1. ANALYSIS AND DESIGN OF FLAT SLAB
BY MOHAMMED FARHAN

2. INTRODUCTION

3. HISTORICAL DEVELOPMENT
Flat slabs were originally invented in the U.S.A in year 1906.
This was the start of these type of construction.
Many slabs were load-tested between in U.S.A.
1914 Nicholas proposed a method of analysis of these slab based on simple statics, this method is know as direct design method.

4. BASIC DEFINITION OF FLAT SLAB
The term flat slab means a reinforces concrete slab with or without drop, supported generally without beams, by column with or without flared column heads.

5. APPLICATION OF FLAT SLAB
In the case of high rise building thinner slabs are required so that additional floors can be added.
The distance that be spanned by post-tensioned slabs exceeds that of reinforced constructions with the same thickness.
For increasing span lengths so as to increases the usable unencumbered floor space in buildings.
For diminishing the number of joints in the structure.
For the speedy construction of the project.
The amount of steel required is much less than in normal RCC structure.
The moulds can be used No. of times as per the demand.
Due to reduce beam section the load transferred to foundation is less compared to that of RCC structure.
The structure is crack free as the whole structure is in compression.
Large span of slab can obtain easily.

6. ADVANTAGES OF FLAT SLAB
Floor to floor height reduction
Faster construction
Early formwork stripping
Water resistant properties
Saving in materials.
Reduced foundation load
Greater column free areas
Architectural freedom
Reduced construction costs

7. LIMITATION OF FLAT SLAB
Careful handling of prefabricated components such as concrete panels or steel and glass panels is required.
Attention has to be paid to the strength and corrosion-resistance of the joining of prefabricated sections to avoid failure of the joint.
Similarly, leaks can form at joints in prefabricated components.
Transportation costs may be higher for voluminous prefabricated sections than for the materials of which they are made, which can often be packed more compactly.

8. CLASSIFICATION OF FLAT SLAB SYSTEM
Solid flat slab(or flat plate)
Solid flat slab with drop panels
Solid flat slab with column heads
Banded flat slab

9. 1.Solid flat slab(or flat plate)

10. 2.Solid flat slab with drop panels

11. 3. Solid flat slab with column heads

12. 3.Banded flat slab

13. METHOD OF CONSTRUCTION SYSTEM
The flat slabs can be cast-in-situ (cast-in-place). Else,the slabs can be precast at ground level and lifted to the final height. The later type of slabs is called lift slabs.
1. POST TENSIONING SYSTEM
2. PRE TENSIONING SYSTEM

14. 1. POST TENSIONING SYSTEM

15. 2. PRE TENSIONING SYSTEM 2.

16. PURPOSE TENSIONING SYSTEM
To reduce the deflection.
To reduce the punching shear.
Reduced structure depth
To Greater column free areas.
Reduces the required number of columns and foundations.
To Increase load bearing capacity.
To bear seismic forces.

17. Component parts of flat slab
Column strip
Middle strip
Panel.

18. ANLYSIS OF FLAT SLAB
The steps of analysis slab is 1. Determine the factored negative (Mu–) and positive moment (Mu+) demands at the critical sections in a slab-beam member from the analysis of an equivalent frame. The values of Mu– are calculated at the faces of the columns. The values of Mu+ are calculated at the spans. The following sketch shows a typical moment diagram in a level of an equivalent frame due to gravity loads.

19. ANLYSIS OF FLAT SLAB
2. Distribute Mu– to the CS and the MS. These components are represented as Mu,–CS and Mu,–MS, respectively. Distribute Mu+ to the CS and the MS. These components are represented as Mu, +CS and Mu, +MS, respectively.

20. ANLYSIS OF FLAT SLAB
3.If there is a beam in the column line in the spanning direction, distribute each of Mu,–CS and Mu, + CS between the beam and rest of the CS.
4) Add the moments Mu,–MS and Mu, +MS for the two portions of the MS (from adjacent equivalent frames).
5) Calculate the design moments per unit width of the CS and MS.

21. Code provision for flat slab
1.Thickness of flat slab
As per IS-456 : 2000

22. Code provision for flat slab
2. For drop
3. For column head

23. Design of flat slab

24. Direct design method Limitations
Slab system designed by the direct design method shall fulfill the following condition.
There shall be minimum of three continuous spans in each direction.
The panels shall be rectangular, and the ratio of the longer span to the shorter span within a panel shall not be greater than 2.0.
It shall be permissible to offset column to a maximum of 10 percent of span in the direction of offset not with standing the provision in (b).

25. Direct design method
The successive span length in each direction shall not differ by more than one-third of the longer span. The end spans may be shorter but not longer than the interior span , and
The design lived load shall not exceed three times the design dead load.

26. Design steps for flat slab
Check preliminary dimension
Check for applicability of DDm
Divide the slab with frame in X and Y directions and obtain dimension of X and Y frames.
Analysis the interior and exterior panel.
a. Longitudinal distribution
b. Transverse distribution
Estimate the design moment in the external column
Estimate the design moment in the internal column
Design for shear
Detailing should be done as per code requirement.

27. Design for shear

28. Punching shear

29. Punching shear reinforcement

30. Detailing of flat slab
1.Plan 2.section

31. Analysis and design of flat slab using etabs software

32. CONCLUSIONS
As per Indian code we are using cube strength but in international standards cylindered are used which gives higher strength than cube.
Drops are important criteria in increasing the shear strength of the slab.
Enhance resistance to punching failure at the junct ion of concrete slab & column.
By incorporating heads in slab, we are increasing rigidity of slab.
In the interior span, the total design moments (Mo).
The negative moment’s section shall be designed to resist the larger of the two interior negative design moments for the span framing into common supports.

33. CONCLUSIONS
According to Indian standard (IS 456) for RCC code has recommended characteristic strength of concrete as 20, 25, and 30 and above 30 for high strength concrete. For design purpose strength of concrete is taken as 2/3 of actual strength this is to compensate the difference between cube strength and actual strength of concrete in structure. After that we apply factor of safety of 1.5. So in practice Indian standard actually us es 46% of total concrete characteristic strength. While in International practice is to take 85% of total strength achieved by test and then apply factor of safety which is same as Indian standard so in actual they use 57% of total strength.
Pre fabricated sections to be integrated into the design for ease of construction.

34. References Indian standards 456,875. S.P 16.
Advanced Reinforced Concrete Design-P.C Varghese.
A.K. Jain - Limit state design of Reinforced concrete.
Reinforced concrete design - S.unnikrishna Pillai, Devdas Menon
S.Ramamrutham & R. Narayan - Design of Reinforced concrete Structures.
V.N, Vazirani & S.P Chandola – Hand book of civil Engineering.

35. FIELD REPORT
During the mini project I visited lanco hills site under which I was done the supervision of IT 6 block in which I was saw the different types of flat slab and construction details ,arrangement of formwork ect.