KEVOS® MAKEOS · Strength
Compression Strut Sizing (Euler or Johnson, Round Bar or Tube)
What is the smallest round bar or tube that carries this compressive load with my safety factor against buckling, and how does my chosen size compare?
Sizing of a pin-ended or end-restrained compression strut in solid round bar or round tube: the slenderness ratio and transition slenderness, buckling by the Euler formula for slender struts or the Johnson parabola for intermediate ones (as the column buckling tool), the least outside diameter that meets the safety factor (found by search), and the safety factor, stress and mass per metre of the size you choose.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Compressive load | kN | Axial compressive load. |
| Length between supports | mm | Length between the end supports. |
| Effective length factor | Effective length factor for the end conditions (1 pinned–pinned, 0.7 fixed–pinned, 0.5 fixed–fixed, 2 fixed–free; designers often use larger practical values). | |
| Elastic modulus | GPa | Elastic modulus of the material. |
| Yield strength | MPa | Yield strength of the material. |
| Safety factor required | Safety factor against buckling you require. | |
| Section | choice | Solid bar or tube. |
| Tube outside diameter ÷ wall thickness | For a tube, the ratio of outside diameter to wall thickness (kept constant while sizing). | |
| Outside diameter chosen | mm | Outside diameter you intend to use. |
| Density | kg/m³ | Density for the mass per metre. |
Method and formulas
- Slenderness λ = K L ÷ r with r = √(I ÷ A); transition C_c = π √(2E ÷ S_y).
- Euler (λ ≥ C_c): σ_cr = π² E ÷ λ²; Johnson (λ < C_c): σ_cr = S_y − (S_y λ ÷ 2π)² ÷ E (as the column buckling tool).
- Safety factor = σ_cr A ÷ P; the least diameter is found by bisection for the required factor.
- Example: 50 kN over 2000 mm needs at least 47.661 mm; a 50 mm bar gives a factor of 3.028.
Limitations
- Concentric load; eccentric loads need the secant formula.
- Thin-walled tubes can also fail by local buckling.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a push rod, brace or prop
- Compare bar and tube for weight
- Check a strut after a length change
Who it is for
- Design engineers
- Structural engineers
- Fabrication supervisors
What you get
- Least diameter for the safety factor
- Slenderness and buckling mode
- Safety factor, stress and mass of the chosen size
Interpreting the result
- For the same mass a tube is far stiffer against buckling than a solid bar.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 281–287. Topic reference; no source tables or recommended settings are reproduced.
WORKOS 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.
Open the Compression Strut Sizing (Euler or Johnson, Round Bar or Tube) calculator
