What is Two Way Slab? Types, Calculations & Details
In structural engineering, two way slabs play a vital role because they give infrastructure and buildings stability and support. In simple terms, “two-way” means the slab is a structural component that offers balanced support for various designs of buildings by equally distributing loads in both the length and width directions. Also read one way slab and two way slab.
What is Two Way Slab?
If loads are distributed in both directions or ways of a slab and these loads are carried by all sides, then the slab is called a two way slab.
In the figure two way slab images, (L) is longe direction and (W) is short direction of the slab. If L/W < 2, then the slab is two way and the deflection occurs in both directions.
Simple Definition of the Two way Slab
In a Two Way Slab, the ratio of longer span (l) to shorter span (b) is less than 2. Because, when longer span (L) to shorter span (W) is less than 2, then the slab will tend to bend in both directions.
Types of Two Way Slab
- Two-way solid slab (with beams)
- Two-way Waffle Slab (with Beams)
- Two-way Waffle Slab (with Integral Beams)
Two Way Slab Formula
Longer span(L) /Shorter span (W)<2
Two Way Slab Example
L= 50
W= 40
L/W=50/40=1.25<2
What are the Two Way Slab reinforcement details?
In two way slab, the main reinforcement is installed on both sides with extra distribution bars to enhance the durability.
- Slab Formwork: During construction, concrete slabs are supported and shaped by a temporary framework referred to as slab formwork.
- Concrete Cover: a layer of protection against environmental factors and corrosion for the concrete reinforcement.
- Main bar: The main straight bars known as bottom bars are provided along a longer span of the slab.
- Distribution or Cross Bars: Distribution bars known as top bars are placed perpendicular to the main bars.
- Detailing at Corners and Edges: To improve strength and prevent corner cracking, extra bars such as bent-up bars are frequently utilized at corners.
- Shear Reinforcement: To resist bending stresses and avoid failures, two-way slabs use stirrups or bent-up bars near supports.
- Clear Cover: For reinforcement to remain rust-free and in the proper location inside the concrete, a clear covering is essential.
How to design the Two Way Slab?
In slab continuously supported on all four sides by stiff beams there are significant bending moments in both orthogonal directions. With the increase in load, the center part of the slab deflects downwards while corners tend to curl upwards.
- If the edges are held down by beams or walls, a negative moment is generated that causes diagonal cracks at corners.
- Variations in bending moments in uniformly loaded slabs simply supported on all four sides are shown here.
- At overload, a pattern of yield lines develops and the slab will collapse with the formation of a mechanism.
What is the Minimum Thickness Two way slabs?
Minimum Thickness of Nonpre-stressed 2 way slabs without interior beams.
Fw MPa(2) | Without drop panels(3) | With drop panels(3) | ||||
---|---|---|---|---|---|---|
Exterior panels | Interior panels | Exterior panels | Interior panels | |||
Without edge beams | With edge beams(4) | Without edge beams | With edge beams(4) | |||
280 | Ln/33 | Ln /36 | Ln /36 | Ln /36 | Ln /40 | Ln /40 |
420 | Ln /30 | Ln /33 | Ln /33 | Ln /33 | Ln 36 | Ln /36 |
520 | Ln /28 | Ln /31 | Ln /31 | Ln /31 | Ln /34 | Ln /34 |
How to calculate the load of the Two way slab?
Calculating the load on a beam from a two way slab through the formula is:
Y = (1/2) x A x B x C
- Y= Total load on the beam
- A = Depth of the slab
- B = Span of the slab
- C= Distribution coefficient. For a simply supported slab distribution coefficient is typically 0.25 and o.20 for a continuous slab.
You can watch the tutorial to better understand the way to calculate the load.
Note: Remember that this formula is only usable for uniformly loaded two way slabs. It could be necessary to perform extra computations if the slab has additional issues or is not loaded uniformly.
Frequently Asked Questions:
What is the span ratio of a two way slab?
The span ratio of a two way slab is less than 2.
What size is a two-way slab?
Conventional spans of 10′-40′ (3.05-12.19 m) and post-tensioned spans of 13′-46′ (3.96-14.02 m) are the two standard sizes for Two Way Slab Concrete Floor construction.
What depth of a Two way slab?
The slab’s depth, as determined by the span-to-depth formula of span/30, is primarily between 4″- 14″ (10.2-35.6 cm).
Wrapping Up:
Two-way slabs transmit the load equally on both sides and are susceptible to two-way bending. Engineers are able to ensure the durability and integrity of structures supported by two-way slabs by observing specifications and applying careful detailing. They know when the two way slab is used to enhance the lifespan of the property.