One-way and two-way slabs are two different types of reinforced concrete slabs used in building construction.
One-way slab
A one-way slab is a type of reinforced concrete slab that is supported by beams on two opposite sides.
The slab is designed to span in one direction only, perpendicular to the supporting beams.
One-way slabs are typically used in structures where the ratio of the longer length of the span to the shorter length of the slab is greater than or equal to 2.
| For example: Consider a rectangular reinforced concrete slab of dimensions 2 m x 6 m. Where Longer span (Ly) = 6m Shorter span (Lx) = 2m we have to find out which type of slab is this. Solution: Put the above values in the formula One-way slab = Longer span(Ly)/Shorter span(Lx) ≥ 2 = 6/2 = 3 As this value is more than 2. The type of slab is a One-way slab. |
One-way slabs are usually thinner than two-way slabs and are easier and less expensive to construct.
Load mechanism in one-way slab
In one-way slab systems, the loads are primarily transferred in one direction, typically along the longer span of the slab. This is because the reinforcement in the slab is placed primarily in one direction, usually perpendicular to the longer span.
As a result, the slab has less capacity to resist loads in the other direction. One-way slabs are typically used for smaller spans and lighter loads, such as in residential or low-rise commercial buildings.
Two-way slab
On the other hand, a two-way slab is a type of reinforced concrete slab that is supported by beams on all four sides. The slab is designed to span in two directions, perpendicular to each other, with the load being distributed in both directions.
Two-way slabs are used in structures where the ratio of the longer length of the span to the shorter length of the slab is less than 2.
| For example: Consider a rectangular reinforced concrete slab of dimensions 4 m x 6 m. Where Longer span (Ly) = 6m Shorter span (Lx) = 4m we have to find out which type of slab is this. Solution: Put the above values in the formula Two-way slab = Longer span(Ly)/Shorter span(Lx) <2 = 6/4 = 1.5 Since the aspect ratio is less than two, the slab is a two-way slab. |
Two-way slabs are usually thicker and more rigid than one-way slabs, and they can support heavier loads.
Load mechanism in Two-way Slabs
In two-way slab systems, the loads are distributed in both directions, along both the longer and shorter spans of the slab. This is because the reinforcement in the slab is placed in both directions, creating a more balanced and uniform distribution of loads.
Two-way slabs are typically used for larger spans and heavier loads, such as in high-rise buildings or industrial structures.
Difference between one-way and two-way slabs
| S. No | Criteria | One-way Slab | Two-way Slab |
| 1 | Span direction | One direction only | Two perpendicular directions |
| 2 | Support type | Supported on two opposite sides | Supported on all four sides |
| 3 | Span-ratio | Greater than or equal to 2 | Less than 2 |
| 4 | Formula | Longer span(Ly)/Shorter span(Lx)2 | Longer span(Ly)/Shorter span(Lx)< 2 |
| 5 | Reinforcement layout | Reinforcement in one direction | Reinforcement in both directions |
| 6 | Depth | Relatively shallower | Relatively deeper |
| 7 | Bending strength | Lesser than a two-way slab | Greater than one-way slab |
| 8 | Load Capacity | Lesser than a two-way slab | Greater than one-way slab |
| 9 | Beam requirement | Requires supporting beams | Requires supporting beams |
| 10 | Formwork cost | Cheaper due to simpler shape | More expensive due to the complex shape |
| 11 | Slab thickness variation | Cannot vary easily | Can vary easily |
| 12 | Moment distribution | More uniformly distributed | Not uniformly distributed |
| 13 | Shear distribution | Less uniformly distributed | More uniformly distributed |
| 14 | Diaphragm action | Cannot provide diaphragm action | Can provide diaphragm action |
| 15 | Maximum span length | Up to 6 meters or less | Up to 12 meters or more |
| 16 | Reinforcement complexity | Simple and easy to install | More complex and difficult to install |
| 17 | Construction cost | Less expensive | More expensive |
| 18 | Lateral stability | Less stable than a two-way slab | More stable than a one-way slab |
| 19 | Load distribution | The load is concentrated in one direction | Load is distributed in both directions |
| 20 | Flexibility in design | Limited design options | More design options are available |
| 21 | Application | Suitable for smaller spans | Suitable for larger spans |
| 22 | Example | Chajja (sun shade/sunblock) and Varandha | Small residential buildings, Large commercial buildings, such as offices or malls |
