One Way and Two Way Slab | Basic Info and Design Aspect

One way and Two way Slab Design Aspect

Slab, In architecture, a flat, monolithic piece of stone or concrete used for a floor or roof. There are various forms of reinforced-concrete slabs: one-way slabs cast with supporting beams, ribbed slabs cast with series of joists, two-way ribbed slabs (known as waffle slabs), flat plates reinforced in two or more directions without beams or girders, and flat slabs thickened at column supports with drop panels and/or column capitals. 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. It is supported by parallel walls or beams, bend in only one direction and transfer their loads to the two opposite support walls or beams.

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

But, 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.

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 slab

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.

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 procedure by the Direct Design Method

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.