Torsion of Cruciform Column
Torsion of Cruciform Column
(OP)
I've got a cruciform shaped column; a W27x84 crossed by a W14x53. I'm trying to check torsion, but am unsure how to determine the torsional constant, J. Do I simply add the two together? Any guidance on what to do would be much appreciated. Thanks!






RE: Torsion of Cruciform Column
RE: Torsion of Cruciform Column
The result would be interesting. Remember the proposed twisted spire building in Chicago?
RE: Torsion of Cruciform Column
RE: Torsion of Cruciform Column
RE: Torsion of Cruciform Column
I do not know what the equivalent section in AISC would be but I would expect it is there. If you can't find it or get access to the Canadian code, I will try to copy it for you.
Best regards,
BA
RE: Torsion of Cruciform Column
shear stress = Tc/R ...look similar to Mc/I? It should.
T is the Torque, c is the distance from the center of the section to the outer fiber and R is the Torsional Resistance.
To determine R(total) of the cruciform it needs to be broken down into it's 6 solid rectangular sections. Then the R for each rectangle can be determined using R=(beta)*b*d^3, where beta is based off the b/d ratio. Then just sum them up!
Hope this helps us all learn something new.
Have a good one!
RE: Torsion of Cruciform Column
Can you elaborate a little on derivation of beta, and its significance. I am somehow getting lost.
RE: Torsion of Cruciform Column
RE: Torsion of Cruciform Column
Enjoy the day!
RE: Torsion of Cruciform Column
I recognize the page from "Design of Steel Structures" by Blodgett. Please note one point, however. The term 'J' in that book represents polar moment of inertia, not the torsional constant which we now call 'J'. In most cases of I-Beams, channels and angles comprised of flat plate elements, the term beta is close enough to 1/3, so that there is no serious error in using J instead of R.
Best regards,
BA
RE: Torsion of Cruciform Column
RE: Torsion of Cruciform Column
According to "Structural Engineering Handbook" by Gaylord & Gaylord, the "torsional stiffness (constant)" for a rectanglar shape is "[(b*t63)/3]*[1-0.63*(t/b)+0.052(t/b)^2]"; "t<b". For b >> t, this eq reduces to b*t^3/3.
K & BA, thanks for the opportunity to review the basics.
RE: Torsion of Cruciform Column
Just one more point respecting this thread. You calculated that the column was not adequate to resist a particular torsion. However, you did not address the axial capacity of the column and it is important to note that there are three ways in which such a column can buckle, about each principal axis and torsionally.
CSA S16-1 addresses torsional buckling for this shape and I am sure that AISC does as well, but I do not have that document so can't say for sure.
Best regards,
BA