Hollowblock slabs.pdf
Short Description
Hollowblock slabs guide...
Description
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
Hollow Block Slabs
Hollow Block Slabs (definitions)
Hollow blocks are used to fill portions of the slab thickness; this results in deeper arm for the reinforcement while saving the amount of concrete and hence the own weight of the slab.
The
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reinforcement is located between the blocks inside the ribs. Blocks may be concrete blocks or styro-foam
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Monday, October 15, 2012 RC Design 2 - Dr. Hany Nessim
Hollow Block Slabs (definitions)
When the ribs are in one direction then it is a one-way hollow block slab, regardless of the rectangularity; r.
When ribs are in both directions then it is a two-way hollow block slab.
Advantages & Disadvantages
Advantages
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
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Reducing slab weight by reducing amount of concrete below neutral axis. Ease of construction, especially when all beams are hidden beams. Economic for spans > 5m with moderate live load: hospitals, office and residential buildings. Improved insulation for sound and heat.
Disadvantages
Not economic for small spans Not suitable for heavy loads or dynamic loads. Difficult to repair or strengthen
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Monday, October 15, 2012
One-Way Hollow Block Slabs ts
RC Design 2 - Dr. Hany Nessim
t
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
5
b
e
Support condition
Simply supported
Continuous one end
Continuous both ends
cantilever
Clear span (Ln/t)
20
25
28
8
Ribs are in one direction only
For spans less than 10m or cantilevers less than 2m long.
St. 400/600; for other grades divide values by 0 . 4 +
f
650
y
Cross ribs
Cross ribs are used when the live load ≥ 3kN/m2 or Span > 5m.
Span
≤ 5m
>5m
4m – 7m
Live Load
≤ 3kN/m2 ≤ 3kN/m2 > 3kN/m2
>3kN/m2
Cross Sibs
No
Three cross ribs
One cross One cross rib rib
>7m
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Monday, October 15, 2012
Minimum Dimensions
RC Design 2 - Dr. Hany Nessim
ts t
b
e
e ≤ 700mm b = t/3 ≥ 100mm ts = e/10 ≥ 50mm
Monday, October 15, 2012
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Loading
RC Design 2 - Dr. Hany Nessim
ts t
b
e
Own weight including blocks’ weight. Flooring Live Load
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Own Weight including blocks’ weight (kN/m2) ts
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
Loading
t
b
e
Concrete Blocks Concrete Blocks Concrete Blocks 400x200x150 400x200x200 400x200x250
Foam Blocks 500x400x200
One way
Two way
One way
Two way
One way
Two way
One way
Two way
3.03
3.36
3.30
3.80
4.10
4.78
0.70
1.20
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
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Load Distribution Factors for Two-way
When LL ≤ 5 kN/m2 or when the compression flange is incomplete.
r
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
α
0.396
0.473
0.543
0.606
0.660
0.706
0.746
0.778
0.806
0.830
0.849
β
0.396
0.333
0.262
0.212
0.172
0.140
0.113
0.093
0.077
0.063
0.053
10
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Monday, October 15, 2012 RC Design 2 - Dr. Hany Nessim
Load Distribution Factors for Two-way
When LL > 5 kN/m2
r
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
α
0.500
0.595
0.672
0.742
0.797
0.834
0.867
0.893
0.914
0.928
0.941
β
0.500
0.405
0.328
0.258
0.203
0.166
0.133
0.107
0.086
0.072
0.059
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
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Calculate Bending Moments
Two spans When LL ≤ DL, and the difference between spans is not more than 20%; then the bending moment could be calculated as following.
w
W
= wu for limit state design method. & w = w for allowable stresses design method.
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6
W
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
Calculate Bending Moments
More than two spans
Spans differences are not more than 20% LL ≤ DL
Using Design Aids Charts (flexure)
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
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Shearing stresses should be less than the concrete shear capacity and, hence, minimum shear reinforcement could be used.
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
Design for Shear
Shearing stress; q u =
Qu b.t
Concrete shear capacity; q cu = 0.16
f cu
γc
Minimum shear reinforcement; A s, min =
≥ q u ( N / mm 2 )
0.4 q u b.s fy q cu
( mm 2 )
RC Design 2 - Dr. Hany Nessim
Monday, October 15, 2012
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Minimum Reinforcement
Slab reinf.
Stirrups Rib reinf.
5φ6mm/m’ perpendicular to ribs. 4φ6mm/m’ parallel to ribs.
In top slab
Bottom reinforcement = rib main reinforcement. Top reinforcement = ½ bottom reinforcement
In Cross rib
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Monday, October 15, 2012 RC Design 2 - Dr. Hany Nessim
Minimum Reinforcement
Slab reinf.
Stirrups Rib reinf.
A s, min
= min.
A s, min
≥
0.225 1.3 A s
0.25 100 0.15 100
f cu bd fy
≥
bd;
for
mild
bd;
for
deformed
1.1 fy
steel steel
In Main rib
(as beams)
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Example
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