Difference Between One-way Slabs and Two-way Slabs

One way Slab and Two way Slab

A slab serves as a flat surface that is typically horizontal. Used in bridges, floors, buildings roofs, and other types of structures, slabs are supported by walls and reinforced cement beams. These beams are typically cast monolithically along with the slab. It carries the load distribution uniformly and its depth is very small compared to its width and length.

Before we get into the difference between one way slab and two way slab, you should know that there are 5 types of slabs:

  1. Flat slab
  2. One-way slab
  3. Waffle slabs or grid slabs
  4. Two-way slab
  5. Circular slab

What is a One-way Slab?

Slabs that have only two rigid supports on either side, carry the load on them by flexure in a perpendicular direction to the rigid supports. When a slab is predominantly loaded uniformly, its plane surface distorts into a cylindrical surface. The curvature and the need for the cylindrical surface develop only in one direction. Such slabs are named one-way slabs.

One-way slabs essentially act as shallow beams having a large width. Slabs that are supported on all 4 sides also behave as a one-way slab if the ratio of their span from longer to shorter is greater than 2 (according to IS 456:2000).

What is a Two-way Slab?

Some slabs receive support on all 4 edges and bear the load by flexure in 2 perpendicular paths, instead of one. They have a less than two longer to shorter span ratio and are called two-way slabs.

Two-way Slab and One-way Slab: How to Calculate Them

Here are the steps to calculate one-way and two-way slabs:

  • If the ratio of their span from longer to shorter is made greater than or equal to 2 then, the slab is regarded as a one-way slab.

 L/b (longer span/shorter span) ≥ 2

  • On the other hand, if the ratio of their span from longer to shorter is made less than 2, the slab is regarded as a two-way slab.

 L/b (longer span/shorter span) < 2

One-way Slab and Two-way Slab: Difference Between Them

Sl. No. Point of Comparison One-way Slab Two-way Slab
Longer span to shorter span ratio (L/b) One-way slabs have an L/b ratio that is greater than or equal to two Two-way slabs have an L/b ratio of less than 2.
The direction of bending Slabs bends only in a shorter span direction. Two-way slabs bend in both directions (longer span and shorter span)
Reinforcement The shorter spans receive main reinforcement while the longer spans are provided with distribution reinforcement. The main reinforcement is delivered in both directions.
The direction of the crank The crank is provided in 2 directions. The crank is provided in all 4 directions.
Beam support Beam support is provided to a one-way slab on its two opposite sides. Two-way slabs are supported by a beam on all 4 sides.
The direction of the load Load is carried in only one direction in a shorter span. Load is carried in both directions.
Deflected shape After the bending, the deflected shape of a one-way slab is cylindrical. After bending, the deformed shape of a two-way slab is a saucer or dish-like shape.
Examples Varanda and Chajja are examples of one-way slabs. Constructive floors of multistory buildings are examples of two-way slabs.
Amount of steel and thickness One-way slabs have less steel in them which is why it has increased depth and thickness. Two-way slabs have more steel in them which is why it has decreased depth and thickness.
Economical span One-way slabs are economical up to a span of 3.6 metres. Two-way slabs are economical for panel sizes up to 6m ✕ 6m.

One-way and Two-way Slab: Reinforcement Details

Reinforcement Details of One-way Slab

Suppose, we take a slab measuring 4m in breadth and 10m in length. Then,

L/B = 10/4 = 2.5 ≥ 2

It is clear from the above example that the L/B ratio is greater than 2, making it a one-way slab. The distribution bars are provided in a longer span while the cranked bars or main bars are provided in a shorter span.

Reinforcement Details of Two-way Slab

Let’s take a slab measuring 6m in breadth and 6m in length. Then,

L/B = 6/6 = 1 < 2

It is clear from the example taken that its L/B ratio is less than 2, making it a two-way slab. Here the cranked bars or main bars are provided at a shorter span as well as the longer span too.

The Bottom Line: One way slab and two way slab

The main point of difference between a one-way slab and a two-way slab is that the ratio of their span from longer to shorter will be greater than equal to two for one-way slabs whereas the span ratio of longer to shorter will be less than two (not equal to two) for two-way slabs. Both have their set of differences apart from the L/B ratio (longer span to shorter span). We hope our article on the difference between a one-way slab and a two-way slab helped you spot their differences.

One way Slab and Two way Slab FAQs:

1. What is the difference between a one-way slab and a two-way slab?

If the ratio of the long side to the short side is greater than 2, then the slab is a one-way slab. On the other hand, if the ratio of the long side to the short side is less than 2, the slab is called a two-way slab.

2. Which slab is stronger, one way or two way?

Both the one-way slab and the two-way slab are not designed to outdo each other in strength but a change in the loading conditions causes a change in the length-breadth ratio of the slab. Whichever slab can carry all the load will be considered strong.

3. What is the maximum span of a slab?

There is no limit to the maximum length of a slab, theoretically. However, practically speaking, the length of a slab is restricted from 6.0m to 9.0m. This also makes it economically feasible.

4. What is the minimum depth of a two-way slab?

The minimum depth of a slab with no drop panel is 125mm. With a drop panel, the minimum depth would be 100mm.

5. What is a cantilever slab?

Cantilever slabs are extended beyond the line of the ground floor to form a cantilever. It is a common feature in buildings where you need to have bigger spaces on the upper floors.

6. Which is more economical, the two-way slab or a one-way slab?

Two-way slabs are economical for panel sizes up to 6m ✕ 6m while one-way slabs are economical up to a span of 3.6 meters. The quality of steel in one-way slabs is less than compared to one-way slabs.

7. What is the advantage of two-way slabs over one-way slabs?

Two-way slabs do not construct the reflection of light. If you use a two-way slab in a room, its height will be more as a flat ceiling will be available because of no beam in the middle of the cell. Two-way slabs give you a beautiful and spacious room.

8. What is the crank length in a slab?

Crank is the term used to define a light bending in the bars at the lap. The bending maintains a clear cover even at the lap positions. The minimum length of a crank is 300mm with a 1:10 slope.
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