Buckling lengths in portal frames 2

[Eagleee]

Well, here it is, yet another thread on buckling lengths. Although I would say this is a very common issue, I have not been able to find clear information until now and I am interested what you guys think of the following situation. I have a simple portal frame in which the rafters have the top flange braced by transverse members at several locations. The bottom flange is unbraced. I am interested in the approach to design for gravity loads (ignore uplift), more specifically on how the critical length of the bottom flange is to be considered. The rafters being fixed to the columns have rather high negative moments at the ends (ignore haunches) so I am uncertain as to how the critical length should be taken. Of course, the conservative approach would be to take it as the full length of the beam, but after the inflexion points in the moment diagram, the bottom flange is in tension, so it would seem like overkill (I am aware that 0 moment points are not to be considered as braced points). Do you do buckling analyses in this type of situation and just extract the elastic critical moment that way?

An additional point for which I have found conflicting information is the in-plane (column) buckling length of the rafters. Ive seen it taken both with 1 x L and 0.5 x L, L being the total span between columns. Also, if the frame is a sway frame, is it also sometimes the case that this buckling length would be larger than the beam length?

Any opinions are much appreciated.

 

[bowlingdanish]

These are just my opinions, but I look forward to KootK weighing in

I typically wouldn't consider this if the top flange is fully restrained, unless the beam was extremely deep. If you are fully restrained along the top flange, and presumably at the column, I don't see how the small region of negative moment can cause an instability in the whole beam, the section just won't be able to globally rotate, which in essence is what LTB is. So my vote is for just checking section capacity (if FLR along the top) and a sturdy beam-column connection.

Depends on the restraints... You probably have a proper brace out of plane at the ridge and haunches, so for buckling about the weak axis, I would take 0.5L (since the ridge brace stops it buckling). About the major axis, I would take 1xL, since the ridge wont help as much with this, and 'buckling' is restrained by the columns tying it to the ground. But I've never seen that govern design anyway.


A portal frame is a sway frame

 

[JAE]

Not sure what country/code you are working under.

In the US - with AISC, that is taken care of with their Cb ratio using the full length of the unbraced flange as Lb.
(specification found HERE (free download).
(Look at Chapter F)

Per bowlingdanish, if the beam is adequately (rigidly) connected to the top flange transverse members, and there are web stiffeners in your beam such that rotation is restricted by the stiffness of the transverse members, then the transverse member intersection point could be considered a brace point. If the transverse members are only simply bolted with questionable rigidity between beam and member, then probably not a brace point.




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[wannabeSE]

If the question is: can the point of inflection be counted as a braced point on a beam when figuring the unbraced length? The answer is no.

 

[JAE]

wannabeSE - he did say in his post this:

(I am aware that 0 moment points are not to be considered as braced points)

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[KootK]

Similar to others:

- LTB length is full rafter length if the purlin connections provide only lateral restraint and not rotational restraint. If rotational restaint is to be provided by the purlins it should be fly bracing or something similarly convincing.

- If this thing has a pitch and a peak, usually you can call the peak a point of LTB bracing, even without explicit bracing there. Kind of another version of being braced at a column. There might be a minimum pitch required, I don't recall.

- IP faux bracing was a myth that we all collectively fell for in the previous millennium. A persistent delusion that needs to die off altogether.

- While IP bracing is a no go, feel free to squeeze every scrap of capacity that you can out of the moment gradient business.

If you're serious about getting into this kind of work, pick up the Aussie book below. It's my goto for this kind of thing. Newman is good to but not as techy.

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.

 

[wannabeSE]

Thanks JAE. I missed that. I need be more careful reading and typing when I am on a bus with my phone.

 

[dik]

KootK... great book.

Dik

 

[dik]

A lot of good technical stuff including portal frames and 'laced built-up columns'. To Eurocodes, but good stuff nonetheless:

http://sections.arcelormittal.com/library/design-manuals-steel-building-in-europe.html

Dik

 

[Eagleee]

Thank you all for the replies. I will definitely check out the literature you point to. Working under Eurocodes, so I'll add the Arcelormittal guide next to the SCI publication dealing with portal frames. I just skimmed through it and I am a bit uncertain of the following piece of information I have found in many other places as well: the in-plane buckling length of columns in portal frames is generally as taken the full height. NCCI doc. SN031 specifies 1.5 x the height. However, it seems to me that if the frame is not very stiff laterally, the (global) sway buckling mode can increase the column buckling length well above these values. In this context I was asking in my original post about the in-plane buckling length of the rafters (@bowlingdanish by sway I meant that the frame is sensitive to second order effects).

@KootK yes, the moment gradient consideration does indeed help a lot. Also, Ive put the book you mention on my list with the highest of priorities. And yes, I would assume the trasnverse members offer only lateral restraint as although I'm not worried about their stiffness, the connection is of questionable rigidity, to use JAE's phrasing.

 

[dik]

OP:
If you search the net, you can find a *.pdf copy of the book KootK mentioned... If it weren't for copyright issues, I'd post a link.

Dik