A cantilever slab's support moment is exact statics, not an approximate coefficient — and because both the moment and the tip deflection grow with the projection length squared and fourth power respectively, a cantilever punishes small errors far more than an ordinary simply-supported slab does.
Every check above runs automatically inside the Structyze Cantilever Slab module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
The Example We'll Use
- Projection 1.2 m
- Thickness 150 mm, cover 20 mm
- Superimposed dead 1 kPa, live 3 kPa
- No edge line load
- Concrete f′c = 4000 psi, steel fy = 60 ksi
Working Through It
The support moment follows closely: Mu = wuℓn²/2 + Puℓn, with the second term (any parapet or railing line load at the free tip) added on top — here zero, so Mu = 0.210(4)²/2 ≈ 1.68 kip·ft per foot (using the book's imperial defaults).
The calculated steel requirement comes out below the ACI 0.0018bh minimum, so minimum steel governs the top bars — #4 at 6 in. The tension face is the top of the slab here, the opposite of a simply-supported slab, and those bars must be fully anchored back into the supporting beam or wall.
Deflection is controlled by a strict thickness rule rather than a calculation: h ≥ ℓn/10 = 4.8 in against the 6 in slab provided — a materially tighter ratio than an ordinary slab's span/depth check, reflecting how much more sensitive a cantilever's tip deflection is to thickness.
What Governs, and Why
Deflection thickness runs tightest at utilisation 0.80, ahead of temperature steel (0.79) — both serviceability-flavoured, with genuine strength checks (flexure 0.33, shear 0.15) carrying much wider margin. A cantilever slab is rarely governed by strength; it's governed by stiffness and detailing.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Cantilever Slab module — this particular module is part of the full desktop catalogue rather than the free browser trial, since the trial only carries a curated flagship set. You can still try the free trial modules right now to see the same calculation style in action on RCC Beam, Column, Slab and a handful of others.
For Cantilever Slab and the rest of the full catalogue — 80+ components across RCC, Steel, Composite, Timber, Masonry and more, across 20+ design codes — buy the full Structyze desktop app or see everything it does on the Software page.
Quick FAQ
Why is a single mis-placed top bar such a serious problem here?
The tension steel in a cantilever runs in the top of the slab, and if it gets pushed down toward mid-depth during the concrete pour, the effective depth shrinks and most of the flexural capacity is lost — a failure mode that's far more forgiving in a simply-supported slab where the bars sit near the bottom, protected by the formwork below.
What changes if there's a parapet or railing at the tip?
Any line load at the free edge factors as dead load and adds directly to the support moment through the Puℓn term — and the extra moment needs matching top steel and anchorage, not just a heavier tip.
Final Thoughts
A cantilever slab's detailing — bar position, anchorage, chair spacing — matters at least as much as the arithmetic, which is just the kind of judgment a spreadsheet can't apply on its own.