported

R < 0.5

CantileverSlab

One-Way Slabs

Span Span

Span

Span


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One-Way Slab (contd) Main steel is only provided parallel to span

One-way slab is designed as singly reinforcedrectangular section.

hminfor the slab is different compared with thebeams.

End Conditions Steel Grades

300 420 fy

Simply Supported L/25 L/20

One end continuous L/30 L/24

Both ends continuous L/35 L/28

Cantilever L/12 L/10

690

f4.0

20L y

690

f4.0

24L y

690

f4.0

28L y

690

f4.0

10

L y


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One-Way Slab (contd)

L = Effective Span. Lesser of the following:

L= Ln+ h/2 + h/2= Ln+ h

h = depth of slab

and

L = c/c distance between supports.


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Examples of One-Way Slab (contd)

Shades in the roofing system (cantilever)

Slab of stairs.

Cantilever retaining walls.

Footings.

Slab of stair

Footing


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Bar Spacing Cover For Slabs (contd)

smaxwill be lesser of following.

1. 3 x h (local practice is 2 x h)2. 450 mm (local practice is 300 mm)

3. (158300/fy) -2.5Cc4. 12600/f

yCc= Clear Cover


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Distribution, Temperature & Shrinkage Steel ForSlabs (ACI-318-7.12)

Shrinkage and temperature reinforcement is

required at right angle to main reinforcement tominimize cracking and to tie the structure togetherto ensure its acting as assumed in design.

Top and bottom reinforcements are both effective

in controlling the cracks.


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Distribution, Temperature & Shrinkage Steel ForSlabs (ACI-318-7.12) (contd)

For Grade 300 0.2% of b x h = 0.002 As= 0.002bh

For Grade 420 0.18% of b x h = 0.0018 ..As= 0.0018bh

For other grades .yf

4200018.0

Temperature steel in no case will be less than 0.0014


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Distribution, Temperature & Shrinkage Steel ForSlabs (ACI-318-7.12) (contd)

smaxshall be lesser of following

1- 5 x h (field practice is 2 x h)

2- 450 mm (field practice is 2 x h)


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Minimum Steel For Slabs

Same as the distribution steel


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Design Procedure for One-Way Slab

1. Check whether the slab is one-way or two-way.2. Calculate hminand round it to higher 10mm

multiple.i. Not less than 110 mm for roomsii. Not less than 75 mm for sunshades.

3. Calculate dead load acting on the slab.

Dead Load = Load per unit area x 1m width.4. Calculate live load acting on the slab.Live load = Load per unit area x 1m width.


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Design Procedure for One-Way Slab (contd)

5. Calculate total factored load per unit strip. (kN/m)6. Calculate the moments either directly (simply

supported) or by using coefficient for continuousslabs.7. Calculate effective depth.

d = h (20 + ()db)

db= 10, 13, 15, generally used.8. Check that

d dmin


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9. Calculate As required for 1m width.

10.Calculate minimum/distribution/temperature &

shrinkage steel.

11.Select diameter and spacing for main and steel.

12.Check the spacing for max. and min. spacing.

smin 90mmif spacing is less than minimum increase the diameterof bar.


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13.For continuous slabs, curtail or bend up the +vesteel. For -ve steel see how much steel is already

available. Provide remaining amount of steel.14.Calculate the amount of distribution steel. Decideits dia. & spacing like main steel.

15.Check the slab for shear.

vVc Vu16.Carry out detailing and show results on thedrawings.

17.Prepare bar bending schedule, if required.


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Approximate of Steel for Estimate

Approximate amount of steel in slab

= 0.07 kg/mm/m2

If slab thickness = 100 mm

steel = 0.07 x 100 = 7kg /m2


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Concluded

lec-23- analysis and design of slabs

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