Transcript Slide 1

Design of Concrete Structure I
Dr. Ali
Tayeh
First
Semester
2009
Lecture 12
One Way Slab
Part 1
Slab – Minimum thickness
One-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.
12.5 cm.
16.0 cm.
1 hour
2 hours
3 hours
Cover for Slab Reinforcement
ACI Sec. 7.7.1 (min. cover for corrosion protection)
( 1.) Concrete exposed to earth or weather.
< f16
= 4.0 cm
> f16
= 5.0 cm
( 2.) Concrete not exposed to earth or weather.
< f32
= 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
As shrinkage  0.0018 b h
Solid slab
Ribbed Slab
Two way slab L  2
S
Two way slab
One-way slab
One-way slab
L
2
S
One-Way Solid Slab Design
Reinforcement
Typical Reinforcement in a one-way slab:
One-Way Solid Slab Design
Cutoffs
If requirements for use of ACI Moment Coefficients:
One-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
Vu  f Vc
Usual use
 fc

f Vc  f 
bw d 
 6



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 1
Using 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
f c  25Mpa
f y  420Mpa
 2 106 M u  
0.85 f c 

1  1  

2 
fy 
 0.85f f c bw d  

d  184m m
b  1000m m
 min  0.0033
 lim  0.0161
 max  0.0184
Max.  ve moment 26.3KN .m


0.85 25 
2 106  26.3
   0.0021 O.k.
1  1  

2 
420 
 0.85 0.9 25  1000184  

 ve As  0.0021 1000184  386.4m m2  3.9cm2
0.79f10
3.9

100cmstrip
S
S  20.25
use f10@20cm
Max.  ve moment 29.0 KN .m


0.85 25 
2 106  29.0
   0.0023 O.k.
1  1  

2 
420 
 0.85 0.9 25  1000184  

 ve As  0.00231000184  423.2m m2  4.23cm2
0.79f10
4.23

100cmstrip
S
S  18.67cm
use f10@15cm
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 f   25Mpa
c
f y  420Mpa
An Example for Ribbed slab Reinforcement