FRP column buckling calculator
Check a pultruded I-beam, square or rectangular tube or round tube as a column: global buckling, local buckling of the flanges, web or walls, and crushing, with the lightest catalog sizes that pass.
- Catalog sections
- 46
- Limit states
- Global · local · crushing
- Screens
- ASCE · CEN/TS · ASD
K values are the recommended design values for ideal ends. Real pinned FRP connections sit between the ideal cases; when in doubt, use the larger K.
FRP EN 13706 E23: EL 23 GPa, ET 7 GPa, GLT 3.5 GPa, compressive strength 200 MPa (EN 13706-3:2002 minimums (G_LT, F_cL assumed)). Replace them with the supplier's certified values for final design.
| Mode | Critical stress | Nominal | Design |
|---|---|---|---|
| Global buckling about x (depth) | 35.1 MPa | 103 kN | 53.7 kN |
| Global buckling about y (width) | 35.1 MPa | 103 kN | 53.7 kN |
| Local buckling, widest wall | 271.0 MPa | 798 kN | 415 kN |
| Crushing (compressive strength) | 170.0 MPa | 500 kN | 260 kN |
Design = nominal × 0.65 × λ 0.8; factored load = service × 1.6.
KL/r 80 about x and 80 about y. Longest length that passes with these inputs: 3.90 m.
- CHS 114×6.4 · 3.3 kg/m · 79%
- SHS 100×100×6 · 3.5 kg/m · 75%
- CHS 127×6.4 · 3.7 kg/m · 57%
- SHS 114×114×6 · 4 kg/m · 50%
Concentric axial load only. Eccentric load, combined bending, flexural–torsional buckling of channels and angles, connections and creep under sustained load are outside this screen.
How the screen works
Global buckling
PE = π²ELI / (KL)² about each axis, reduced for shear: P = PE / (1 + PE / GLTAv). Weak-axis bracing shortens only the weak-axis length.
Local buckling
Flange outstand b = bf/2: σ = GLT(t/b)². Web or tube wall at its centerline width: σ = (π²/6)(t/b)²[√(ELET) + νLTET + 2GLT], with νLT = 0.3.
Crushing and factors
Compressive strength × area, with the environment knockdown. All modes take the bending resistance factor of the chosen screen, and the ASCE-style screen adds λ.
| End conditions | K |
|---|---|
| Pinned at both ends | 1 |
| Fixed base, pinned top | 0.8 |
| Fixed at both ends | 0.65 |
| Fixed base, free top (flagpole) | 2.1 |
These are the recommended design values for ideal end conditions in the AISC 360 commentary, widely used for other materials too. Beams are checked in the FRP profile calculator, whose methodology page lists the design codes by market.
Frequently asked questions
Why does an FRP column need more checks than a steel one?
Pultruded glass FRP is about a tenth as stiff as steel lengthwise, and its shear and crosswise stiffness are lower still. Columns therefore buckle long before the material crushes, and the thin flanges and webs of open sections can buckle locally at low stress. Wide-flange sections are the classic case: the flange outstands buckle before the column as a whole.
Which equations does the calculator use?
Global buckling is the Euler load about each axis with the Engesser shear correction. Local buckling treats each wall as a long orthotropic plate: a flange outstand with one free edge buckles at G_LT (t/b)², and a web or tube wall held on both edges at (π²/6)(t/b)² [√(E_L E_T) + ν_LT E_T + 2G_LT]. The junctions are taken as simple supports, which ignores the restraint the flanges and webs give each other and so gives a lower bound. Crushing is the compressive strength times the area.
Which resistance factors are applied?
Every mode takes the factor the FRP profile calculator uses for bending: φ = 0.65 with the time-effect factor λ on the ASCE/SEI 74-23 style screen, 1/1.5 on the CEN/TS 19101 style screen, or a factor of safety of 2.5. ASCE/SEI 74-23 is understood to allow higher factors for buckling, but they could not be confirmed against the published text for this tool, so the lower bending factor is used. That keeps the screen on the safe side; the project design applies the code factors.
What is not covered?
Eccentric load and combined axial force and bending, flexural–torsional buckling of channels and angles, the interaction between local and global buckling where their loads are close, initial out-of-straightness, connections and bearing at the ends, and creep under sustained load. The result flags interaction and slenderness above 200. Channels and angles are left out because they twist as they buckle.
Which material values should I use?
The presets use the EN 13706-3 minimums for E23 and E17 with an assumed shear modulus and compressive strength, because EN 13706 gives neither. Buckling scales with the moduli, so the supplier's certified E_L, E_T and G_LT matter more here than the strength. Ask for them with the quotation, and use the wet-service knockdown where the column stands in water.
Send the column check with your RFQ
Send the section, length, end conditions and loads; engineering confirms the design values for the supplied profile. We reply within one business day.