Yet Another Unbraced Length question

[slickdeals]

I am sure this has been beaten to death already......but thought I should ask anyways.

Assume you have a 2' high concrete pedestal spaced on a grid of 30' x 20'. A W24 steel beam spans between the pedestals in the 30' direction. There are end stiffeners welded to the W24 when it bears over the column to prevent rotation @ support. In the other direction, steel joists spaced at 5' o.c. span into the top flange of the W24.

For gravity loading only, is the unbraced length of W24 :
1. 5' ?
2. 30' ?
3. Something in between?

 

[csd72]

Yes it seems to be the way codes are going these days virtually forces you to use a computer package to cover all these idiosyncrasies.

 

[slickdeals]

Thanks for this discussion.
Please see revised sketch and provide your comments.

 

[ToadJones]

I am going to hi-jack this thread.

Unbraced length of column encased in masonry?

 

[slickdeals]

Toad,
Don't muddy the water...........start a new thread. POR FAVOR!!!

 

[ToadJones]

will do,
Sorry.
I will title it "yet ANOTHER unbraced length question"

 

[Lion06]

Slick-
The answer to the last question in the sketch is yes. That provides rotational restraint with or without the presence of the deck.

 

[BAretired]

Sorry Lion, I am going to have to disagree with your last post.

The question on the sketch was "If a kicker is provided as shown and the deck is removed, is the unbraced length 5'?"

If a kicker is provided at both ends of the joist, I agree that rotational restraint has been provided to both beams. I do not agree that lateral restraint has been provided to the top flange of either beam and I do not agree that the unbraced length is 5'.

The top flange of both beams will behave as a column with variable axial load. The unbraced length is 30' and it will buckle over a 30' length. Failure will not be by flexural torsional buckling, but elastic column buckling, modified slightly by the tension in the bottom flange trying to hold it in line (something I would ignore in design).

BA

 

[hokie66]

...And modified by tension in the top flange trying to hold it in line?

 

[Lion06]

BA-
Fair enough. I looked at the one end and assumed that detail would be typical at both ends.

As far as the lateral restraint of the top flange, AISC doesn't require lateral restraint of the top flange to brace the beam for LTB. It can be accomplished through rotationally restraining the beam section.

Once the one end is restrained rotationally, I think the entire section (in weak-axis bending) is effective in restraining the top flange of the other beam.

I would analogize this to a moment frame where one beam/column connection is a moment connection and the other is a shear connection. Both columns are stable even though only one is rotationally restrained.

 

[Lion06]

I'm going to call the W24 with the stiffeners and kickers BEAM 1, the opposite W24 without the stiffeners and kickers BEAM 2, and the infill beams that stack BEAM3.

I think that the stiffeners and kickers on BEAM 1 constitute rotational restraint and will act as a brace. Beam bracing doesn't have to be lateral restraint of the top flange, it can be accomplished through rotational restraint alone. Additionally, the bracing accomplished via rotational restraint is not affected by what's going on at the other end of BEAM 3. The only way for BEAM 1 to buckle (LTB) is by overwhelming BEAM3 in strong axis bending.

For BEAM 2, that's not quite as clear. It would depend on the weak axis bending stiffness of BEAM 1. If BEAM 2 is to buckle (LTB), then BEAM 1 would have inadequate stiffness in weak axis bending to brace BEAM 2. I see BEAM 3 acting as the brace back to BEAM 1, which has the entire section engaged in weak axis bending via the kickers and stiffeners.

 

[hokie66]

For BEAMs3 to restrain rotation of BEAM 1 between the supports, the connections have to be stiff, and in the sketch, it doesn't look like that is the case.

 

[Lion06]

I think any positive connection between the bottom flange of BEAM 2/top flange of BEAM 1, kicker to BEAM 3, and kicker to the stiffener on BEAM 1 would restrain rotation of BEAM 1.

He just sketched something there. Obviously, if there's no positive connection between the parts then everything is a moot point, but almost any positive between those different pieces would restrain BEAM 1 rotationally.

 

[BAretired]

The main point of contention seems to be that the spacing of torsional restraints alone defines the unbraced length of a simple span beam. AISC appears to say that but CSA S16 does not.

I consider the unbraced length to be the spacing of lateral bracing members. Rotational restraints are required at supports.

BA

 

[hokie66]

Check out the photos in this thread for some beam buckling.
thread607-269140

 

[JennyNakamura]

I'm coming into this kind of late and haven't read through all the responses, but I believe the answer to the original question is 5' without the deck and 0'(continous lateral support) with the deck.

Lateral torsional buckling is a strong-axis bending phenomenon. When you brace the compression flange against "torsion" the "lateral" movement will not follow.

This global lateral movement that is being described can only be due to weak axis bending, with a load being applied horizontally. Lateral tosional bucking occurs in strong axis bending only and the uniform lateral displacement described in some of the posts will not occur due to a vertical load -- only if a horizontal force is applied.

 

[csd72]

Jenny, Sorry to be blunt but I disagree with your entire post.

I think you may have misread the post.

 

[JennyNakamura]

csd72, Thanks for being polite. I think the thread kind of drifted.

The OP's 1st question was for "gravity loading only." For that scenario, the failure mode for strong-axis flexure would be LTB, for which I still stand by my answer -- section 6.3 of the AISC appendix clearly supports that as azcats pointed out earlier.

Later, the OP posted a sketch showing stacked framing with an applied lateral load. In that case, the unbraced length would be 30' with the stiffeners at the supports being the only thing resisting "lateral deflection," as others have pointed out. With the deck, there would be no reduction in weak axis bending capacity, as the deck would act as a diaphragm (assuming the deck had sufficient shear capacity and was fastened to the steel properly).

In reality, the deflection shown in the sketch would never happen, because the designer would probably have web stiffeners on the main beams under the joists which would minimize the rotation effect depicted on the sketch and the beam would bow outward, rather than tilt linearly.

 

[csd72]

Jenny,

I would suggest you have another look at it.

You say the effective lenth is 5' without any decking, what is to stop the top flange of both beams translating together?

You say that it is continuously restrained if there is deck but there is still only restraint at 5' centres and not continuous as you have stated (the joists are continuously restrained but not the main beams).

RE"Lateral torsional buckling is a strong-axis bending phenomenon. When you brace the compression flange against "torsion" the "lateral" movement will not follow." I agree to a certain extent though I am not sure where the torsional restraint is that you are referring to. Unless the stiffeners are piut in which you assume every designer would - good steel designers only put stiffeners in when they are required (thus the original post).

RE"This global lateral movement that is being described can only be due to weak axis bending, with a load being applied horizontally. Lateral tosional bucking occurs in strong axis bending only and the uniform lateral displacement described in some of the posts will not occur due to a vertical load -- only if a horizontal force is applied. "

If the beam was perfectly straight and vertical you would be correct, but as soon as there is any twisting then a component of the applied force becomes a lateral one. The reason why rotational restraint is not because there is not possibility of lateral translation but rather because the tension in the bottom flange acts to help keep it in place.

I have worked wuth unrestrained beams quite a lot and find that most engineers do not understand buckling as well as they think they do.
I believe it is important for all of us to occasionally question our own knowledge.

 

[slickdeals]

Folks,
I must clarify a point. The arrow that I put in the sketch was not a lateral load, but the direction the system might move under gravity loading once the LTB kicks in.

I apologize for the confusion.

 

[Lion06]

Jenny,

I agree with csd. I think you should re-think it. My take on the sketch was that the arrow was merely pointing in the direction that the assumed buckling was taking place. The horizontal load isn't gravity (as mentioned in the OP) and wouldn't cause LTB (at least not in the manner we typically think about it, through strong axis bending).

Regarding the 0' unbraced length comment - can you explain how you arrived at that conclusion? I would have never come to an answer of 0' unbraced length, and, frankly, don't see how it's possible. See my attached sketch. I don't see how the unbraced length could ever = 0'.


Where was anyone describing a uniform lateral displacement?



CSD-
I agree with you that most engineers don't understand buckling and stability issues as well as they think they do. Elastic stability has always sort of fascinated me. That's why I bought several decent books on the topic (including Timoshenko's Theory of Elastic Stability and Golambos' Stability of Structural Steel). I fall into a different camp. I don't understand it quite as well as I would like, but I think I have a pretty good grasp of my actual level of understanding on the topic (i.e. I know that I don't understand it as thoroughly as I would like). The last Grad class I took was Advanced Structural Analysis. I asked the professor there if they have a class dedicated to stability, but unfortunately the answer was NO.
I wouldn't mind getting my hands on Yura's notes from his stability seminars.