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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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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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Design Procedure for One-Way Slab (contd)
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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Design Procedure for One-Way Slab (contd)
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