warping of steel I beam 1

[NS4U]

I've read S&J's section on Torsion and AISC's DG 9 on torsion.

If I understand torsion of I beams correctly, the warping phenomenon is essentially result of the top and bottom flange rotating about the web's axis. So, the top and bottom flanges have normal and shear stress distributions similar to a rectangular beam in bending.

AISC DG 9 says, multiple times, that warping in I beams is only possible if warping is restrained. What constitutes a warping restrained connection? Here's how i envision this "restraint": If the top and bottom flanges are assumed to be behave as "beams" when the I shape warps, then there needs to be a "shear" support (transverse to plane of web) at the end of these "beams"; therefore, some sort of flange connection is needed to provide the required restraint. This connection does not need to provide flange rotation restraint. Am I thinking about this correct?

I'd like some simple examples of warping restrained vs unrestrained connections. For example, are simple shear connections "unrestrained" for warping? Are Moment connections considered "restrained" for warping? Any other examples people want to give would be much appreciated!

 

[AuEng99]

You just need to use lateral bracing at certain intervals. This prevents I-beam warping. Also, cross bracing of beams is also a good practice, as long as there is room and it will not interfere with maintenance.

 

[MikeHalloran]

For a time, I worked as a drafter in a shop that makes bridge cranes.
I was astonished the first time I saw them lift one corner of a single girder crane by a couple of feet, while the other three corners stayed on the shop floor. I'd estimate the twist at 20 degrees over 40 feet. ... of a damn big beam.
That's why the hoist runs on the bottom flange.

I-beams have many fine qualities, but resistance to torsion is not one of them.

Do not underestimate the importance of lateral bracing.
If you overdo it, no one will ever know.
If you under-do it, everyone will know.





Mike Halloran
Pembroke Pines, FL, USA

 

[WillisV]

AISC Design Guide 9 uses the unfortunate terminology of torsionally "pinned" and torsionally "fixed". This can misleading for someone trying to figure things out. If something is truly torsionally pinned, then it cannot resist any torsion (i.e. it would just spin in space with applied torsion). I would put a small shear tab or some other fairly weak simple shear connections solidly into this category. So, anything that has to resist applied torsion must be able to take the torsion out at the connections, therefore it cannot be truly torsionally pinned, it is must be some form or torsionally fixed. The more proper terms are warping restrained (equivalent to DG 9 Torsionally Fixed) and warping unrestrained (equivalent to DG 9 Torsionally Pinned).

For something to be warping restrained, the connection must prevent the flanges from rotating in plan. This is fairly difficult to achieve without a full penetration weld, a very stiff end plate, or some other type of heavy moment connection. An example of a connection that you might count on to resist torsion but is warping unrestrained would be a deep double angle connection (good luck finding a design example for taking torsion through this type of connection, but it can be done by using basic statics).

So, in summary, most typical shear connections should not be counted on to resist any torsion (i.e. they do not even qualify as torsionally "pinned" in DG9 terminology). Some sturdier connections such as full depth double angles can be designed to resist the torsion applied, but do not resist warping of the flanges, thus being warping unrestrained (or torsionally pinned in DG9 terminology). Connections that prevent twisting of the flanges in plan such as full pen welding can be considered warping restrained (or torsionally fixed in dG9 terminology).

I would agree with others; however, in that the best way to deal with torsion is to first, find a way to avoid it, second, use a closed shape where possible, third, use kickers and bracing to take it out, and fourth as a last resort design the wide-flange beam to resist the torsion keeping in mind the above discussion.

 

[NS4U]

Thanks, Willis. Good info. Exactly what I was looking for.