Difference Between One Way Slab and Two Way Slab | What is Slab
Civil Design Difference

Difference Between One Way Slab and Two Way Slab | What is Slab

Difference Between One Way Slab and Two Way Slab | What is Slab

What Is a Slab?

A slab is constructed to provide flat surfaces, typically horizontal, in building roofs, floors, bridges, and other types of structures. The slab could be supported by walls, by reinforced concrete beams normally cast monolithically with the slab, by structural steel beams, either by columns or from the ground.

A slab is a plate element having depth (D), very small as compared to its length and width. A slab is used as floor or roof in buildings, carry distribution load uniformly.

Type of Slab

Type of Slab b

Slab May Be

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Type of slabs based on support conditions are: 

    1. One Way Slab
    2. Two Way Slab
    3. Flat Slab Resting Direction on a Column Without Beam 
    4. Grid Slabs or Waffle Slab
    5. Circular Slab and Other Shapes 

What Is a One-Way Slab?

One way slab

One Way Slab

    • The most straightforward routine structural element for illustration of design provisions from the Code is that the one-way slab.
    • A one-way slab is defined for functions of the book as a flexural member with thickness small relative to other dimensions, sup-porting (gravity) loads applied normal to and directly above its surface, a span in one direction between parallel supports, and fortified for flexure in this direction only.
    • For purposes of analysis, one-way slabs might be restrained to some degree in the supports or possibly unrestrained. A number of Code provisions reference to “flexural members,” including one- and – two-way slabs, beams, girders, footings, as well as where bending is present together with the axial walls, load, and columns.
    • In general, when this code provision is intended to use to one-way slabs, the term is going to be utilized in the sense of this definition herein.
    • If the is supported on two opposite sides, it is called a one-way spanning slab. In this type of slab, loads are transferred on two opposite as per the above figure.
    • If the slab is supported at four sides, and if Ly/Lx ≥ 2 one way spanning slab.
    • For any slab, if Ly = Lx, the slab has a tendency to bend in both directions. With an increase of Ly, the tendency of bending along Ly is reduced and that on Lx is increased.
    • When Ly/Lx ≥ 2, the slab bends only in X-directions
    • When Ly/Lx ≥ 2, the slab is called a one-way slab. In one-way slab, the main reinforcement is provided along Lx ( Short-Span)

Also, read: Total Station in Surveying | Operations | Advantage & Disadvantage | Types

What Is a Two-Way Slab?

Two way slab

    • The design considerations of wall-supported two-way slabs are similar to those pertaining to one-way slabs.
    • The thickness of the slab is generally based on deflection control criteria, and the reinforcements in the two orthogonal directions are designed to resist the calculated maximum bending moments in the respective directions at the critical sections. [Additional reinforcement may be required at the corners of two-way slabs in some cases, as explained later].
    • The slab thickness should be sufficient against shear, although shear is usually not a problem in two-way slabs subjected to uniformly distributed loads.
    • If the slab is supported at all four edges and if  Ly/Lx < 2,
    • The tendency of the slab is to bend in both directions. Such slab is called two-way slab as per above figure  c
    • In two way slabs, main reinforcement is provided along Lx as  well as Ly direction

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What Is the Flat Slab?

Flat Slab

Flat Slab

    • When the slab is directly supported on a column, without beams, it is known as a flat slab.
    •  Flat slab is provided to increase the floor height and to permit a large amount of light which might be obstructed by the depth of beams.

What Is Grid Slab?

Grid Slab

Grid Slab

    • When the slab is supported on beams with column only on the periphery of the hall, the slab is called grid slab.
    • Sometimes, in large halls, public places, marriage halls, auditoriums, etc. a large column-free area is required.
    • In these cases, large deep beams may be permitted, but the columns are permitted only on the periphery.

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Analysis of Slab:

    • Slabs are primarily flexural members as beam and are analyzed and designed in the same manner as the beams. The analysis may be carried out as follows:
      • Elastic Analysis:

        • A strip of 1 m width of the slab is considered, and loads are found on this strip. This strip id analyzed as a beam 1 m width.
      • Code Coefficients:

        • This is a semi-empirical method of analysis based on yield line theory. The coefficients give in code may be directly used to analyze the slabs.
        • However, the redistribution of moments is not permitted in this case.
      • Yield Line Theory

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Difference Between One Way Slab and Two Way Slab

One Way SlabTwo Way Slab
One way slab two opposite side support beam /wall Two Way Slab four side mins all side supported beam /wall
One way slab is bend only in one spanning side direction while load transfer Two way slab is bend both spanning side direction while load transfer
Main Reinforcement is in provide short span due to banding Main Reinforcement is in provide short span due to banding
Ly/Lx ≥ 2 one way slab spanning   Ly/Lx < 2 two way slab spanning
One-way slab near about 100mm to 150mm based on the deflection two-way slabs is in the range of 100mm to 200mm depending upon
one way slab economical near about 3.5 m Two-way slab may economical for the panel sizes near about 6m x 6m.

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Design Considerations: 

  • One Way Slab

1. Effective Depth (d)

      • For deflection control 
      • L/d = 20 X M.F
        • M.F. Modifiction factor from— IS: 456, p.38.Fig-4
        • Assume % steel 0.3 to 0.6%
      • Fs =  0.58 Fy X (Ast requierd / Ast Provied)
      • Initially assume that Ast reqierd = Ast Provided 
      • Fy = 250 N/ Sq.mm —– Fs = 0.58 X 250 = 145 N/ sq.mm.
      • Fy = 415 N/ Sq.mm —– Fs = 0.58 X 415 = 240 N/ sq.mm.
      • Fy = 500 N/ Sq.mm —– Fs = 0.58 X 500 = 290 N/ sq.mm.

2. Effective Span:

      • ClearSpan + d
      • c/c of Supports

Whichever is smaller ——–  as per IS 456-2000 P. 34, CI 22.2.a

3. Reinforcement Requirements

    • Minimum reinforcement 
      • For Fe-250     Pt = 0.15 % of total C/s area (d x D)
      • For Fe-415     Pt = 0.12 % of total C/s area (d x D)
      • For Fe-500     Pt = 0.12 % of total C/s area (d x D) ——–  as per IS 456-2000 P. 48, CI 26.5.2.1
    • Maximum diameter (Sp 34)
      • For minbar:
        • Plain bars———–10 mm Ø min dia
          Deformed bars—–8 mm Ø min dia
      • For Distribution bars:
        • Plain bars———–6 mm Ø min dia
          Deformed bars—–6 mm Ø min dia

4. Check for Cracking

    • For Min Steel:
      • 3d ——— Where. d = Effective depth
      • 300mm

Spacing should not exceed smaller these two values.

    • For Distribution steel:
      • 5 d
      • 450mm

Spacing should not exceed smaller these two values. ——- IS: 456-2000, P.46

5. Check for Deflection:

    • Allowable L/d  = 20 X M.F.
      • M.F is Obtained from IS:456-200 P-38 Fig 4
    • Find actual, L/d
    • If Actual L/d < allowable  L/d ———- Ok

6. Check for Development Length (Ld)

IS 456-2000,P.44, Cl. 26.2.3.3 C

    • Ld should be ≤ 1.3  (M1/V) + L0

Where

    • Ld = (Ø.σs / 4 τ bd )—————–σs = 0.87 fy          As per IS 456-200, P.42

 

    • 50 % of steel is bent up near support. Therefore find M.R for 50 % of steel only
    • M1 = M.R. for 50% steel support
    • V = Shear force at the support 
    • L0 = Sum of anchorage beyond the center of support
    1.  d
    2. 12  Ø

Take L0 as the smaller of two values.

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  • Two Way Slab

1. Effective Depth (d):

      • For deflection control 
      • L/d = 35 X M.F X 0.8
        • M.F. Modifiction factor from— IS: 456, p.38.Fig-4
        • Assume % steel 0.3 to 0.6%
      • Fs =  0.58 Fy X (Ast requierd / Ast Provied)
      • Initially assume that Ast reqierd = Ast Provided 
      • Fy = 250 N/ Sq.mm —– Fs = 0.58 X 250 = 145 N/ sq.mm.
      • Fy = 415 N/ Sq.mm —– Fs = 0.58 X 415 = 240 N/ sq.mm.
      • Fy = 500 N/ Sq.mm —– Fs = 0.58 X 500 = 290 N/ sq.mm.

2. Effective Span:

      • ClearSpan + d
      • c/c of Supports

Whichever is smaller ——–  as per IS 456-2000 P. 34, CI 22.2.a

3. Load Calculations:

      • Total Load = D.L. + F.L. + L.L.
        • Dead load of slab = (d x 25)
        • Floor Finishing load = (as floor finishing near 1 kn/sq.mm)
        • Live load = ( as per calculation)
      • Factor Load = 1.5 x Total load

4. Mid Span Moment:

Corners not held down conditions is given as per IS: 456-2000 P-90 CI D-2

    • Mx = ax . w . lx . lx
    • My = ay . w . lx. lx
      • ax and ay coefficientare obtained from IS: 456 table -26, fig 10.3 shoe nine separate possible arrangement for two way restrained slab.

5. Effective Depth of Flexure:

    • Mu = 0.138 . fck . b.d.d
      • Heaer find d
      • Mu = Sp 16 P 10 Table C
      • Fck = strength of concrete
      • b = 1 m area required load

6. Reinforcement in  Mild Strip :

    • Along Lx 
      • Pt=  50 (fck/fy)x( 1-√(1-(4.6xMu/Fck b.d.d)))
        • fck = strenth of concrete
        • fy = 415 N/Sq.mm
        • Mu = Sp 16 P 10 Table C =0.138 . fck . b.d.d= Mx
    • Along Lx 
      • Pt=  50 (fck/fy)x( 1-√(1-(4.6xMu/Fck b.d.d)))
        • fck = strenth of concrete
        • fy = 415 N/Sq.mm
        • Mu = Sp 16 P 10 Table C =0.138 . fck . b.d.d= My

7. Check for Cracking:

    • Along Lx:
      • 3d ——— Where. d = Effective depth
      • 300mm

Spacing should not exceed smaller these two values.

    • Along Ly:
      • 3d ——— Where. d = Effective depth
      • 300mm

Spacing should not exceed smaller these two values.

8. Check for Deflection:

    • Allowable L/d  = 35 X M.F.X 0.8
      • M.F is Obtained from IS:456-200 P-38 Fig 4
    • Find actual, L/d
    • If Actual L/d < allowable  L/d ———- Ok

9. Check for Development Length:

IS 456-2000,P.44, Cl. 26.2.3.3 C

    • Ld should be ≤ 1.3  (M1/V) + L0

Where

    • Ld = (Ø.σs / 4 τ bd )—————–σs = 0.87 fy          As per IS 456-200, P.42

 

    • 50 % of steel is bent up near support. Therefore find M.R for 50 % of steel only
    • M1 = M.R. for 50% steel support
    • V = Shear force at the support 
    • L0 = Sum of anchorage beyond the center of support
    1.  d
    2. 12  Ø

Take L0 as the smaller of two values.

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Hey, I am Krunal Rajput. The Man Behind CivilJungle. I started this site to spread knowledge about Civil/Mechanical/Electrical Engineering. I am a Degree Holder in Civil Engineering.

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