A one way slab design example shows how a reinforced concrete slab strip is checked for loads, bending, reinforcement, shear and deflection. A rectangular slab supported on four sides generally behaves as one way when the longer-to-shorter effective span ratio exceeds 2, subject to support conditions.
Illustrative design data
Consider a 1 m wide simply supported strip with effective span 3.0 m, thickness 125 mm, M20 concrete, Fe500 steel, floor finish 1.0 kN/m² and imposed load 3.0 kN/m². These are teaching assumptions, not a project design.
Step 1: Calculate loads
- Self-weight = 0.125 × 25 = 3.125 kN/m².
- Floor finish = 1.0 kN/m².
- Imposed load = 3.0 kN/m².
- Total service load = 7.125 kN/m².
- Illustrative factored load = 1.5 × 7.125 = 10.69 kN/m².
Step 2: Calculate bending moment
For a simply supported strip under uniform load, Mu = wuL²/8 = 10.69 × 3²/8 = approximately 12.03 kN·m per metre width. This expression does not directly apply to cantilevers or continuous slabs.
Step 3: Design reinforcement
Determine effective depth from thickness, cover and bar diameter. Calculate required steel using IS 456 limit-state flexural equations. For HYSD bars, minimum slab tension reinforcement is generally 0.12% of gross cross-sectional area. The calculated flexural steel may govern.
Step 4: Complete design checks
- Flexural capacity against design moment.
- One-way shear at the critical section.
- Deflection and crack control.
- Development length, anchorage and cover.
Main bars generally run along the shorter span direction; distribution bars run perpendicular. Continuous slabs also need top reinforcement over supports according to design moments.
Related reading and references
See one-way slab reinforcement and beam span calculation. Refer to IS 456:2000 and the NPTEL RC design course.
FAQs
When is a slab one way?
A four-side-supported rectangular panel generally behaves as one way when the longer-to-shorter effective span ratio exceeds 2.
Can this example be used at site?
No. It is an educational illustration; a qualified structural engineer must design and approve the actual slab.