unbraced length (seemingly very fundamental) question 6

[Lion06]

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?

 

[jike]

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.

 

[JadeRabbit]

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.

 

[frv]

StructuralEIT,

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

 

[JAE]

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.

 

[jechols]

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

 

[jechols]

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

 

[271828]

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.

 

[JAE]

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.

 

[vincentpa]

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.

 

[Lion06]

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.

 

[JAE]

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."

 

[vincentpa]

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.

 

[csd72]

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.

 

[vincentpa]

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

 

[frv]

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.

 

[Ussuri]

This is interesting.

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?

 

[msquared48]

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.

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

 

[JJMT]

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?

 

[vincentpa]

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.

 

[vincentpa]

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.