IS 800 treats a base plate as a genuinely two-way spanning plate, not the one-way cantilever strip several sibling codes in this series use — and the two-way formula mathematically reduces to the familiar one-way case when the smaller projection goes to zero, confirming the mechanics are consistent at that limit.
Every check above runs automatically inside the Structyze Base Plate 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
- Column 250 × 250 mm
- Plate 450 × 450 × 32 mm (provided)
- Axial: dead 300 kN, live 200 kN
- Pedestal concrete grade M25
- Plate material Fe410 (E250), fy = 250 MPa
Working Through It
Factored load Pu = 1.5(300+200) = 750 kN, spread over the plate gives bearing pressure w = 750,000/(450×450) ≈ 3.704 MPa — well under the concrete bearing limit of 0.45fck = 11.25 MPa (IS 800 itself defers this check to IS 456's own Cl 34.4 rather than setting its own value).
With a square column centred on a square plate, both projections coincide: a = b = 100 mm. The SP:6 two-way formula, ts = √(2.5w(a²−0.3b²)γm0/fy), credits some genuine stiffening benefit from the biaxial action coefficient 2.5 (rather than a one-way formula's 2) — giving ts,req ≈ 16.89 mm against the 32 mm provided.
What Governs, and Why
Plate thickness governs at utilisation 0.528 over concrete bearing's 0.329 — the provided 32 mm plate is nearly double the 16.89 mm the two-way formula actually requires, a common practical outcome since base plate thickness is often set by the minimum available stock size or anchor-bolt edge-distance needs rather than by the bearing calculation alone.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Base Plate 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 Base Plate 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
How does the two-way formula reduce to a one-way cantilever?
Setting b → 0 in ts = √(2.5w(a²−0.3b²)γm0/fy) leaves just √(2.5wa²γm0/fy) — structurally a one-way cantilever of span a, just with IS 800's biaxial coefficient 2.5 in place of the simpler codes' coefficient of 2, confirming the two-way formula's mechanics collapse correctly to the one-way case at that limit.
Why does IS 800 defer the concrete bearing check to IS 456?
IS 800 is a steel code and doesn't set its own concrete bearing permissible stress — Cl 7.4.1 explicitly refers to the applicable concrete code (IS 456 Cl 34.4) for that part of the check, a clean division of responsibility between the two codes rather than a gap.
Final Thoughts
IS 800's truly two-way base plate formula is a small but real mechanics difference from the one-way cantilever approach used elsewhere in this series — worth applying explicitly rather than defaulting to the simpler one-way method out of habit.