Column Slenderness 2

[penpe]

The software that we use for structural steel analysis, (Tekla TEDDS), will in some cases result in adequate column strength in shear strength, compressive strength, flexural strength, and combined forces. However, it issues the warning that "member slenderness exceeds 200". (In this particular case the KL/r = 210). Reading the AISC steel manual 13th edition, with regard to column slenderness, "KL/r should preferably be limited to a maximum of 200". Then "Note that this recommendation does not apply to members that are primarily tension members..."

When they use words like "recommendation" and "preferably" I think that it's okay to exceed 200. But would it be considered inadequate?

If it was only a single column being considered I'd just upgrade the column so the preference is satisfied. But there are numerous columns for this project, and there's some motivation to make the column sizes the same for fabrication and construction simplicity. And I don't want to upgrade all 20 for the one outlier.

Thanks for your input.

 

[phamENG]

No, it is not a hard and fast requirement. It used to be - it was a practical limit to set a sort of baseline reliability for columns and other compression members. There are other reasons for it as really slender elements become difficult to handle without damage. Have you read the commentary? As I recall there's a paragraph or two dedicated to explaining where that came from and how it's changed over the past couple of the revisions of the Specification.

 

[jayrod12]

How conservative are you being on the height of the columns? I often calculate it as u/s of baseplate to t/o steel for conservative numbers, and then if necessary I pare that down to the actual clear distance. Maybe that will get you closer.

 

[JoshPlumSE]

There are multiple interpretations for this section of the AISC code.
1) I agree with you that the language is not compelling. That there are circumstances where using a KL/r > 200 would be satisfactory.

2) What's the liability? This section has created a de-facto "standard of practice" of not using compression members with KL/r values of 200 or more. For braces, I tend only to do this for members where I don't rely on their compression strength. For columns, I think this would be difficult to justify in a lawsuit. Even if the columns aren't the problem that initiated the lawsuit, it could be used to demonstrate that your design fell below the "standard of practice" for engineers in the industry.

3) For industrial structures that support equipment that is cheap and easily replaceable I could certainly see using a more slender column, but only if it has essentially zero occupancy or danger to the public.

 

[dik]

Just a caution... there's, generally, not much redistribution with compression members.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[271828]

A column with a high KL/r would be more sensitive to initial out-of-straightness and load eccentricity. Some simplifying assumptions might no longer be valid.

For example, I typically design columns for concentric axial load even though there is practically always eccentricity due to unequal beam or girder reactions. If KL/r exceeds 200, I'd include an axial and flexure interaction check. I might also consider a moment at mid-height equal to the axial load times the max initial out-of-straightness from the Code of Standard Practice Section 6.4.

I don't know if this would make a difference, but I would investigate to make sure.

 

[driftLimiter]

To piggy back off @JoshPlumSE and @dik... we don't like to mess around with our columns here. Most of the time we are trying to keep column UC below 80%. It is such an important element. Reducing the slenderness form 210 to 200 or less shouldn't cost too much weight of steel. And when it is all said and done how much cost % of the project are your columns going to be. A failure of a column is always a very bad thing, and almost everyone out there is using KL/r < 200 as a hard limit to their design. Also slender members like this can often end up becoming more costly to handle during installation because they are not so slender they are subject to instabilities during transportation and handling. Tonnage is not the only indicator of cost. AISC DG 23 By David Ruby has some info on this, and I have seen other webinars by David Ruby that cover this in detail. He makes several cases for additional weight of steel to provide actual cost savings to the project.

 

[80PercentTruth]

Regarding KL/r > 200, it is a code recommendation rather than a requirement as echoed above. As you begin to exceed KL/r for an axially loaded member the design begins to get impractical. Often times I see engineers unnecessarily bump up beam sizes (due to program flagging) to get under the 200 slenderness limit, but you have to use engineering judgment...if the member doesn't see much axial I am generally OK with it.

 

[milkshakelake]

I agree with @JoshPlumSE about legal ramifications. I wouldn't really mess with KL/r>200 and the extra attention to detail that comes with it. Just increase the size a tiny amount and be done with it. If things turn ugly, like with a peer review or legal thing, you'll be at less of a self-imposed handicap from the beginning.

If it was only a single column being considered I'd just upgrade the column so the preference is satisfied. But there are numerous columns for this project, and there's some motivation to make the column sizes the same for fabrication and construction simplicity. And I don't want to upgrade all 20 for the one outlier.

You don't want to upgrade 20 columns because one of them has KL/r>200? I don't really understand that, because all of them would be KL/r>200, unless it's one tall column or they're braced in different ways. In that case, maybe you could mess around with the K by adding fixity.

 

[WinelandV]

Honestly, I wouldn't personally worry about it.

I'm reading out of the AISC360-16. Chapter A. A1. 2nd Paragraph:

This Specification includes the Symbols, the Glossary, Abbreviations, Chapters A through N, and Appendices 1 through 9. The Commentary to this Specification and the User Notes interspersed throughout are not part of this Specification. The phrases "is permitted" and "are permitted" in this document identify provisions that comply with this Specification, but are not mandatory.

The user note [edit: in E3 of the spec] itself says "preferably". If AISC wanted to make kl/r <=200 apart of the spec, they certainly could have done so, but went out of their way to specifically not make it apart of the spec. Based on the AISC being the national organization, and the spec being a legal document via adoption of the IBC in most jurisdictions, I wouldn't be too worried if no other engineer in your vicinity does not use kl/r > 200. (I am not a lawyer, and this is not legal advice - I'm just a guy on the internet. Please consult an attorney, yadda yadda yadda) Maybe other members have specific instances in mind when they say this is covered under the Standard of Practice.

Concerning constructability, how many of us have used a 40' stick of column that's braced every 12'? Unless the kl/r for that 12' section was around 57 or below, you now have a long spindly member that needs to be erected before the rest of the building framing. My point being, the special handling ended up being req'd despite our best slenderness intentions.

As for the sensitivity to initial out of straightness, if this is gravity only, you should certainly be including your notional loads.

Finally, if this is a single stick, going from kl/r = 200 to kl/r = 210 is only a 5% length increase. I posit that a column with a slenderness ratio of 210 is going to behave similarly to a column with a slenderness ratio of 200. That is, the 200 is a nice round number that was picked for the "this is the line for too slender" mark. It could have just as easily been set at 205, or 210.

 

[KootK]

Concerning constructability, how many of us have used a 40' stick of column that's braced every 12'? Unless the kl/r for that 12' section was around 57 or below, you now have a long spindly member that needs to be erected before the rest of the building framing.

#MeeToo. That is a fine point that I had not previously considered.

 

[driftLimiter]

@winelandv fair points on unbraced 40' sticks during erection and 210 being close enough. And since it's only a recommendation of the code, OP can rest assured on strength checks because they include column slenderness ratio's affect beyond kl/r over 200.

 

[human909]

Honestly, I wouldn't personally worry about it.

That is my general response but I would comment that be wary of serviceability vibrations. When you are dealing with members this slender even incidental lateral loads can induce significant flex.

I got bitten recently by having a horizontal bracing member with that was too slender and had end connections that behaved too much like REAL pins. The members vibrated in moderate wind badly and rectification was needed.

 

[dold]

re: construction

+1

Here's what I would assume to be a code-compliant spindly one for you
(I was acting as chief passerby on this project).

This always makes me remember the OSHA column vs post anchorage thing. Column is anything over 300 lb self weight I think? And requires 4 anchorbolts and must be checked for erection loads. Which is also a 300 lb load 18" off face of column. To my understanding, that load*distance is supposed to be an ironworker scaling the column. But could be a not-far-off representation of a spindly column, sans the ironworker.

 

[WinelandV]

dold,

"Means and methods by the contractor. Contractor to ensure stability during erection." ;-)

The columns in your pic are surely thin. There *appears* to be a bunch of plates welded to them (possibly for siding girts?), and so maybe they're not so bad once the full building is up.