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    Concrete / Torsion

    Concrete Torsion Check

    Whether torsion can be ignored and, if not, how much of the concrete capacity it consumes. ACI 318 lets you neglect torsion below a threshold, which is why most beams are designed without it.

    Threshold Tth10.9 kN·mDesign torsion30.0 kN·mAcp²/Pcp23 ×10⁶ mm³
    Threshold Tth
    10.9 kN·m
    Below this, neglect torsion
    Design torsion
    30.0 kN·m
    Must be designed
    Acp²/Pcp
    23 ×10⁶ mm³
    Section property
    Verdict
    Design stirrups
    ACI 318 §22.7.4
    Threshold by section
    Tth = 0.083·λ·√f'c·Acp²/Pcp; below it, torsion may be neglected.
    Threshold by section
    Width × depthTth (kN·m)
    300×5006.6
    350×60010.9
    400×70016.7
    500×80028.9
    600×100052.8
    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Threshold against section size
    The threshold grows with the square of the area over the perimeter.
    Threshold against section size0.00011.8323.6635.5047.3359.16300×500300×500350×600350×600400×700400×700500×800500×800600×1000600×1000Concrete Torsion Check
    Canonical equations
    The exact relationships implemented by this engine.

    Tth = 0.083·λ·√f'c·Acp²/Pcp · ACI 318 §22.7.4.1.

    Tu < Tth · Then torsion is ignored.

    engineering methodology

    A clear path from inputs to results.

    Whether torsion can be ignored and, if not, how much of the concrete capacity it consumes. ACI 318 lets you neglect torsion below a threshold, which is why most beams are designed without it.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set width (mm).
    2. Set overall depth (mm).
    3. Set concrete f'c (mpa).
    4. Set design torsion (kn·m).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    Tth=0.083⋅λ⋅√f′c⋅Acp2/PcpTth = 0.083·λ·√f'c·Acp²/Pcp

    Reference: ACI 318 §22.7.4.1 torsion threshold.

    Worked example

    At the default inputs

    Inputs
    • Width (mm): 350; Overall depth (mm): 600; Concrete f'c (MPa): 32; Design torsion (kN·m): 30
    Calculation path
    1. 1

      Threshold Tth: 10.9 kN·m · Design torsion: 30.0 kN·m · Acp²/Pcp: 23 ×10⁶ mm³ · Verdict: Design stirrups

    Result

    Threshold Tth: 10.9 kN·m · Design torsion: 30.0 kN·m · Acp²/Pcp: 23 ×10⁶ mm³

    Interpreting the result

    The threshold is 10.9 kN·m against 30.0 kN·m of design torsion.

    Assumptions & limitations

    • Solid section, no openings.
    • No prestress.

    Frequently asked questions

    When can torsion be ignored?

    Below the threshold Tth, which is why most beams are designed without it.

    What if it exceeds the threshold?

    Closed stirrups and longitudinal steel are required.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.