A rigid-block machine foundation isn't sized for strength at all — it's sized so its own natural frequency stays safely away from the machine's operating frequency, avoiding resonance, using a different soil property (dynamic modulus) than the static bearing pressure an ordinary footing check would use.
Every check above runs automatically inside the Structyze Machine Foundation 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
- Block 13 × 10 ft × 48 in
- Machine weight 67 kip, operating speed 1500 rpm
- Soil dynamic modulus Cu = 30,000 kN/m³
- Allowable bearing 3.0 ksf
- Steel fy = 60 ksi; #6 at 5 in each way
Working Through It
Static bearing is checked first, almost as a formality: total weight (machine plus block self-weight) 145 kip over 130 ft² gives q = 1.12 ksf against 3.0 ksf allowable — wide margin, utilisation 0.37.
The real calculation is the vertical natural frequency: modeling the block as a rigid mass on an elastic soil spring (Barkan/Richart), kv = CuA, fn = (1/2π)√(kv/m) ≈ 11.8 Hz.
Comparing against the machine's operating frequency fop = 1500/60 = 25 Hz: the ratio fn/fop = 11.8/25 ≈ 0.47, just outside the resonance danger band of 0.5–1.5. The block is under-tuned — the machine runs above the foundation's natural frequency, which is the standard target for a high-speed machine, keeping the transmitted dynamic force small.
What Governs, and Why
The frequency ratio passes at 0.47, just clearing the 0.5 threshold — a real, load-bearing result, not a formality like the static bearing check (0.37 margin) might suggest. Minimum reinforcement, at utilisation 0.98, runs surprisingly close to its own limit. The thick block's nominal temperature steel is nearly what's provided.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Machine Foundation 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 Machine Foundation 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 check only the vertical vibration mode?
This module models the simplest and most commonly critical mode for a symmetric block foundation — horizontal sliding, rocking, torsion, soil damping and the machine's actual dynamic force amplitude are all real effects a complete design needs, but they require a fuller soil-dynamics analysis (per IS 2974 or equivalent) beyond this vertical-mode screening check.
What's the difference between Cu and the static allowable bearing pressure?
They're measuring genuinely different soil properties — Cu, the coefficient of elastic uniform compression, comes specifically from a dynamic soil test and describes how the soil responds to oscillating load, while allowable bearing pressure describes static load-carrying capacity. Using a static bearing value in place of a real Cu would give a meaningless frequency result.
Final Thoughts
A machine foundation is one of the few RCC checks in this whole series where strength is almost beside the point — the natural-frequency calculation is the actual design, and it needs a soil property most ordinary footing designs never touch.