Torsional Stiffeners
Torsional Stiffeners
(OP)
I need either confirmation or correction.
The situation I've got is a skid with C12 girders and C8 joists with an all around fillet at joist ends. The 4" below is space for cables and conduit to be installed. The floor plate above also ties it all together.
The person checking has asked about torsion because the shear center is not at the joist/girder interface.
The school of thought I was taught was that if you have torsional restraints that are at least 2/3 the depth of the beam (which I have) then that was adequate torsional restraint. I'm being challenged on it now to provide a source for that.
Well, I've looked and I can't find anything that speaks to the minimum depth of torsional restraint required at any depth. Does anyone know?
The situation I've got is a skid with C12 girders and C8 joists with an all around fillet at joist ends. The 4" below is space for cables and conduit to be installed. The floor plate above also ties it all together.
The person checking has asked about torsion because the shear center is not at the joist/girder interface.
The school of thought I was taught was that if you have torsional restraints that are at least 2/3 the depth of the beam (which I have) then that was adequate torsional restraint. I'm being challenged on it now to provide a source for that.
Well, I've looked and I can't find anything that speaks to the minimum depth of torsional restraint required at any depth. Does anyone know?






RE: Torsional Stiffeners
RE: Torsional Stiffeners
The C8's, fully welded as you suggest, certainly resist torsion in the C12 at the locations of the C8's. But not between them.
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RE: Torsional Stiffeners
For the purposes of bracing, Appendix 6 of AISC (13th edition) addresses this. IIRC, there is no required minimum depth or mandatory location for attachment. But torsional bracing can be negated if the web lacks distortional stiffness. You also have to run a variety of localized checks on the sections involved.
In any case, whatever "restraint" you have.....you still have to check the member for the torsional forces developed. At the very least, there will be compatibility torsional stresses as the members displace.
And by the way, your subject line mentions "stiffeners".....be advised: plate stiffeners do virtually nothing for a beam in torsion. (I know you are using channels as your "stiffeners" but I wanted to make that point in case I am misunderstanding something.)
RE: Torsional Stiffeners
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Torsional Stiffeners
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Torsional Stiffeners
BA
RE: Torsional Stiffeners
JAE, I always like reading your comments.
The C8 joists are at 24" o.c. But the only C12 are at the end points (all perimeter members are C12). So, his argument would make the torsional span about 30 feet long. This is a ridiculously long span for a channel to resist torsion. But if we count the C8, then the torsional span is only 24" long. Not only that, but there is no other load other than from the joists. So, if the joists themselves provide torsional restraint, there is virtually no torsional load.
WARose,
Unfortunately I only have the 9th and 14th editions. But that idea makes sense. I'm pretty sure other minor details at that connection will be sufficient to diminish the extreme fiber load to the point the web will be ok.
KootK,
Yes, that was my knee jerk reaction. But I always have to think in terms of "if it ever goes to court..." And we did take into account the opposing loads. But with a 30ft torsional span, that simply didn't do anything.
BARetiered,
If it were only a shear load, that would be true. But if it is a moment ocnnection, then it is not, correct?
RE: Torsional Stiffeners
RE: Torsional Stiffeners
This is addressed in the 14th edition as well. (Same Appendix # IIRC.)