One-way and two-way slabs
A slab supported on two opposite sides bends in one direction, like a wide beam; this is a one-way slab. A slab supported on all four sides bends in both directions; if its long span is less than about twice its short span, both directions carry significant load, and it is designed as a two-way slab. If the ratio is larger, almost all load goes the short way, so even four-side-supported long panels are designed as one-way.
A one-way slab is designed as a 1 m wide beam strip, with main bars along the span and distribution bars across it to spread point loads and control shrinkage cracks. Thickness is usually set by deflection, using span-to-effective-depth ratios with modification for steel stress and percentage. Shear rarely governs in ordinary slabs.
Two-way slabs share load between the two directions. The short span, being stiffer, takes more. Codes give moment coefficients depending on aspect ratio and edge conditions (continuous, discontinuous) so designers can find moments without plate theory. The corners of a two-way slab tend to curl up; where they are held down, twisting moments develop, so torsion reinforcement is placed at top and bottom in both directions at corners where the slab is discontinuous on both edges (and a reduced amount where only one edge is discontinuous). Short-span bars go in the outer layer to give them the larger effective depth. Flat slabs, which rest directly on columns without beams, need separate checks for punching shear around each column.
One-way if ly / lx > 2Strip design: Mu = w lx² / 8 (simply supported)Two-way: Mux = αx w lx², Muy = αy w lx² (coefficients from code by ly/lx and edges)Minimum steel (HYSD): 0.12 percent of gross area
- The ly/lx > 2 rule is a guide; four-side supported slabs are still checked for corner and edge effects.
- Annex D coefficients assume uniformly loaded rectangular panels; concentrated loads and openings need separate analysis.
Problem. A simply supported one-way slab spans 3.6 m, 150 mm thick, d = 125 mm, total factored load 12 kN/m². M20, Fe 500. Find steel required.
- Mu = 12 × 3.6² / 8 = 19.44 kNm per metre.
- Mu / (b d²) = 19.44 × 10⁶ / (1 000 × 125²) = 1.24 MPa.
- Solve Mu = 0.87 fy Ast d (1 − Ast fy / (b d fck)): Ast ≈ 388 mm²/m.
- Use 10 mm bars at 200 mm (393 mm²/m). Minimum 0.12 percent = 180 mm²/m, satisfied.
Answer. About 388 mm²/m; 10 mm at 200 mm c/c
Quick quiz
1When is a four-side-supported slab treated as one-way?
When long span / short span exceeds about 2.
2Why are short-span bars placed in the outer layer?
To give them larger effective depth, since the short span carries more moment.
3What extra reinforcement do two-way slab corners need where both edges are discontinuous and the corner is held down?
Torsion reinforcement, top and bottom, in both directions.
- IS 456 (India): Plain and reinforced concrete: general rules for materials, durability, design and construction.
Clauses and documents to read
- IS 456:2000 cl. 24 (slabs) and Annex D (two-way slab coefficients)
- IS 456:2000 cl. 23.2.1 (span to effective depth ratios)
- IS 456:2000 cl. 26.5.2.1 (minimum slab reinforcement)
- IS 456:2000 cl. 31.6 (punching shear in flat slabs)
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- S. Unnikrishna Pillai, Devdas Menon, Reinforced Concrete Design, McGraw Hill Education (India), latest edition.
- J. K. Wight, J. G. MacGregor, Reinforced Concrete: Mechanics and Design, Pearson, latest edition.
- A. H. Nilson, D. Darwin, C. W. Dolan, Design of Concrete Structures, McGraw Hill, latest edition.
- R. Park, T. Paulay, Reinforced Concrete Structures, Wiley, 1975.
Free and official sources
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