Torsional Constant 2

[davidjohnson]

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.

Dave

 

[dbuzz]

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

So for an I-beam
J = (2bt_f³ + (d - 2t_f)t_w³) / 3
where
b = flange width
t_f = flange thickness
d = beam depth
t_w = web thickness

 

[rob768]

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

 

[UcfSE]

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.