A waffle slab's solid head around each column has to pass two separate punching checks at two different depths — through the thick solid head near the column, and through the thin surrounding flange further out — and the second, easy-to-forget check is the one that can fail.
Every check above runs automatically inside the Structyze Waffle 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
- Panel 9 × 8 m, square column 500 mm
- Flange 100 mm, rib depth 400 mm; rib width 150 mm, clear spacing 600 mm
- Solid head 4000 × 4000 mm; cover 20 mm; C25/30, B500
Working Through It
The void volume is smeared into an equivalent uniform self-weight, giving a design load w ≈ 14.76 kN/m². The panel static moment M0 ≈ 1066 kN·m splits to a column-strip rib moment of about 91 kN·m per rib, requiring 2φ20 bars against a calculated 500 mm² (utilisation 0.80).
Rib shear, checked at VEd ≈ 49.8 kN against a bare-concrete capacity of only 39.0 kN, genuinely needs design links — 8 mm at 150 mm provides enough shear reinforcement to clear the demand with margin.
Punching at the column face, through the deep 470 mm solid-head depth, passes comfortably (utilisation 0.67). But punching at the solid-head edge, checked through just the 76 mm flange depth alone, comes out at vEd/vRd,c = 0.74/0.50 — exceeded, at utilisation 1.49.
What Governs, and Why
Punching at the head edge fails outright at nearly 1.5 times the limit, even though punching at the column itself passes with real margin. The check conservatively takes only the thin 100 mm flange as the effective depth at that perimeter, ignoring the ribs that actually cross it — in practice, enlarging the solid head, adding a perimeter of ribs at that line, or running the check on the composite section would all restore margin.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Waffle 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 Waffle 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 would punching fail at the head edge but not at the column?
The column-face perimeter runs through the full depth of the solid head and benefits from that extra thickness; the head-edge perimeter sits entirely within the thin flange zone, with none of the solid head's depth to help — a materially weaker section geometrically, even though the applied shear there is lower.
Is ignoring the crossing ribs at the head-edge perimeter realistic?
It's a conservative simplification — the ribs crossing that perimeter do add real shear capacity in practice, but quantifying that composite contribution accurately is more involved than the flange-only check shown here, so the simplified method deliberately errs on the safe side.
Final Thoughts
A waffle slab's two-perimeter punching check is a genuine trap for a hand calculation — it's easy to verify punching at the column, see it pass, and stop there without checking the head edge at all.