Design of Concrete Structure IDesign of Concrete Structure I
Dr. AliTayeh
First Semester 2009
Dr. AliTayeh
First Semester 2009
Lecture 12One Way Slab
Part 1
Slab – Minimum thickness
One-Way Slab DesignOne-Way Slab Design
Thickness of One-Way Slabs
Fire Rating
This is equal to the number of hours for unexposed surface to rise a set amount usually 120 o C
9.0 cm. 1 hour 12.5 cm. 2 hours 16.0 cm. 3 hours
Cover for Slab Cover for Slab ReinforcementReinforcement
ACI Sec. 7.7.1 (min. cover for corrosion protection)
( 1.) Concrete exposed to earth or weather.
( 2.) Concrete not exposed to earth or weather.
< 16 = 4.0 cm
> 16 = 5.0 cm
< 32 = 2.0 cm
Min. covers for fire ratings should also be considered.
Minimum Reinforcement
• According to ACI-Code 7.12.2.1 and for steels yielding at , the shrinkage reinforcement is taken not less than 0.0018 of the gross concrete area, or
hbA shrinkages 0018.0
Solid slab
Ribbed SlabOne-way slabTwo way slab
One-way slabTwo way slab
2L
S2
L
S
One-Way Solid Slab DesignOne-Way Solid Slab DesignReinforcement
Typical Reinforcement in a one-way slab:
One-Way Solid Slab DesignOne-Way Solid Slab Design
Cutoffs
If requirements for use of ACI Moment Coefficients:
One-Way Solid Slab DesignOne-Way Solid Slab Design
Need to confirm thickness is adequate for one-way shear. Difficult to place shear reinforcement in a slab. Minimum area of shear reinforcement required in slabs if
dbf
V
VV
wc
c
cu
6
Usual use
Moment Diagram: Approximate Method of Continuous Beams and One-Way Slabs:
More than two slabs
Two slabs
Shear Diagram: Approximate Method of Continuous Beams and One-Way Slabs:
More than two slabs
Two slabs
Summary of One-way Solid Slab Design Procedure
• Select representative 1-m wide design strip/strips to span in the short direction
• Choose a slab thickness
• Calculate the factored load by magnifying service dead and live loads according to this equation . The dead load includes own weight of the slab in addition to the covering materials it supports. The live load is dependent on the intended use of the slab.
• Draw the shear force and bending moment diagrams for the strip
Example 1Using the ACI-Code approximate structural analysis, design for a warehouse, a continuous one-way slab supported on beams 5.0 m apart as shown in Figure . Assume that the beam webs are 30 cm wide. The dead load is 3 kN/m2 in addition to the slab own weight, and the live load is 2 kN/m2. Use
Mpafc 25
Mpaf y 420
2
6
85.0
10211
85.0
dbf
M
f
f
wc
u
y
c
10@20cm
25.20
79.0
100
9.3
9.34.38618410000021.0
0021.01840001 52 .90 85.0
3.2610211
420
2585.0
.3.26moment .
10
22
2
6
use
S
Scm
cmmmAve
mKNveMax
strip
s
O.k.
0184.0
0161.0
0033.0
1000
184
max
lim
min
mmb
mmd
10@15cm
67.18
79.0
100
23.4
23.42.42318410000023.0
0023.01840001 52 .90 85.0
0.2910211
420
2585.0
.0.29moment .
10
22
2
6
use
cmS
Scm
cmmmAve
mKNveMax
strip
s
O.k.
Ribbed Slab
One-way Ribbed Slab Design Notes
• According to ACI-Code 8.11.6.1, topping slab thickness is not to be less than 1/12 the clear distance between ribs, nor less than 5.0 cm.
• According to ACI-Code 8.11.8, shear strength provided by rib concrete Vc may be taken 10 % greater than those for beams. It is permitted to increase shear strength using shear reinforcement or by widening the ends of ribs
• Ribs are designed as rectangular beams in the regions of positive moment between the supports and as T-shaped beams in the regions of negative moments at the supports. Effective flange width beff is taken as half the distance between ribs, center-to-center
Summary of One-way Ribbed Slab Design Procedure
• The direction of ribs is chosen.• The overall slab thickness h is determined
based on deflection control requirement. • The factored load on each of the ribs is
computed.• The shear force and bending moment
diagrams are drawn.• The strength of web in shear is checked.• Design positive and negative moment
reinforcement.
Example 2
Design a one-way ribbed slab to cover a 4 m x 10 m panel, shown in Figure below. The covering materials weigh 2 kN/m2, the live load is 2 kN/m2 Mpafc 25
Mpaf y 420
An Example for Ribbed slab Reinforcement
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