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unbraced length (seemingly very fundamental) question
6

unbraced length (seemingly very fundamental) question

unbraced length (seemingly very fundamental) question

(OP)
I want to explain a situation that recently happened in my office and I would appreciate some feedback.
I am working on a project for which I am the main contact for our firm.  I am doing the design and coordination with the architect.  I check in periodically with a senior engineer to go over what I am doing.  
I had an exchange this week that troubled me a bit.  
There is one situation where we have a four sided, self supporting "closet" (so to speak) to house some sliding doors.  There are some very small members that span wall to wall and bear on top of the wall.  They are supporting little load.  I decided to make them small HSS so that I could attach a light gauge clip angle to both sides of the tube where it bears on the wall so that these two clips would prevent twist of the member at this location to provide a brace point.  I told him the reason I did this was that if I couldn't brace the end, I wouldn't be able to calculate an unbraced length because it wasn't restrained anywhere.  Even if I provided one clip for a channel, it could still twist at the ends and just flop over.  
He proceeded to tell me that if you have a beam, set it on a wall (without restraining the top flange at a single location and without restraining the ends against torsion) that the unbraced length would be the actual length of the beam.  I expressed some concerns about this, and even stated that AISC would not allow this (pointed to the example of a seat angle having the requirement for the top angle to brace the ends).  He proceeded to just tell me I was wrong and that the unbraced length is never larger than the actual length (which I agree with as long as it is actually braced at its ends).  
He also talked about how he was talking about strength and not overall stability of the member.  Isn't lateral-torsional buckling a stability failure and NOT a strength failure.  As far as I am concerned that is a stability issue.
Anyway, I think he came away from that meeting with the impression that I am lacking some fundamental understanding of behavior, but I am not seeing it that way.  
Can anyone here comment on my thought process?
This person has been with the company for about 10 years and has about 20 years of experience.  He only recently came to our office and this was one of the first interactions I've had with him.  He is well-respected in the office and I hate the idea of him thinking that I am lacking fundamental knowledge.
It ended up being a very akward situation for me, because I felt like he was missing the fundamental idea I was trying to get across (which I thought I did pretty clearly), but how does a guy with 1.5 years experience say that to a 20 year guy?
Any thoughts?

RE: unbraced length (seemingly very fundamental) question

If you set a beam on a wall and have no connection other than direct bearing and the load applied to the beam is perfectly concentric, then the beam will simply span from wall to wall and the unbraced length would theoretically be the span length because:  the compressive bearing stress at the end would inhibit one side of the tube lifting off the wall.

However, there is no such thing as a purely concentric loading or perfectly straight beam so there will always be some level of lateral torsion or twist in the member.

So with some level of torsion (due to beam sweep or load eccentricity) the beam would have to have some sort of load path to resist this torsion.  Again - lifting of one flange tip or beam edge off the wall would be resisted to some extent by the compression bearing stress.  When the beam tips sideways, the bearing reaction moves to the other side and you have an effective resistance to the torsional moment in the other direction.

Example:  take it to a crazy extreme....you have a 20" x 20" tube sitting on the wall.  With a l lb load how would it ever twist off the wall?  It just intuitively wouldn't.

But your main question was unbraced length.  

The question then is:  Does the bearing compressive stress in the end of the beam serve to inhibit section rotation at that point?  

I don't think, in normal terms, that it does so in such a manner that can be checked, quantified or relied upon.  

Even with my crazy extreme example above, as a structural engineer (with 29 years experience) would I count on it except for very small loads or members...and even then, what about wind uplift, diaphragm transfer to the wall, etc.?

AISC 13th edition spec - in Appendix 6, section 6.3 states:
"At points of support for beams, girders and trusses, restraint against rotation about their longitudinal axis shall be provided."

That's pretty clear to me.

RE: unbraced length (seemingly very fundamental) question

(OP)
JAE-
I believe even if you bolted the flanges down or welded them that this would still not constitute a brace point.  It is certainly more braced than not welding or bolting, but AISC require stiffeners along with the bolts where beams continue over columns (or other beams) in addition to the bolts, correct?
For a seat angle where the beam is welded to the seat, you still are required to put an angle at the top flange (or near the top of the web), correct.
I can see the theory of a perfectly concentric load as you noted above, but that situation doesn't occur in real life so why go into discussion about it during a meeting?
As for a 20"x20" tube, that is inherently stable because it is a section with equivalent MOI in both directions, correct?  If you have weak axis bending there are no lateral-torsional buckling considerations, correct?
I agree with the example you gave about the 20" x 20" tube with a 1 lb load, but as you point out that just isn't realistic.
Thanks for the input.

RE: unbraced length (seemingly very fundamental) question

but AISC require stiffeners along with the bolts where beams continue over columns (or other beams) in addition to the bolts, correct?

AISC Section K1.5 requires a check against sidesway web buckling.  Theoretically, sometimes you do NOT need stiffeners at continuous beams over columns but every collapse I've ever seen has involved the lack of these stiffeners so I always use them no matter what.

For a seat angle where the beam is welded to the seat, you still are required to put an angle at the top flange (or near the top of the web), correct.

Yes but I think it is simply a good practice, not necessarily implying that in every case it would fail if you didn't.  

As for a 20"x20" tube, that is inherently stable because it is a section with equivalent MOI in both directions, correct?  If you have weak axis bending there are no lateral-torsional buckling considerations, correct?

Correct.  I agree.  I was just using the 20 x 20 example to help describe the force couple developed when the tube tries to overturn.  It would be applicable for a 20" tall by 18" wide tube.

Keep in mind my post:  I agree that a connection is required...AISC requires it as I indicated in my last paragraph.

RE: unbraced length (seemingly very fundamental) question

(OP)
JAE-
I appreciate your input.  The last thing I would like you to comment on is this - He talked about lateral-torsional buckling (and the associated unbraced length) and overall stability as if they were mutually exclusive.  
My understanding is that lateral torsional buckling IS a stability failure.  When the moment strength of the beam is limited by lateral torsional buckling, then you have stability issues that require reducing the amount of load a beam can handle compared to its full strength.

RE: unbraced length (seemingly very fundamental) question

I think the disconnect might be due to the type of member.  I don't have my 3rd Ed. LRFD or 13th Ed. Spec. handy, but I'm pretty sure one does not ever have to check an HSS for LTB.  This leaves only global stability.  

You took care of global stability, IMO, with your connections.

"He proceeded to tell me that if you have a beam, set it on a wall (without restraining the top flange at a single location and without restraining the ends against torsion) that the unbraced length would be the actual length of the beam."====>This is totally wrong.  Actually, one buckling mode might give the same Mcr, but there's a lower load: the rigid body one.

"He proceeded to just tell me I was wrong and that the unbraced length is never larger than the actual length (which I agree with as long as it is actually braced at its ends)."====>You are correct.  He needs to review the underlying theory behind the LTB Mcr in Salmon & Johnson, for example.  

"Isn't lateral-torsional buckling a stability failure and NOT a strength failure.  As far as I am concerned that is a stability issue."===> It's both as far as I'm concerned.  I don't see any usefulness in a distinction.

I think your thought process is fine.  There's no way you're gonna convince him of anything, though.  

JAE typed: "AISC Section K1.5 requires a check against sidesway web buckling.  Theoretically, sometimes you do NOT need stiffeners at continuous beams over columns" ===>That's interesting.  I thought SSWB was for something else entirely.  I'll have to look that one up for myself.  

Like you, I'd never go without the stiffeners.

RE: unbraced length (seemingly very fundamental) question

I know that the LTB equations are based on the ends of a beam beaming restrained. But I also agree that it should be restrained even when LTB is not required to be checked just based on global instability coming from tolerances, etc.. and also since it is stated to be required AISC.

RE: unbraced length (seemingly very fundamental) question

StructuralEIT:

I would entertain the idea of the required torsional restraint being provided at the ends of a W section, or HSS for that matter, if the ends were bolted to the top of the wall through bearing plates welded to the beam end, and an additional end plate welded to the beam and bearing plate to transfer the torsional forces to the bearing plate.  The bolts would then take the torsional force to the wall structure.

This is essentially the scenario for a cantilever beam bolted to a concrete wall, but the torsion is resisted by the bolts in tension as opposed to shear for the cantilever.

Mike McCann
McCann Engineering

RE: unbraced length (seemingly very fundamental) question

(OP)
So what should I do now?  It seems like he has the impression that I don't know what I'm doing (maybe thinking) and I know his opinion will be valued when it comes time for my review.  I wouldn't be as concerned if this weren't my first interaction with him of any reasonable length (you know what they say about first impressions).

RE: unbraced length (seemingly very fundamental) question

JMHO:

You might just print off this thread and show him that intelligent, experienced, structural engineers disagree about seemingly basic questions. If everything in our field was really black and white, its doubtful this board would exist.

Our office consists of several very expereinced engineers and 1 EIT. The EIT often shows the rest of us the errors of our ways. The experienced engineers are wise enough to realize that they don't know everything ( and really only dimmly grasp the outlines of what they don't know in a lot of cases), and can always learn something new.

RE: unbraced length (seemingly very fundamental) question

Not everyone will like you, or agree with you, or think of you as you think of yourself.  Deal with it and move on with life.  

RE: unbraced length (seemingly very fundamental) question

(OP)
UcfSE-
I'm not sure exactly what you think my concern is.... but I am only concerned about how his view of my work will affect the projects I am given in the future as well as my review (and consequently, my raise).
I couldn't give two hoots if he likes me, agrees with me (as long as I know my thought processes are reasonable), or if he thinks of me how I think of myself (not really sure what that one even means).
When his opinions (which, IMO, are based on some flawed ideas of his own) of me will affect my work environment, then I do have a problem with that.

RE: unbraced length (seemingly very fundamental) question

I wonder if your continued hard work, concientious efforts, improved skills over time, willingness to learn, quality of work, etc. wouldn't overcome any brief one-day episode such as this?

It may be possible that the engineer doesn't even remember your conversation as so earth-shattering as you do.

RE: unbraced length (seemingly very fundamental) question

Doesn't anyone go to lunch with coworkers anymore???

StructuralEIT, offer to take him to lunch....bring a notepad and a pencil and you guys draw up the structure together and work through the force and stability analysis.  You'll both benefit a great deal.

ZCP
www.phoenix-engineer.com

RE: unbraced length (seemingly very fundamental) question

It can be a nightmare once ego becomes an issue. Senior engineers sometimes do not want to listen a "rookie" engineer. If that is the case, don't even bother trying to explain things 'cos that can only make things worse. And again, StructuralEIT......don't take it personally and I am sure you are just over-reacting the situation. I don't think your raise is going to be affected, 'cos mistakes happen all the time (not that I am saying you are wrong or he is wrong).

RE: unbraced length (seemingly very fundamental) question

I agree with ikjh345.  

28 years ago when I was an EIT, there was another EIT in the office that pointed out a simple truss problem in detailing that had beewn going on for years in the office without the principles thinking about the ramifications.  It had to do with the effect of providing non intended interior support points to roof trusses underlain with non-bearing walls by not allowing the trusses to deflect as designed over these same walls.  The future use of Simpson DTC clips at non-bearing walls alleviated the problem and substantially minimized the liabliity of the firm.  It took the new blood and lack of "experience" of an EIT to see things as they were.  

Never sell anyone short.

Mike McCann
McCann Engineering

RE: unbraced length (seemingly very fundamental) question

Don't confuse curt with refusal to spoon feed.  I send people where they need to go rather than spend hours retyping posts that get answered about once a year or so, in general, before you joined for instance.  You probably haven't been here long enough to notice.

I question why you seem to need to go to a committee when you have a decision to make, whether it's here or in another area of the site.  What you need to do is learn to deal with problems and make decisions on your own rather than seek reassurance.  The root of your issue in this thread is something many of us, including me, run into or have run into at some point.  So it intereferes with work.  Deal with it man.  You ask what you should do.  What do you think you should do?

This can apply elsewhere as well.  I would rather tell you to "suck it up, Nancy", rather than give you a cookie and milk and a hug, and that's basically what I posted.  You'll be better off for it in the end, imo.

RE: unbraced length (seemingly very fundamental) question

StructuralEIT:

One person does not know all the answers.  I firmly believe that the only stupid question is the one that is not asked.  Therefore, never hesitate from asking others their opinion.  But, in the end, it is true that you will have to make the decision that is right for you.

Mike McCann
McCann Engineering

RE: unbraced length (seemingly very fundamental) question

StructuralEIT,

As others have said, your thought process is sound, but I wouldnt worry too much about the buckling of a hss as these are torsionally very strong.

I have often found that many old school engineers have very poor awareness of buckling issues. I believe this is because when they began, the codes were conservative enough that this was not so much of an issue.

I would suggest the following approach when dealing with this type of issue:

1. When having the initial discussion, do not be confrontational. Instead us phrases such as "Could you please explain this as I would like to understand your justification for this.."

2. If you feel your approach is correct, and your approach is more conservative, then use your approach. If their approach is more conservative, then use theirs.

RE: unbraced length (seemingly very fundamental) question

StructuralEIT,
This suggestion is about the "process" of dealing with bosses, peers, clients, etc.
Pick up a copy of a book that was written in 1989 by Stephen R. Covey titled "The 7 Habits of Highly Effective People". Pay attention to habit #5, which is: Seek first to understand, Then to be understood". If one's focus is on listening to understand (someone else) as opposed to listening to respond to that individual it sometimes effects a very different end result.
Of course, if the person that you are dealing with doesn't suscribe to the same Principle then you might have to read the rest of the book to pick up a few more "habits".

Good luck

RE: unbraced length (seemingly very fundamental) question

I remember a failure years ago on steel beam shoring for a bridge project in Minnesota and the failure was directly related to simple beam supported on a scaffold tower without the top flange of the beam being braced. As others have said, it is probably less of a problem with a square tube.

As far as stiffeners required for a continuous beam over a column, I do not believe the numerous AISC equations to check to see if stiffeners are required cover the issue of why I require stiffeners in this instance. It has to do with the top of the column being unbraced (at the bottom of the beam). The other way it can be handled is to extend the bottom chord of the joist to the beam or column.

Even though I have 36 years experience, I sometimes will ask an EIT for their opinion on a particular problem. It helps both of us. Their prospective is often different than mine, but no less valuable.  

If there is one thing I have learn in all those years is that I do not know everything.

RE: unbraced length (seemingly very fundamental) question

My suggestion:

Work out all the calculations, and with each check / equation, site the source or code.

Once you have your calculations prepared... always refer back to them. Or make a copy and ask your "buddy" / senior engineer to review them. This is a simple way to eliminate the confrontation, and will allow a calm / non-verbal dialog to take place.

If he disagrees with your calculations, have him mark them up and ask him to provide a reference that you can look at more closely for clarification.

I've come across many Engineers that do things by rule of thumb and many architects that think one design is good for everything.

If I provide something that is "code"/reference supported, then they're not arguing with me... they're arguing with the code. This works great for shop drawings also... (i.e. SEE ACI Sect. 7.1)

Hope that helps you move forward.

RE: unbraced length (seemingly very fundamental) question

StructuralEIT,

I don't think there is any gray area here. You are right. Read p. 2-13 in the 13th edition.

RE: unbraced length (seemingly very fundamental) question

jike,
I would agree with you that the AISC chapter K check may not deal with the column moving relative to the floor/roof structure framing.  That "sidesway web buckling" in Chapter K seemed to pop up a few years ago.

My own mentors from years ago (one started his career in 1927) taught me to "just put the dang stiffners in there no matter what".  I've done so ever since - as I said, many of the roof collapses I've seen over the years have almost always had lack of stiffners as a feature.

RE: unbraced length (seemingly very fundamental) question

So is everyone saying that if a loose beam (not connected) is supported on each end by a wall that there are some shapes that would fail before the applied moment reached Mp even if the actual length was less than Lp for the given shape?  Or is Lb for all shapes infinite and Mn would be limited to Fcr x Sx but less than Mp?  This thread has got me thinking too much for a Monday!

Thanks for the post StructuralEIT!

j

RE: unbraced length (seemingly very fundamental) question

Disregard the second part of my post above.  Fcr is a function of Lb and looks like if Lb is infinite Fcr would be zero.

j

RE: unbraced length (seemingly very fundamental) question

jechols, the question is mainly about what end restraints must exist for Lb to equal L.  StrlEIT's co-worker argues that Lb <= L for ANY beam, regardless of boundary conditions.  In reality, the LTB Fcr is derived assuming torsional restrain at each end of hte unbraced length.

If the unbraced segment is not braced against LTB at its ends, then it can just flip over on its side.  This is more of a global stability failure, not LTB.  If you tried to calculate the buckling load for this, I think you'd find a rigid body mode that doesn't take any load to cause it.

RE: unbraced length (seemingly very fundamental) question

271828 - agree that it is a global thing, not necessarily a LTB condition.  But what the "older" engineer was getting at, in my opinion, was that in MANY cases the loading and beam are such that the beam won't flip over due to the width of the beam.  

What I posted above was that if there was a lateral couple formed by beam sweep, eccentricity of load, etc., then there would be a torsional load on the beam.

Resisting this would be the bearing reaction which, as the beam rotated slightly under the torsion, would move laterally and become eccentric to the centerline of the beam axis and resist that twisting effect.

What I also stated was that I sure wouldn't count on it and I would add the positive connection in most all cases.

RE: unbraced length (seemingly very fundamental) question

If I am incorrect, LTB doesn't need to be checked for a tube.  It is inherently stable.  Somewhere, and I can't remember where, maybe in the masonry code, maybe in AISC, it states that steel beams bearing on masonry walls must be restrained.  It is in one of the codes. I think angles are exempt.  We either bolt the beam to the top of the wall or we weld angles and grout the pocket solid.

When I first started, I worked with and engineer that wasn't too bright on the theory side but he was still and excellent engineer and the best PM I ever worked with.  You have to handle these guys delicately.  Secondly, you have to remember, you do only have 1.5 years of experience.  He has 20 years.  If you were being even slightly confrontational, you probably offended him.  And quite frankly, he probably has a right to be offended.  It is better, when dealing with an older engineer to ask to have it explained as if you are the dummy, even if you know you are right.  Then say, "I thought it was like this... can you tell me why I'm wrong?"  You might have to eat a little goat sometimes but it is better than offending someone by being disrespectful.  I wouldn't be concerned of what he thinks of your abilities. He knows you only have 1.5 years of experience. He will take that into consideration. He is probably angry because he believes you are being disrespectful. You should eat goat and apologize to him for being disrespectful.  Don't show him why you are right, even if you are. It will make matters worse.  ALWAYS REMEMBER, RESPECT YOUR ELDERS! I'm 35 and I still respect older engineers even when I know I'm right.

RE: unbraced length (seemingly very fundamental) question

(OP)
I may have seemed a little frustrated (at leat in my own mind - not necessarily outwardly) at his lack of understanding what I feel I expressed exceptionally clearly.  That being said, I would agree that most HSS are relatively stable, but not as a hard rule - I wouldn't say that of an HSS 20x4 in strong axis bending with no end restraint.
Also, he was making his case using a channel, not a HSS.  I told him that was the reason I used an HSS instead of a channel and that is how the conversation started.

RE: unbraced length (seemingly very fundamental) question

As I posted above:

AISC 13th edition spec - in Appendix 6, section 6.3 states:
"At points of support for beams, girders and trusses, restraint against rotation about their longitudinal axis shall be provided."

RE: unbraced length (seemingly very fundamental) question

A CHANNEL!  You were definitely correct. However, that is not going to save you from the awkwardness of the situation in your office.  Eat some goat.  You'll even feel better.  Take it from me; I've done it before.

RE: unbraced length (seemingly very fundamental) question

make sure it is Jamaican style curried goat that you eat - yum!

But seriously, as many have said, this is more a human issue than a technical one. Dealing with unreasonable people is a skill that you definately need to master as many architects fall into this category.

RE: unbraced length (seemingly very fundamental) question

I have a preference for Spanish goat. My wife is from Spain. At home and in the office. I can't win!

RE: unbraced length (seemingly very fundamental) question

BTW..

LTB DOES occur in HSS members. However, the limiting length of Lr is so long that the reduction in capacity for any length approaching reasonable span to depth ratios is insignificant. Taking into account that Cb is rarely actually 0, the code simply ignores LTB.

RE: unbraced length (seemingly very fundamental) question

This is interesting.

Quote (JAE):

AISC 13th edition spec - in Appendix 6, section 6.3 states:
"At points of support for beams, girders and trusses, restraint against rotation about their longitudinal axis shall be provided."

The British Standard gives for beams without intermediate support gives an effective length for LTB assessment of (1.4xL) + (2x Section Depth), if the load is destabilizing (applied to top flange).  

This is for the condition where the compression flange is laterally unrestrained and both flanges are free to rotate on plan; and where partial torsional restraint against rotation about the longitudinal axis is provided only by pressure of bottom flange on supports.

So according to the UK codes you  effective length can be greater than the physical length of the member.  This relationship looks like a rule of thumb to me and gives no method of actually quantifying the torsional resistance at the support.  

Does this suggest the UK codes allow something the US codes prohibit?

RE: unbraced length (seemingly very fundamental) question

StructuralEIT:

Your supervisor may be a goat, but you do not have to eat him.  That would not be good for either your digestive system, your reputation, or your career.  bigsmile

That being said, as your supervisor, and possibly the one stamping and signing the drawings, he does have the final say and responsibility.  I would recommend that you log your objections to the job file in writing though in case there are any issues with the decision in the future.  Wnem you get your PE and have the responsibility of stamping the drawings, you will be able to do things your way.

Good hunting.

Mike McCann
McCann Engineering

RE: unbraced length (seemingly very fundamental) question

I think what you need to take away from this is the difficulty in explaing ideas to people when they seem obvious to you. (i'm sure everyone here has had a brillent professor who was a poor teacher)  Its a life skill, you use everyday, coworkers, aritechects, significant others.  Personally my approch is to ask questions until it resolves itself.  If you feel like you need to address the issue (like if he says use the channel anyway) go in and ask him to explain the problem to you.  Since you know the awnser, you have a good idea of the 'right' questions to ask .  Otherwise move on, keeping the big picture in mind, you'll have plent of opportunity to show your skills and most likly engineers being engineers, he'll investigate it on his own and see what you were saying.  
Respecting elders is what old people say to keep from being put on the ice flow.  Respect everyone, and value the opinions of the ones with skills.

More importantly, if you have a beam suspended at the midpoint from a cable and cables at the ends carrying your load, whats your unbraced length?

RE: unbraced length (seemingly very fundamental) question

J10.7 AISC 13th Edition
"At unframed ends of beams and girders not otherwise restrained against rotation about their longitudinal axes, a pair of transverse stiffeners, extending the full depth of the web shall be provided."

This is basically refering to beams over columns and beams bearing on concrete and cmu.  The AISCM, 13th edition has a picture but for some reason the AISC, 13th edition does not have a picture.  I have uploaded the picture.  I would still intuitively use anchor bolts for the baseplate to wall connections as well as the stiffeners. I forgot about this detail. I have to change our typical detail. We just weld angles to the side of the beam and grout it, which is not to code it appears.

RE: unbraced length (seemingly very fundamental) question

Page 2-13 of AISC 13th Edition and Appendix 6 talk about this situation also.  I believe the answers you are looking for lie in these pages along with my previous post. I am only just getting familiar with the 13th Edition. It is pretty good.

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