Cb and AISC Beam buckling revisited - [Steel design and Stability] 2

[lexpatrie]

thread507-459248

I got a little sidetracked into this thread.... I was actually scanning to see if anybody mentioned the Zuraski paper in that discussion (Zuraski, discussion of Cb from Engineering Journal. I didn't find it. Oh, well. I think I read it years ago, but don't seem to have a paper copy of it laying about (or it's not listed in my inventory...).

Not sure why this got labeled as a "rafter" since it looks like everything is structural steel and it all looks flat with metal deck, but that's not where I'm going with it. A lot of posts on this site start out with some really odd title and you've got no real idea what it's about, just saying. Just thought I'd mention that. I think there's a lot of "standard" terminology that's not actually standard.. To me it's a beam, not a rafter and "fly brace" is a new term to me as of today.

While I didn't read the whole thread (let alone read and understand it/replicate it), one thing did seem worthy of mention and I didn't see it mentioned in that thread.

It's this picture, where the beam above a column has buckled. This is a stability issue (of course), and it was discussed at a Stability seminar from AISC I remember going to way back when.

As a side note, "Steel Design After College" (on youtube and with a PDF), Link coincidentally has a fair bit of coverage on Cb that is worth watching/reading, particularly for uplift, and less-so for things like "cantilever roof framing" or Gerber Girders, etc. (they're about same thing, Gerber is an older / Canadian name for it from what I've seen.

Anyway.

You're supposed to put a web stiffener here. from the look of the deflected shape, this is inelastic buckling due to compression on the bottom flange of at the column. While the text in the original post stated the joists are 'normally considered as brace points' (that's perhaps true from a how people designed things back in the day), it's untrue to reality, as the beam looks like the bottom flange was in compression, for this to count as a brace point, the bottom flange can't be in compression, so the analysis is using a brace where there really isn't one. It is similar to the "inflection point considered as a brace" that has since been highly discouraged. Keep in mind there are still structures out there that may use this design approach and be aware of them, particularly when looking at cantilever beam framing systems.

Regards,
Brian

 

[human909]

Some of that is just different terminology due to a different locality. People from all of the world post on this forum. Rafter and fly brace are the normal descriptions for those members for those working with AS codes.

Regarding your post. Yes I'd expect the bottom of the beam to require lateral restraint. I'd also be somewhat concerned that a web stiffener alone might be insufficient restrain for the bottom flange.

To further explain the above I'd expect that this is a case of LTB and not a case shear causing web buckling.

 

[lexpatrie]

It doesn't look like web crippling, but that's another possible reason to have a stiffener there. To me there's typically a stiffener there in most standard details.

Here's some rendering of web buckling/cripling via Abacus.

 

[dik]

That looks really bad...

A plate stiffener for the web should have been provided at the column. The splice distance at the right appears to be close to the column. If the steel beam is designed plastically, the splice location moves closer to the column. This may have been the manner of design and you want a plate stiffener at the column because it is the first hinge that forms and you want to maintain the stability of the section.

It's not web crippling. The bottom flange should have been braced to the roof joists and there should have been a web stiffener.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dauwerda]

Sure looks like there is a web stiffener to me.

 

[human909]

Agreed regarding the stiffener. I was thinking the same thing.

A web stiffener has limited benefit for lateral retrain unless you have a very stiff connection at the opposite end.

 

[dik]

That slight shaded part near the blue arrow... my apologies, I missed that; I thought it was a shadow. I don't know why the bottom flange of the beam would have flattened as it appears to have done. The top of the beam could have be translated horizontally and the eccentric load caused the flexure to have it fail. Need more info.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[human909]

LTB

The compression flange lacks lateral restraint. A cantilevered column doesn't cut it. Not to mention the propensity to buckle is increased by reaction load on the bottom flange as opposed to at the centroid.

This could have been readily prevented by connecting the truss to the bottom flange of the beam.

(This design might be common in some localities and maybe it manages to work a fair bit of the time. Those designing to AS 4100 usually would restrain the bottom flange.

Which, for sake of avoiding calculating dozens of load combinations, would likely be along the full length at every truss or every second truss as required to obtain suitable bending capacity.)

 

[Smoulder]

"the joists are 'normally considered as brace points' (that's perhaps true from a how people designed things back in the day), it's untrue to reality, as the beam looks like the bottom flange was in compression, for this to count as a brace point, the bottom flange can't be in compression,"

Complicated by being a cantilever. Top flange is best for lateral bracing of cantilevers but doesn't do much unless right at the splice point. That's reality but the Aus code doesn't let lateral bracing increase cantilever buckling capacity so this cantilever would be designed as unbraced. If the splice was fully braced at top and bot flanges then the compression flange would be the place to brace even though it's a cantilever as far as bending moments go.

 

[DaveAtkins]

I don't think there is a web stiffener. I think that is the shadow from those two white pipes below.

If there were a properly designed pair of web stiffeners that could in effect extend the column to the top flange of the beam, then I would treat this location as a brace point. But the unbraced length of the cantilever beam would be the entire length of the back span, since the bottom flange is in compression and is unbraced.

DaveAtkins

 

[lexpatrie]

I think this is where the photo came from, it's from the link in that prior thread where the photos were posted.

https://www.llbc.leg.bc.ca/public/pubdocs/bcdocs/43459/commissionerinquirystationsquare.pdf

Station Square in Burnaby, Canada, circa 1988. It appears the cantilever is 6' this is a parking deck being supported above, some of the report calls them trusses other portions open web steel joists (O.W.S.J.). But i don't think it's a "truss", strictly, (meaning fabricated from structural steel/under AISC / Structural steel design code versus an SJI product/somewhat proprietary code) since there are so many of them, plus the spacing of what I believe are joists.

Looking at the photo in the source document, I don't know what that dark portion where a web stiffener would be, but the document itself (which is very detailed, has quite a few calculations or multiple versions of the same calculations), says there isn't a web stiffener and it also ruled out (via calculation) web crippling. It's unclear to me because it seems there's some measure of judgement in the web crippling calculation the way it's being discussed (page 83), versus the old web crippling calcs I've come to know and love that are standardized in AISC as of at least the 1st edition LRFD. I thought they had those in the 9th ASD as well. Maybe those showed up in that edition as a result of some Canadian research around the time (after Burnaby?). Don't know.

You can't see the connection better but in Appendix B, page 109 there's more of an overview shot showing where the column ended up.

 

[dik]

The past creeps up.

Peter Jones, How Kwong, and Dick Kishi are names from the past. I worked with Kwong and Kishi out of the Vancouver RJC office about 40 years ago. Peter Jones was the Jones in the Read Jones Christoffersen. The three left RJC and formed Jones Kwong and Kishi. They were top line engineers.

In addition to web crippling, there's a requirement that determines the maximum compression allowable on the web based on h/w ratio. In addition if stiffeners are required for the h/w issue, their thickness should be 1/2 the flange thickness. I had a recent project where this was required. I had to design the reactions for 67% of the equivalent UDL. The beams were large (W18x130 size) and the spans were short. The reactions were very large. I used 1/2" thick stiffeners where the flange was slightly over 1" thick. The calculated load on the cruciform column formed by the beam web and the stiffener load was 3x the design reaction (my rational).

With beams continuous over columns, I always provide a web stiffener, even if not needed. In the event of an overload, the stiffener maintains the beam section against instability in the event the section goes into the plastic design range.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[lexpatrie]

Just as a side note for those interested, the Zuraski article is currently no charge to download.

https://www.aisc.org/The-Significance-and-Application-of-Cb-in-Beam-Design