Slabs are the one of the most widely used structural elements whose depth is considerably smaller than rest of the dimensions. Basically slabs are used as roofs and floors in buildings, roof and bottom on water tanks, on bridges etc.
Slabs support and transfer load i.e. Dead load and live load, to columns by shear, flexure, and torsion. Slabs also help in reducing the effects of lateral wind loads and earthquake loads.
Depending on the load carrying behavior of slab slabs are classified into two types namely one-way slab, and two-way slab. On actual construction site selecting the type of slab to be used depends on economy, ease of construction, loading conditions also effective length and span.
One way slabs are the slabs in which most of the loads are carried on the shorter span. The ratio of longer span to shorter span is equal to or greater than two or when the slab is supported by beams only along two opposite sides slab then the slab behaves as a One-way slab.
In easier worlds one way slab are spanned in shorter side means supported on the beams of longer side as shown in figure above. Because of much greater length, longer side i.e.longer beam, carries most of the loads compared to shorter side.
That is why main reinforcement is provided along shorter span and distribution reinforcement along longer span.
The direction in which the load is carried i.e. for one way slab-short side, is called the span. The direction in which the slab bends i.e. for one way slab-short side, is called as the Main direction. One way slabs may be continuous or even cantilever.
Generally one way slabs are used where longer side is too longer than shorter side that is waranda’s.
The figure above shows the detailing of reinforcement in simply supported single span one-way slab with dimensions Ly/Lx = 9/3 = 3. Which is greater than 2 hence we adopted one way slab.
You can notice that alternate main reinforcement bars are bent-up near supports at a distance of 0.1L from support, as per Clause D-1.6 of code, to resist tension that can appear because of partial fixity of support.
You can check out a full detailed article on Reinforcement detailing of One Way Slab here for detailed explanation.
Two-way slabs are the slabs in which loads are carried on both of the spans. The ratio of longer span to shorter span is less than two and when the slab is supported by beams along all the sides then the slab behaves as a two-way slab.
In two way slab as the floor load transferred in both directions, the flexural reinforcement has to be designed in both directions.
Generally two way slabs are used as floors in a multistory buildings.
The figure above shows the detailing of reinforcement in simply supported double-span two-way slab with dimensions Ly/Lx = 4/4 = 1. Which is smaller than 2 hence we adopted two-way slab.
As torsional stresses and torsional moments developed near corners of simply supported slabs, eventually resulting in lifting up of corners. Hence torsional reinforcements are needs to be provided in the corners of the slabs to resist torsional moments and stresses.
You can check out a full detailed article on Reinforcement detailing of Two Way Slab here for detailed explanation.
|One Way Slab||Two Way Slab|
|Aspect Ratio Ly/Lx is greater than equal to two.||Aspect ratio Ly/Lx is smaller than two.|
|One way slabs are supported on beams only on two opposite sides.||Two way slabs are supported on beams on all the sides.|
|In one way slab, the main reinforcement needs to be provided along shorter span due to bending.||In two way slabs, the main reinforcement is provided along both the spans.|
|Loads are carried along shorter span.||Loads are carried along longer span.|
|Distribution Reinforcement needs to be provided along longer span.||There is no need to provide distribution reinforcement as the main reinforcement already provided in both directions.|
|One-way slabs when loaded will deflect into the cylindrical surface.||Two-way slabs when loaded will deflect into dished surface.|
|Generally, one-way slabs are used where the longer side is too longer than the shorter side that is Varanda’s.||Generally, two-way slabs are used as floors in a multistory building.|
Generally, one-way slabs are used where the longer side is too longer than the shorter side that is Varanda’s.
Generally, two-way slabs are used as floors in a multistory building.
For one-way slabs Aspect Ratio i.e. ratio of longer span Ly and shorter span Lx is greater than equal to two.
Also, One-way slabs are supported on beams only on two opposite sides.
One way slabs are more economical upto span 3.6m as it requires less reinforcements.