One way slab & Two way slab


One Way Slab

One way slabs are such slabs in which the loads are shared only by the shorter direction of the slab. In above figure (a) represents one way slab. Here ratio of longer span of slab to the shorter span of slab is greater than 2. One way slab being supported on two opposite sides only leads to the structural action in single direction.

Analysis and Design of One-way Slab:

Design steps:

Step 1: Preliminary Design:

Find depth of slab using deflection control criteria.

Take Span/d = 25 for simply supported slab.

= 30 for continuous slab.

D ≥ Dmin = 100mm for normal design.

= 125mm for earthquake resistant design.

Step 2: Analysis of slab:

Analyze the slab considering the slab as beam of 1m width and find respective loads, maximum design bending moment and maximum design shear force.

Step 3: Design of Slab:

  1. Verify depth of slab:

  1. Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.

Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

  1. Check slab for shear.

Step 4: Check slab at limit state of serviceability in deflection.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement.

  • Curtail or bent surplus bars.
  • Check development length at support.

Ld ≤ 1.3Ml / Vu + ld

  • Provide min. Ast as distribution bars in longer direction of slab.

Two Way Slab

Two way slabs are such slabs in which the loads are shared by both the shorter and longer direction of the slab. In above figure (b) represents one way slab. Here the ratio of longer span of slab to the shorter span of slab is less or equals to 2. Two way slab being supported on four sides the load will be carried in both the directions.

Analysis and Design of Two Way Slab:

Design Steps:

Step 1: Preliminary Design.

Find depth of slab using deflection control criteria.

Take span/d = 28 for simply supported slab and 32 for continuous slab.

D ≥ Dmin = 100mm for normal design and 125mm for earthquake resistant design.

Step 2: Analysis of Slab.

Slab is analyzed by IS code method. (Ref. Annex D-1 and Annex D-2).

Step 3: Design of Slab.

  1. Verify depth of slab:

  1. Determine ‘Ast’ and ‘Φ’ and number of bars in shorter direction of slab.

Slab section is normally designed as Singly Reinforced Under Reinforced Rectangular Section.

Ast = Mu/(0.87fy(d-0.416xu)) which should not be less than Astmin.

  1. Check slab for shear in shorter direction.

Step 4: Check slab at limit state of serviceability in deflection in shorter direction.

l/d ≤ αβγδγ

Step 5: Detailing of reinforcement:

  • Curtail/bend the surplus bars as per detailing rules of code. (Ref. Annex)
  • Check development length at support.

Ld ≤ 1.3Ml / Vu + ld

  • Provide torsion reinforcement at the discontinuous edge of slab as per codal provision.

 

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