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Torsional Constant

Torsional Constant

Torsional Constant

I saw a thread on calculating the torsional constant, J (listed in table 1-25, page 1-89 of the AISC Manual) for shapes not listed in the manual.  Although, it seemed like only how to evaluate torsion was explained.  Does anyone know how to calculate this value.  I need it only for equations F1-8 and F1-9 in the manual.  I only need it to analyze the lateral torsional buckling limit state or an I-Beam.  Does anyone know how i would calculate this?  Thanks.


RE: Torsional Constant

For an open section, the torsion constant is as follows:
  J = Σ(bt3 / 3)

So for an I-beam
  J = (2btf3 + (d - 2tf)tw3) / 3
    b = flange width
    tf = flange thickness
    d = beam depth
    tw = web thickness

RE: Torsional Constant

Some CAD programs (and any FE preprocessing program) will do the math for you. PErhaps this is an option.

RE: Torsional Constant

To add to dbuzz's thread, note that bt^3/3 is good when b/t>10.  For b/t<10, you will have a different coefficient from the 1/3 above.  

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