1. Plain & Reinforced Concrete-1 CE3601
Lecture # 25 , 26 & 27 20th April to 8th May 2012
Analysis and Design of Slabs

2. Plain & Reinforced Concrete-1
Isolated One Way Slab

3. Plain & Reinforced Concrete-1
Continuous One Way Slab

4. Plain & Reinforced Concrete-1
Negative Moment
Positive Moment
Continuous One Way Slab

5. Plain & Reinforced Concrete-1
Minor Bending
Continuous One Way Slab

6. Plain & Reinforced Concrete-1
Example: Design a cantilever projecting out from a room slab extending 1.0m and to be used as balcony (LL = 300 kg/m2). A brick wall of 250 mm thickness including plaster of 1m height is provided at the end of cantilever.
fc’ = MPa fy = 300 MPa
Slab thickness of room = 125 mm. Slab bottom steel 1in the direction of cantilever is # 190 mm c/c.

7. Plain & Reinforced Concrete-1
Solution:
cantilever
125 mm 1m

8. Plain & Reinforced Concrete-1
Solution: (contd…)
Let we use the same thickness as of the room d
Main steel in cantilever is at the top

9. Plain & Reinforced Concrete-1
Solution: (contd…)
Slab Load
Self weight of slab
75 mm brick ballast/ screed
60 mm floor finishes
Total dead load

10. Plain & Reinforced Concrete-1
Solution: (contd…)
Slab Load
Live Load
For a unit strip

11. Plain & Reinforced Concrete-1
Solution: (contd…)
1.063m
Per meter width

12. Plain & Reinforced Concrete-1
Solution: (contd…) d
# 380 mm c/c already available in the form half the bent up bar from the room slab

13. Plain & Reinforced Concrete-1
Solution: (contd…)
Remaining steel required at the top
Use
Distribution steel

14. Plain & Reinforced Concrete-1
Solution:
380 c/c
Slab bottom steel
1500 mm
280 c/c
380 c/c

15. Plain & Reinforced Concrete-1
Two-Way Edge Supported Slabs

16. Plain & Reinforced Concrete-1
Two-Way Slabs
Slab resting on walls or sufficiently deep and rigid beams on all sides. Other options are column supported slab e.g. Flat slab, waffle slab.
Two-way slabs have two way bending unlike one-way slab.

17. Plain & Reinforced Concrete-1
Isolated Two Way Slab

18. Plain & Reinforced Concrete-1
Positive Moment
Negative Moment
Continuous Two Way Slab

19. Plain & Reinforced Concrete-1
Design Methods
ACI co-efficient method
Direct design method
Equivalent frame method
Finite element method
Notes
In two-way slabs shorter direction strip carry greater %age of load.
Steel will be more in shorter direction.
Shorter direction steel will be placed near the outer edge to get more “d” means more lever arm to get more flexural capacity. Ly Lx

20. Plain & Reinforced Concrete-1
ACI Co-efficient Method
Unit width strip is taken in both directions. The strip is designed separately for +ve and –ve moment.
C = ACI co-efficient ωu = Slab load “C” depends upon the end conditions of slab and the aspect ratio. Three tables are available for “C”
Dead load positive moment
Live load positive moment
-ve moment
M+ coefficients are increased by 25 % and M- coefficients are reduced by 10 % to get the result more closer to accurate solution.

21. Plain & Reinforced Concrete-1
Minimum Depth of 2-Way Slab for Deflection Control
According to ACI
hmin = (inner perimeter of slab panel)/ ≥ 90 mm
For fy = 300 MPa
For fy = 420 MPa
According to ACI
Ln = clear span in short direction

22. Plain & Reinforced Concrete-1
Example: Design the 4 marked slab panels of an ordinary house. Use US customary bars. fc’= MPa fy = 300 MPa 1 2
4500 x 7000
6000 x 7000
Wall thickness = 228 mm 3 4
6000 x 6000
3500 x 6000

23. Plain & Reinforced Concrete-1
Solution: Panel Edge Conditions
Panel # 1 Lx = 4.5m , Ly = 7.0m m = 0.64 > 0.5, 2-way slab
Panel # 2 Lx = 6.0m , Ly = 7.0m m = 0.86> 0.5, 2-way slab
Panel # 3 Lx = 3.5m , Ly = 6.0m m = 0.58 > 0.5, 2-way slab
Panel # 4 Lx = 6.0m , Ly = 6.0m m = 1 > 0.5, 2-way slab

24. Plain & Reinforced Concrete-1
Solution: (contd…)
Slab Thickness Generally same depth is preferred for one monolith slab. Calculate hmin for all the panels and select the largest value.
Panel # 1
Panel # 2

25. Plain & Reinforced Concrete-1
Solution: (contd…)
Panel # 3
Panel # 4

26. Plain & Reinforced Concrete-1
Solution: (contd…)
Effective depth d2 d1
Long direction steel
Short direction steel
For short direction steel
For longer direction steel

27. Plain & Reinforced Concrete-1
Solution: (contd…)
Slab Load
Self weight of slab
75 mm brick ballast/ screed
60 mm floor finishes
Total dead load

28. Plain & Reinforced Concrete-1
Solution: (contd…)
Slab Load
Live Load

29. Plain & Reinforced Concrete-1
Solution: (contd…)
Minimum Steel
For a unit strip

30. Concluded