Saturday, January 14, 2017

Concrete Slab

Concrete Slab


The Concrete slab provides a horizontal surface and is usually supported by columns, beams or walls. Slabs can be categorized into two main types: one-way slabs and two-way slabs.
One-way slab is the most basic and common type of slab. One-way slabs are supported by two opposite sides and bending occurs in one direction only. Two-way slabs are supported on four sides and bending occurs in two directions. One-way slabs are designed as rectangular beams placed side by side.

Concrete Slab
However, slabs supported by four sides may be assumed as one-way slab when the ratio of lengths to width of two perpendicular sides exceeds 2. Although while such slabs transfer their loading in four directions, nearly all load is transferred in the short direction.
Two-way slabs carry the load to two directions, and the bending moment in each direction is less than the bending moment of one-way slabs. Also two-way slabs have less deflection than one-way slabs. Compared to one-way slabs, Calculation of two-way slabs is more complex. Methods for two-way slab design include Direct Design Method (DDM), Equivalent frame method (EFM), Finite element approach, and Yield line theory. However, the ACI Code specifies two simplified methods, DDM and EFM.

Types of Slabs

One-way slabs
⇪ Two-way slabs

P ≠ Q


S = Y

One-way slabs

1. One-way Beam and slab / One-way flat slab:

These slabs are supported on two opposite sides and all bending moment and deflections are resisted in the short direction. A slab supported on four sides with length to width ratio greater than two, should be designed as one-way slab.

2. One-way joist floor system:

This type of slab, also called ribbed slab, is supported by reinforced concrete ribs or joists. The ribs are usually tapered and uniformly spaced and supported on girders that rest on columns.

Two-way slabs

1. Two-way beam and slab:

If the slab is supported by beams on all four sides, the loads are transferred to all four beams, assuming rebar in both directions.

2. Two-way flat slab:

A flat slab usually does not have beams or girders but is supported by drop panels or column capitals directly. All loads are transferred to the supporting column, with punching shear resisted by drop panels.

3. Two-way waffle slab:

This type of slab consists of a floor slab with a length-to-width ratio less than 2, supported by waffles in two directions.

Design Procedure

• One-way slab design

1. Decide the type of slab according to aspect ratio of long and short side lengths.
2. Compute the minimum thickness based on ACI Code.

3. Compute the slab self-weight and total design load.

4. Compute factored loads (1.4 DL + 1.7 LL).

5. Compute the design moment.

6. Assume the effective slab depth.

7. Check the shear.

8. Find or compute the required steel ratio.

9. Compute the required steel area.

10. Design the reinforcement (main and temperature steel).

11. Check the deflection.

• Two-way slab design

1. Decide the type of slab according to aspect ratio of long and short side lengths.

2. Check the limitation to use the DDM in ACI Code. If limitations are not met, the DDM can not be used.

3. Determine and assume the thickness of slab to control deflection.

4. Compute the slab self-weight and total design load.

5. Compute factored loads (1.4 DL + 1.7 LL).
6. Check the slab thickness against one-way shear and two-way shear.

7. Compute the design moment.

8. Determine the distribution factor for the positive and negative moments using ACI Code.

9. Determine the steel reinforcement of the column and middle strips.

10. Compute the unbalanced moment and check if it is adequate.

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