BIZOMAT MAKEOS · Mechanics
Machine Foundation Block Sizing
How big a concrete block does this machine need, will the soil carry it, and is the block clear of resonance?
Preliminary sizing of a concrete inertia block for a machine: block mass as a multiple of machine mass, depth for the plan area, soil bearing pressure against the allowable, vertical stiffness from the soil’s subgrade modulus, natural frequency of the machine and block on the soil, and the ratio of running frequency to natural frequency against the resonance band.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Machine mass | kg | Mass of the machine on the block. |
| Block mass ÷ machine mass | Block mass as a multiple of machine mass (often 2–3 for rotating machines, 3–5 or more for reciprocating and impact machines). | |
| Concrete density | kg/m³ | Density of reinforced concrete. |
| Block length | m | Plan length of the block. |
| Block width | m | Plan width of the block. |
| Allowable soil bearing pressure | kPa | Allowable bearing pressure of the soil (from a geotechnical report). |
| Soil subgrade modulus (vertical) | MN/m³ | Vertical modulus of subgrade reaction for the soil (from the geotechnical report). |
| Lowest running speed | rpm | Lowest steady running speed producing vibration. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Machine mass
- What it is
- Mass of the machine resting on the foundation block, from the machine maker’s data.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Block mass ÷ machine mass
- What it is
- Mass of the concrete block as a multiple of the machine mass, chosen by the designer or from the machine maker’s foundation guidance.
- Units and format
- Enter a plain number with no unit. The form accepts 0.5 to 20. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Block mass and depth rise in proportion.
What this means: Concrete density
- What it is
- Density of the reinforced concrete used for the block, from the concrete specification.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 5000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2400 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A denser concrete gives a shallower block for the same mass.
What this means: Block length
- What it is
- Plan length of the block, normally the machine footprint plus a working margin.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 100 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Block width
- What it is
- Plan width of the block, measured at right angles to its length.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 100 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Allowable soil bearing pressure
- What it is
- Allowable bearing pressure of the soil under the block, from the geotechnical report for the site.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 10000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Soil subgrade modulus (vertical)
- What it is
- Vertical modulus of subgrade reaction of the soil: the pressure needed per metre of settlement, from the geotechnical report.
- Units and format
- Enter a number in MN/m³: MN/m³. The form accepts 0.000001 MN/m³ to 1000 MN/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 MN/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- MN/m³; it depends on footing size, so use a value given for a block of this size.
What this means: Lowest running speed
- What it is
- Lowest steady running speed of the machine that produces vibration, compared with the natural frequency of the block on the soil.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Block mass = ratio × machine mass; depth = mass ÷ (density × length × width).
- Bearing pressure = (machine + block) g ÷ plan area.
- Vertical stiffness k = subgrade modulus × plan area; natural frequency f_n = √(k ÷ m_total) ÷ 2π.
- Frequency ratio r = running frequency ÷ f_n; keep clear of 0.5 < r < 1.5.
- Example: a 5000 kg machine needs a 1.0417 m deep block, bearing 32.689 kPa on the soil.
Limitations
- Preliminary rule-of-thumb sizing; rocking modes, embedment and dynamic loads need a foundation design by a structural engineer.
- Subgrade modulus depends on footing size and soil; use project data.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a foundation block for a press, compressor or generator
- Check soil bearing before pouring
- Avoid a foundation that resonates at running speed
Who it is for
- Plant engineers
- Civil and structural engineers
- Installation engineers
What you get
- Block mass, volume and depth
- Soil bearing pressure
- Natural frequency and frequency ratio
Interpreting the result
- A block running well above its natural frequency (r > 1.5) isolates best; well below (r < 0.5) is stiff and also acceptable.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3237–3238, 197–198. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Foundations, baseplates and grout
In Shaft Alignment by the Numbers
What it covers: Why alignment starts with the foundation: a first estimate of a concrete block for mass, bearing pressure and resonance, and how baseplates, grout and holding-down bolts hold alignment.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.