Transverse Web Stiffeners for Rotation?

[ClarksonEng]

As seen in the example attachments, several engineers in the utility industry (where members experience significant moment/torsional loading) believe the use of stiffeners assists the overall structural design. It is believed, on the other hand by myself and few others that stiffeners do little in terms of preventing or limiting rotation at all, and create excessive fabrication work (i.e., welding of 12+ stiffeners on each beam). Also, even if stiffeners had been placed under the load centers, they are not bearing. Another place that excessive stiffening is thought to be used is in the column to top-plate area. My thought is that it may have originated as “flange stiffeners”, for which I do not see calculations on how to dis-prove their need. It has been such a legacy practice that the misconception continues. I am interested in your views. Thanks.

 

[tempeng]

If you do the calcs to prove that local web buckling can be resisted purely by the chosen section alone (without compromising economy by over-sizing the section), then stiffeners can be omitted. I don't see what the problem is.

 

[daranguiz]

Stiffners dont help for the stability of a beam (LTB). It's a common misconception.
For flexural stability read the theory behind LTB and it's pretty clear.
For torsion, the shear stress due to torsion is not travelling around the section in all the length of the beam but maybe it's helping a little.

Stiffners are useful for limiting the buckling associated with shear of the web and also to restraint the flanges Under concentrated loads.

Here is a Q&A from modern steel:

Transverse Stiffeners as Stability Bracing
It seems that full-depth transverse stiffeners prevent rela-
tive movement of a beam’s flanges. Can the location of
transverse stiffeners in a beam be considered a brace point?

We receive this question on a regular basis. The answer is
no. Transverse stiffeners are simply along for the ride as the
section rotates and provide no resistance to lateral-torsional
buckling on their own. As a result, they do not affect the
unbraced length of the beam, Lb. Stiffeners can be used to
reduce the web deformation and improve the efficiency of
torsional braces. However, used alone, web stiffeners are inef-
fective in enhancing the stability of members.
Bo Dowswell, PE, PhD

 

[wannabeSE]

On the second sheet, the stiffeners at the foundation provide rotational restraint and I wouldn't remove all of them (at least one at each foundation is needed). And, I don't have a problem with the stiffeners at the plates on the top of the beam; they may be needed to stiffen the flanges, web or plate. The stiffer at the middle of the beam on the second sheet looks like its just along for the ride - I don't any benefit.

 

[KootK]

The only instance in which I could see the midspan stiffeners providing meaningful torsional help is if you've got tranverse framing tying in at those locations to provide rotational restraint. The stiffeners don't participate in St. Venant torsion and really looking aren't stiff enough out of plane to assist with the longitudinal flange strains associated with warping torsion.

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.

 

[WARose]

Some years ago, I got into this a bit because I had some beams that had some huge torque loads on them and I also had stiffeners there (for other purposes). My concern was about the stiffener welds to the flange and the web (especially the flange): what kind of loads would develop in them from the torque?

To be on the safe side, I modeled a portion of one of the beams (including the stiffeners) in STAAD using all plate elements. The results showed that not only did the stiffeners have virtually no impact on the amount of rotation......but also, very little force developed between the stiffeners and the flange (or web) from the torsional forces.

That being said, I do typically include stiffeners where I have a torque applied......if nothing else, it helps to "get it into" the whole beam.

 

[mronlinetutor]

the stiffeners are definitely needed at the end of the beam to prevent flange and web crippling. the interior stiffeners locates at 1/3 distance from the column and looks like the designer wants to create plastic hinges at that location so that the beam acts as cantilevers from the column. in so doing, the designer turns an indeterminate structure into a determinate one which is solvable by hand.