Beam bracing 19

[NewEngineerHere]

Hi everyone:

I am a recent graduate and trying to understand the beam bracing analysis/design.
I read in many texts that the beam flange needs to be braced otherwise it would buckle ( like a wave shape as we see in a column under axial load.)

Now my questions are:

If I have a beam supporting floor slab, (top flange is fully braced- great, no worries) Now, bottom flange needs to braced so it does not rotate (torsional buckling?)
how do I know if I really need to brace or not ? OK, AISC manual gives a Cb equation I can calculate my un-braced length

1) How can I correlate Cb equation with the moment applied to the beam, or stability of the beam ? Cb should tell me how much more moment capacity I am gaining for providing that brace at that distance (correct?).... what does it have to do with stability ?
2) What If I have moment connection (double curvature moment diagram) is it different than simply supported ? ( Do I always need to brace both flanges Tension and compression ?
3) How much load will be brace see? how much should be designed for, is this load coming from the moment or out of plane load ( wind, seismic )?
4) When do I need to brace/add stiffener to the web of the beam ?

My first thoughts are: If the beam works for giving load for moment, shear, and deflect - perfect.....But know I have stability issues I need to understand.

I am just trying to make sense of equations, rules, and develop a sense of what is right and what is wrong (how can I prove by calculation).

Thank you

 

[RFreund]

Isn't allowing tension buckling into your camp's argument the same thing as agreeing on compression buckling of the flange... oh boy.

EIT

http://www.HowToEngineer.com

 

[KootK]

I welcome tension buckling into the discussion. In fact, I've been trying to garner acceptance of it all along as a way too refute the notion that buclking is always a compression phenomenon.

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.

 

[MacGruber22]

RFreund - I do not follow your logic. I am reading "tension instability exists, therefore compression instability exists". And..so?

"It is imperative Cunth doesn't get his hands on those codes."

 

[bowlingdanish]

Cheers KootK for your well thought out responses, swell. I've always thought of buckling as a mathematical instability. We accept that standard euler compression buckling is a mathematical bifurcation. Why can't we accept that LTB is a different kind of bifurcation? Why does it have to be a product of the one we already blindly accept? I'd propose in an arbitrary 3 dimensional space, an arbitrary 3 dimensional object subject to an arbitrary load in all 3 axes would exhibit a shitload of instabilities that wouldn't conform to either LTB or euler compression buckling. It's just maths yo. Mind you I'm a graduate and know nothing

 

[MacGruber22]

When designing a new structure, it is easier to put in bracing than to perform the calculation showing it is not necessary

Yes. Yes Yes. Structural engineering fees are not high enough to spend all sorts of time proving that bracing (among other things) is not required. And besides, the money it costs to fight tooth and nail over bracing for typical structural elements is often handily greater than the money to install the disputed item(s).

Ultimately, I see these discussions as a passionate attempt to understand the behavior, if only for our enjoyment...and maybe for that unique project which requires pulling a sophisticated engineering rabbit out of our hat.

That said, I believe the KootK is much closer explaining the general LTB behavior than others, even with some of the digression. It is clear that the phenomenon is a coupling of numerous components (torsional stiffness, minor axis flexural stiffness, and others).

"It is imperative Cunth doesn't get his hands on those codes."

 

[hokie66]

This took a bit of reading, and I may have learned a thing or two. But I think we should be careful of our language. Calling tension instability "buckling" is going a bit far, IMHO.

 

[MacGruber22]

The researcher in that video I posted also mentioned that it may not good practice to refer to it as "tension buckling". I feel the same way. Particularly when trying to teach the concept. Though, I think it is very easy to say, because the term "buckling" is associated so strongly with the image of "a sudden change in geometry which causes a collapse/failure"


"It is imperative Cunth doesn't get his hands on those codes."

 

[RFreund]

@Mac / @KootK - Sorry that was said in jest (hard to convey tone across the interwebs sometimes). Interesting video for sure.

I've have also thought about LTB being mostly a torsional problem, so I'm a bit surprised with all the buckling talk. If the bottom flange can't move relative to the top flange than you don't have LTB (or atleast that's my understanding). So I see why simply attaching the top flange to the deck (i.e. only bracing the compression flange) may not be sufficient as the bottom flange can still move relative to the top flange. I suppose this is where that tension buckling demonstration would help one visualize why.

EIT

http://www.HowToEngineer.com

 

[Jerehmy]

Cool video, but why are we calling this tensile buckling when the failure occured in the joint/connection and not the member. I guess it could be called tensile buckling of the system?

Also, kootk, if the beam in your model was loaded on it's weak axis, would it still want to dlop over? It's already in its most stable orientation so where's it going to go? It seems in your model it would still twist. But you wouldn't call this twist LTB would you? If it's loaded on its strong axis you would call it LTB. What about the weak axis?

Does you interpretation of LTB allow for weak axis LTB? I've still been mulling this over.

 

[MacGruber22]

Also, kootk, if the beam in your model was loaded on it's weak axis, would it still want to dlop over? It's already in its most stable orientation so where's it going to go?

1. LTB cannot be generated for weak axis oriented I-shapes, unless I am misreading your post.
2. How is it already in it's most stable orientation? What does that mean? His sketch shows a post rigidly connected to the beam - that is it; the post is free to rotate above. And, besides, if the post was not very flexurally stiff, it could be restrained out of plane to the sketch at the top and still allow the eccentric instability in the beam to occur.



"It is imperative Cunth doesn't get his hands on those codes."

 

[Jerehmy]

I'm well aware weak axis members don't have LTB concerns.

The sentence before, I said what if it's loaded on its weak axis. The weak axis is more stable than the strong axis for transverse loading. I said nothing about the post so I'm not sure why you brought it up. And I was specifcally asking kootk about his interprtstion of LTB with his diagram and how weak axis bending is incorporated into his interpretation.

If the beam in the diagram is unstable whether the beam is loaded on its weak or strong axis, is it still LTB? Thats what I'm getting at.

 

[MacGruber22]

I said nothing about the post so I'm not sure why you brought it up.

I obviously misunderstood what you said. My apologies - I will leave the rest of the thread to you and KootK.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Jerehmy]

Text isn't a good way to convey tone! Don't leave! This is a thread for everyone I didn't mean to insult you. If I did I apologize!

 

[MacGruber22]

I interpreted your response as "get out of your way". I appreciate your clarification.

Anyways - I think I am losing interest in the thread, as I am still confused about the initial argument regarding the compression flange buckling dominating the onset of LTB. Maybe I will check back in to this thread in a few years to see what has happened.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

@RFreund: not to worry, I took your comment in the spirit that you intended

@Macgruber22: don't leave! I don't really have a "camp" without you.

@Jerehmy: I think that I see your point regarding weak axis LTB. The two point litmus test that I proposed above requires a third point, making it:

1) Whole section flopping over needs to result in the applied load moving closer to the earth.

2) Whole section flopping over would generate additional stain energy by way of some combination of beam rotation and/or beam lateral sway.

3) Lost potential energy from #1 >= strain energy stored from #2.

With this addition, the weak axis example would not meet my proposed LTB definition. It would fail test #3 and would just be combined flexure and torsion. And I think that's what both of us, and MacGrubber, believe should be the case.

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.

 

[KootK]

Comments -- and videos -- posted in this thread have caused me to think a little harder about tension buckling than I have in the past. I believe that I need to adopt a more nuanced view of tension buckling. Here's where I'm at:

1) A system may buckle due to insufficient stiffness of a member in tension if, and only if, the systems also contains a member in compression.

2) Unlike a compression member, a tension member cannot buckle internally in non-system fashion.

I'd like very much to hear any arguments or examples that may contradict these statements. Here is my analysis of the tension example that we've discussed so far:

1) Tension chord buckling. Here a lack of tension chord flexural stiffness leads to a system buckling mode that I contend is analogous to beam LTB. The compression chord and webs are in compression.

2) MacGruber's videos. Again, a lack of flexural stiffness in the tension members leads to system buckling. Compression would be present within the sliding mechanism that connects the tension members.

3) A tension only brace failing under P-Delta effects. Here, insufficient axial stiffness of a tension member leads to system failure. The compression members are the braced frame columns.

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.

 

[hokie66]

In my opinion, the video shows an unstable system rather than any type of buckling. Two rollers, tied together by a type of spring, were introduced at the ends of two independent members, and they rolled in different directions.

 

[Hokie93]

I agree with that opinion, Hokie66. Further, it seems to me the instability observed in the tension system examples is likely a product of, or certainly exacerbated by, unintended eccentricities (imperfections, misalignments, etc.) in the system. Has anyone ever observed or read about a real-life structural member in pure tension failing in this manner? I have not.

 

[KootK]

In my opinion, buckling is a problematic label for a theoretical discussion of this nature. As this thread has made abundantly clear, "buckling" doesn't have a generally accepted, precise definition. I'd much prefer to restrict ourselves to "instability" which has a tidy "maths yo" definition. Besides, as designers, we design to preclude mathematical instability. Buckling simply represents the myriad of physical consequences that may occur once a point of instability is reached.

I don't think that the videos were posted to suggest tension buckling as a common, practical design consideration. Rather, they were posted to support the claim that stiffness deficiencies in compression free members can dominate the response of systems prone to instability. And, certainly, the videos accomplish that.

Further, it seems to me the instability observed in the tension system examples is likely a product of, or certainly exacerbated by, unintended eccentricities (imperfections, misalignments, etc.) in the system

Are there forms of buckling/instability for which this is not true?

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.

 

[RFreund]

Well maybe we should define buckling vs instability. Although I understand what Hookie(s) are saying.

@kootk - what about the square tension ring example deforming into a parallelogram? I suppose whatever is loading the the tension ring would probably be in compression.
Although maybe not. What if you had a pool. To support the pool you had a ring of beams. This could be a tension ring. If square then you have may have LTB problems as well as bracing at the corner problems. hmm....



EIT

http://www.HowToEngineer.com