Cantilever slab design is used for a reinforced concrete slab projecting beyond a beam, wall or other support without a support at its free edge. Common applications include balconies, canopies and sunshades. Under downward gravity loading, the top face near the support is generally in tension, so main reinforcement is placed near the top and anchored into the support.
Design information required
- Projection length and support dimensions.
- Slab thickness, concrete and reinforcement grades.
- Self-weight, finishes, live load, parapet and railing loads.
- Exposure, cover, fire requirements and anchorage space.
Step 1: Calculate loads and actions
Calculate self-weight and add waterproofing, finishes, parapets, services and imposed loads. For an ideal cantilever of length L under uniformly distributed factored line load w, fixed-end moment Mu = wL²/2 and support shear Vu = wL. Add point loads and other actions separately.
Step 2: Design main reinforcement
Design the section for the support moment using the applicable concrete code. Main bars generally run parallel to the projection near the top face. Check required steel area, minimum reinforcement, spacing, effective depth, crack control and development length.
Step 3: Check anchorage
Main bars must extend into the support sufficiently to develop the design force. Check embedment, bends, support dimensions and congestion. Bars cut short or displaced downward can compromise capacity.
Step 4: Check shear and deflection
Verify shear capacity, service-load deflection, crack control, cover and durability. Coordinate balcony drainage, waterproofing and railing details. Do not assume a perfectly fixed support unless the connection justifies it.
Site checklist
- Top main bars are in the correct direction and level.
- Anchorage matches the approved drawing.
- Cover blocks and chairs maintain specified cover.
- Distribution and support reinforcement are present.
- Formwork and propping remain until approved removal criteria are met.
Related reading and references
See slab reinforcement guide and beam span calculation. Refer to IS 456:2000 and the NPTEL RC design course.
FAQs
Where is main steel placed?
Generally near the top face and parallel to the cantilever projection.
Why is anchorage important?
It transfers tensile force into the support and helps develop the intended moment capacity.