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

 

[Jerehmy]

Kootk - Yeah I didn't get much work done yesterday lol.

 

[NewEngineerHere]

@BAretired,

Here are a few examples I found on google search. Not necessarily shear plate connection, but what gets me thinking is long span beams without bracing against rotation.....

http://dellacooks.com/beautiful-for...rlooks-the-beautiful-zen-garden-design-ideas/

http://zainteriora.net/2009/11/25/suppose-design-office’s-concept/

http://versehead.com/steel-and-conc...t-home-interior-design-with-floor-to-ceiling/

 

[BAretired]

Number 1.

Are those steel beams? Hard to see. Looks like glulam beams to me. But in any case, there would have to be fixity between the columns and the beam. Otherwise, the structure is unstable (four hinges in the column).

BA

 

[BAretired]

Number 2.

Again, they could be glulam beams...not clear.

BA

 

[BAretired]

Number 3.

Aha! Now we have some steel beams; but the two grey looking beams are not carrying any load. Since they are attached to a column, they may be (probably are) prevented from rotating about their longitudinal axis.

The beams supporting the floor structure are continuously braced on the top flange. So where's the problem?

BA

 

[NewEngineerHere]

I thought Picture No.1 and No.2 are steel beams (looking at the I shape ! ) .... My mistake if they are not...
As for Picture No.3 : I was under the impression that both flanges must be braced (Am I wrong?) .. If the top is braced continuously by the top flange, shouldn't the bottom flange should have some sort of support against rotation too ? (Logically I think no we don't need since the top flange is assumed to be wide enough to stable the beam..... I may be wrong ....)


What about if the beam loaded with a glass on the top (Picture No.3) was to support a floor slab instead of the glass, would it require a brace for the bottom flange ?
Thank you !

 

[MJB315]

Ok, wow. Lots of passion on this one. But I'm with Jerehmy on this.

Buckling occurs when things are in compression. The compression flange of a beam in bending is...well in compression. So it wants to buckle.

In LTB, the compression flange is buckling. That triggers the show. But since the tension flange is not buckling (things in tension don't buckle), it stays put. The compression flange wants to "walk", the tension flange wants to "stay", and the web ties the two flanges together.

Like a dog tied to a tree, the compression flange is trying to run off but it is getting tugged by the web and pivoting around that tension-flange of a tree.

That's LTB. The rest are details.

Koot -- I dig your swagger and energy in this post. But I don't understand why you think that explanation of LTB isn't the start of the LTB story.

"We shape our buildings, thereafter they shape us." -WSC

 

[KootK]

All. Challengers. Welcome. Seriously, I'm grateful for your participation MJB315. Be sure to read my signature if you haven't already.

Buckling occurs when things are in compression...things in tension don't buckle

These statements are incorrect or, at the very least, misleadingly imprecise. And it is exactly this misunderstanding of instability that leads to all of the confusion regarding LTB.

Compression is not required for buckling. All that is required for buckling is a load and resisting stiffness that drops to zero. Here are some examples:

1) When a tension brace is a in a wall fails due to P-Delta effects, that's buckling.

2) Bottom chord bracing is often required on trusses to prevent the tension chords from buckling (Link).

3) The example that I posted above with a beam loaded from above but with every fiber in tension (2 Jun 15 14:46). That's compression free buckling.

In LTB, the compression flange is buckling. That triggers the show

So riddle me this:

You can brace the bottom, compression flange of a cantilever continuously but, if the top flange isn't restrained, it will still LTB under enough load. The top, tension flange will roll over the bottom flange like a dog tied to a tree (love that).

If compression flange buckling initiates LTB, and the compression flange is continuously restrained in this example, how is it that LTB is still a viable failure mode? My theory of LTB explains this. The compression flange buckling theory clearly does not.

I don't understand why you think that explanation of LTB isn't the start of the LTB story.

To some extent I agree with you. To quote myself:

For the most common cases, the story of LTB is primarily the story of tje compressed portion of the beam trying to do something akin to buckling about the tension portion of the beam...It's just that that story is incomplete and can lead to poor bracing decisions in the less common cases.

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.

 

[BAretired]

@NewEngineerHere:

No. 1 is described thus: "Warm Corridor With Timber Deck And Wooden Exposed Beam Ceiling Also Glass Wall Panels And Overlooks The Beautiful Zen Garden Design Ideas". Doesn't sound like steel beams.

No. 2 is too dark to discern much of anything. If the beams are steel, the top of the upper beam is braced by roof joists. The lower beam, if steel, would need to be prevented from torsional rotation at each end. The beam would need to be capable of carrying the design moment with an unbraced length equivalent to the span.

The beams in No. 3 appear to be simple spans. The bottom flange is in tension and does not require bottom flange bracing whether it supports a floor or not.

Continuous beams may require bottom flange bracing in the vicinity of inflection points but not necessarily. If the beam section is stocky enough to carry the load without bracing, then bracing is not required but torsional rotation must be prevented at the supports.

BA

 

[KootK]

Those houses are sexy! I'd live in any of them, unstable death traps or not.

With all my prosthelytizing, I almost forgot to send you this NewEngineerHere: Link. Despite being old as dirt, this is still my goto document for practical beam bracing guidance. Spend an hour with it and all will be revealed... Well, lots anyway.


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.

 

[nonplussed]

For the benefit of the original poster let me just add my thoughts to this topic. When considering restraining a member against LTB you need to first determine which flange is the 'critical flange'. This will be the flange which you will use to determine the effective length from and restrain accordingly. This may or may not be the compression flange depending on the situation, as illustrated below and in the attachment.
From AS4100:
5.5 CRITICAL FLANGE
5.5.1 General
The critical flange at any cross-section is the flange which in the absence of any restraint at that section would deflect the farther during buckling. The critical flange may be determined by an elastic buckling analysis (see Clause 5.6.4) or as specified in Clauses 5.5.2 and 5.5.3.
5.5.2 Segments with both ends restrained
The critical flange at any section of a segment restrained at both ends shall be the compression flange.
5.5.3 Segments with one end unrestrained
When gravity loads are dominant, the critical flange of a segment with one end unrestrained shall be the top flange.
When wind loads are dominant, the critical flange shall be the exterior flange in the case of external pressure or internal suction, and shall be the interior flange in the case of internal pressure or external suction.

Once you have determined the critical flange you can then calculate the effective length of the critical flange based off factors in the code. These factors simply depend on whether the critical flange is unrestrained, partially restrained, laterally restrained, or fully restrained. For example, the first picture in your previous post where the beam above the sliding doors is supporting the roof is fully restrained at each end by the perpendicular steel beam. You can see the beam outside and they are probably welded at the intersection. The other beams you show are also fully restrained at each end by intersecting beams or columns.

 

[KootK]

Yeah, the Aussie code blows the North American codes out of the water when it comes to providing clear guidance in this area. And prescriptive guidance is surely the way to go. Relying on designers' understanding of the underlying, very complex theory is just asking for trouble.

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.

 

[nonplussed]

Now to add to the rest of the discussion

2) Bottom chord bracing is often required on trusses to prevent the tension chords from buckling (Link).

This link actually refers to restraining buckling of the diagonal compression member in the truss. It states that the bottom chord of a truss may not provide adequate restraint against lateral movement of the diagonal compression member, thus you cannot use the diagonal member length as the effective compression length in all cases. Nothing to do with LTB.

3) The example that I posted above with a beam loaded from above but with every fiber in tension (2 Jun 15 14:46). That's compression free buckling.

This type of failure is similar to a tension cable moving to the side when a tight rope walker falls off. Although this, by simple definition, may be LTB since the beam deflects laterally and exhibits torsion, I don't think it is the same LTB definition that is broadly used by the engineering community. If you use this same example but with the load applied below the centre of gravity then it will not fail in LTB but will fail by tension in the bottom flange.

 

[KootK]

Well, well, well... Like the Spartans in 300, I face an innumerable horde of challengers. So be it! To hell with billable hours!

This link actually refers to restraining buckling of the diagonal compression member in the truss. It states that the bottom chord of a truss may not provide adequate restraint against lateral movement of the diagonal compression member, thus you cannot use the diagonal member length as the effective compression length in all cases. Nothing to do with LTB.

I disagree. Firstly, your only two choices for the diagonal compression member are a) K=1 and b) K=infinity. So the tension chord has to brace the compression web. The compression web leans on the tension chord for its stability which, in turn, makes the tension chord itself the next stability issue. One way to tell is by the equations proposed to evaluate the tension chord. They're all about the lateral stiffness of that chord. For the most part, stiffness only comes into play in capacity calculations when stability is an issue.

This is entirely analogous to the situation where you use a diagonal kicker to brace a beam that might LTB buckle. Once you've dealt with the beam, the next problem is to design the kicker itself to ensure that it won't buckle.

So, the tension chord issue is definitely a buckling issue. But is it a lateral torsional buckling issue? More on that later. For now, I'll point out that a) tension chord buckling represents a global rotation of a truss, just like LTB and b) what is a truss, really, other than a solid section beam with some extraneous web material removed? Tension field theory anyone?

If you use this same example but with the load applied below the centre of gravity then it will not fail in LTB but will fail by tension in the bottom flange.

Not true. With the load applied below the centre of gravity, the beam is less likely not to buckle but not guaranteed not to buckle. Just look at picture "B" in the document that you posted.




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]

This type of failure is similar to a tension cable moving to the side when a tight rope walker falls off. Although this, by simple definition, may be LTB since the beam deflects laterally and exhibits torsion, I don't think it is the same LTB definition that is broadly used by the engineering community.

The lack of a precise definition of LTB really is a problem in this debate. My two analogies make perfect sense to me because I hold a certain model of LTB in my head that makes the comparison obvious to me. You, and others, have different models in your heads that lead you to question the validity of my analogies. Bridging that gap is difficult.

To address this, I propose an unconventional definition of LTB to be our gold standard here. Usually, we deal in bifurcation theory when discussing LTB, similar to Euler buckling. Trouble is, that's the result of 4th order DiffEQ manipulation and doesn't really speak to anyone's intuition other than perhaps Galambos, Yura, Timoshenko, and Bazant. They'll probably be drinking buddies in non-denominational heaven.

A much simpler definition of LTB is the energy definition in my opinion. Here's how I see LTB when viewed from the energy perspective:

1) As a consequence of entropy, all systems tend to minimize embodied energy. When physical movement of a structure would result in a lower embodied energy, you get that physical movement and we call it buckling.

2) When LTB occurs, a beam flops to the side and starts to resist load about its weak bending axis rather than its strong bending axis. This increases deflection and moves the load closer to mother earth. This reduces the potential energy.

3) When LTB occurs, additional strain energy accumulates in the beam in the form of beam twist (J/Cw) and beam lateral sway (Iy). These represent the "lateral" and "torsional" in lateral torsional buckling.

3) If the loss of potential energy mentioned in #2 exceeds the gain in strain energy mentioned in #3, you get LTB. The end. And nowhere has compression been mentioned as requisite.

In my mind, this definition means that only two criterion need to be satisfied for a buckling mode to be considered LTB.

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.

Both of my analogies, the truss and the top loaded beam, satisfy both of these requirements.


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]

Dear God! This thread. KootK - I feel bad for you.

I believe I am way too late in the game, but I do not understand what all the fuss is. You can easily prevent a beam from failing in LTB by bracing the web only (a sufficient depth that is). The compression flange will be left to do whatever is next in line in the limit states (local buckling, yielding). All you need is a couple of forces with an adequate lever arm to resist the LTB instability. The compression flange may or may not be involved.

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

 

[KootK]

Dear God! This thread. KootK - I feel bad for you.

Don't cry for me Argentina! Seriously, it's exactly this kind of rare debate that I come here for. And, obviously, I played a rather active role in dragging a simple question out into near triple digit posting.

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]

Well, I am very curious about the resistance to your arguments, KootK. Frankly, you are putting much more effort into your argument with examples, sketches, etc. than the others who initiated the conflict.


Maybe this will open your mind about structural instability - Link

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

 

[KootK]

Star for you Macguber22. Not for the support, which is also much appreciated, but for the nifty tension buckling video clip. The next time that this comes up and I'm feeling inflammatory, I'll just open with that!

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]

You're very welcome. And the cool thing, is that tension instability is not purely theoretical. That failure can happen in a real structure that is not braced at the potential bifurcation point, e.g. truss chord joint.

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