1. Reinforced Concrete Design II
Lecture 10
Dr. Nader Okasha

2. Two way slabs Introduction

3. Introduction: One way slab:
When the ratio of the longer to the shorter side (L/S) of the slab is at least equal to 2.0, it is called one-way slab. Under the action of loads, it is deflected in the short direction only, in a cylindrical form. Therefore, main reinforcement is placed in the shorter direction, while the longer direction is provided with shrinkage reinforcement to limit cracking.
Strip 4
Strip 2

4. Introduction: Two way slab:
When the ratio (L/S) is less than 2.0, it is called two-way slab, shown in Figure 8.2. Bending will take place in the two directions in a dish-like form. Accordingly, main reinforcement is required in the two directions.
Strip 4
Strip 1,2

5. Introduction:
Moments in rectangular slabs supported at the corners:

6. Introduction:
Moments in rectangular slabs supported at the corners:

7. Introduction: Two way slabs Types: Flat Plates (simple slabs)
A flat plate floor is a two-way slab with no supporting beams, only columns. The slab has smooth under surfaces without column capitals or drop panels. This type of two-way slabs is suited for light loads associated with apartment construction. Use of flat plates is limited by their punching shear capacities where large thickness is required in case of heavy loads or large spans, thus rendering the system uneconomical. Deflection may be excessive.

8. Introduction: Two way slabs Types: Flat Slabs
A two-way slab with column capitals or drop panels, or both. This system is appropriate for heavier loads encountered in office or industrial buildings.

9. Introduction: Two way slabs Types: Edge supported solid slabs
In this system, beams provide moment interaction with the columns especially when moment resisting frames are used to
resist lateral loads.

10. Introduction: Two way slabs Types: Waffle Slab
The waffle slab is capable of providing the largest spans of the conventional concrete floor systems. Waffle slab construction consists of orthogonal sets of ribs with solid parts at the columns. The ribs are formed with fiberglass or metal dome forms (pans). The ribs are usually 0.60 to 0.90 meter on center. Shear is transferred to the columns by using beams or shear heads.

11. Introduction: Two way slabs Types: Edge supported Ribbed Slabs
It is similar to the waffle slab but the voids between ribs are filled with hollow blocks. Hidden or drop beams can be used with this system depending on their spans.

12. Introduction:
Deciding the type of floor system:

13. Introduction:
Design methods:

14. Minimum thickness of two way slabs to control deflection

15. Minimum thickness of two way slabs to control deflection

16. Minimum thickness of two way slabs to control deflection

17. Minimum thickness of two way slabs to control deflection

18. Minimum thickness of two way slabs to control deflection

19. Minimum thickness of two way slabs to control deflection

20. Minimum thickness of two way slabs to control deflection

21. Minimum thickness of two way slabs to control deflection

22. Minimum thickness of two way slabs to control deflection
Equivalent beam:

23. Minimum thickness of two way slabs to control deflection
Standard drop panel:

24. Minimum thickness of two way slabs to control deflection

25. Minimum thickness of two way slabs to control deflection

26. Minimum thickness of two way slabs to control deflection

27. Minimum thickness of two way slabs to control deflection

28. Minimum thickness of two way slabs to control deflection

29. Minimum thickness of two way slabs to control deflection

30. Minimum thickness of two way slabs to control deflection
For SI Units