Slenderness Ratio 3

[aswierski]

Is there a preferable range for slenderness ratio? I.E. is it preferable to have my slenderness ratio fall into a specific range like, say, 90-150? I have a column that checks out ok, but the slenderness ratio is at 190. My tendency would be to simply bump up a size (increase the radius of gyration). What do you guys think?

 

[slickdeals]

I think there used to a limiting slenderness ratio of 200 for compression members. I think this was based on construction economy, ease of handling etc. It is no longer a limit in the AISC code, but rather a recommendation to keep Kl/r < 200.

 

[aswierski]

I've attached my calc for this simple design. It is a simple pipe support inside a building, so there won't be any wind load. I didn't attach the base plate or sonotube calcs as they don't really pertain to my question. Let me know if I'm doing anything wrong.

 

[aswierski]

slickdeals,

I know that AISC recommends not exceeding 200, but my question is what if you're approaching it? If you see my attachment in the previous post, you will see that my slenderness ratio is 188. Is it good practice to bump up my column size to lower the slenderness ratio? Thanks in advance.

Adam

 

[rb1957]

this thing's going to be pretty wobbly (side-to-side), no ? (with only a cantilevered base) ... if somebody pushed on it at the top, say 50 lbf *17' = 850 ft.lbs ... i'd be looking to brace the support somehow rather than making it a slightly bigger section.

if your limit slenderness is 200, but you in practice avoid it (by designing to say 160), then your limit is 160, no?

 

[aswierski]

RB1957,

Actually the base connection will be pinned (base plate). I have it shown incorrectly on my load diagram... Bracing is out of the question as the client wants as much open space as possible, in addition to it blocking key access ways.

I'm a little lost as to what you mean by the last line, in which you wrote, "if your limit slenderness is 200, but you in practice avoid it (by designing to say 160), then your limit is 160, no?"

 

[rb1957]

how about extending it up to the roof ?

how about a lateral brace up at the pipe level ?

what's going to keep the top of the support directly above the base ?

how are you attaching the pole to the base ? clevis ? edge weld ? there'll be some moment stiffness there, just not much. as your calc show, the load is off-set from the pole's axis, so statically you need the moment reaction. how is the off-set accomodated in your column calcs ? (Fa is pretty darned small, 4ksi)

you're supporting insulated pipes .. these'll impart no (ie zero) lateral load on the pole ?

but even if the calcs based on intended loads and geometry show it good, what unintended loads could produce an unintended geometry that isn't good ?

 

[aswierski]

RB1957,

These supports are inside a compressor building within a large Hydrogen Plant. The client does not want any bracing of these supports. With that said, the piping itself will add some rigidity, but the pipe stress engineers tell me there is no side load. I cannot go to the roof as it is another 30 feet or so (doesn't make much sense to extend the column an additional 30'). Client said no to lateral brace at piping elevation. The column will be welded to a base plate, with the plate having 4 cast-in-place chemical adhesive anchors (1" grout underneath the plate). I designed the base plate to account for the moment reaction. The only unintended loads I can think of is if someone runs into one of these supports with a forklift.

 

[rb1957]

hey, you're the guy signing for it ... to me it looks pretty wobbly ...

 

[rb1957]

just noticed your moent sum is way off (too high, by miles) ...
P1*17' ? should be P1*0.75'
same for others

what about weight ? (17'*7.6in2*0.3 = 465lbs

 

[Lion06]

You can't design a frame with pinned bases and pinned beam connections. It's inherently unstable. Whether the pipe engineer says there is a lateral load or not, there will be some lateral load (even is it only nominal), be it from being built out-of-plumb, someone bumping into it, etc. You can't completely neglect a lateral force.

 

[aswierski]

rb1957,

Wow, I don't know why I did that (or how), but thanks for the catch. And yes, I accounted for the column weight (27.41# x 17') in the Pmax under B.3 Column Design.

EIT,

The base is pinned, beams will be fixed. So if no lateral load is specified from the piping group, what should I be using as my side load?

I'm in a difficult spot being the only civil/structural guy in my whole office. We have a satellite office that houses all of the civil/structural engineers (other than myself) in our company, but it's difficult getting a hold of them. Usually I would ask my dad (a civil P.E.) but he is out of town. And StructuralEIT, I noticed that you post quite a bit (and good tips at that). You seem more like a PE than an EIT...

 

[kslee1000]

"Actually the base connection will be pinned (base plate)"

Yet you have used k = 2.1 (fixed) in the cal.
I think you need to address column base reinforcing rather that worry about slenderness ratio (188 is ok in this case, since fa/Fa is very small, and the interaction is far less than 1.0).

However, the moment capacity provided alone can not validates the fixed column base assumption. The column base will rotate due to elogation and shortening of the anchor bolts. You need to limit such displacement to a minimum by using larger bolts, more bolts (2 at a corner), and thicker base plate (avoid distorsion).

Or you might need do nothing, if the stresses (in bolts) are small, and amount of displacement is neglegible. But you need to check that out.

 

[Lion06]

Thanks for the compliment. I only have 3 years of experience right now, so I'll be an EIT for the next year and a half.
I tend to agree with kslee on this. Because the axial stress is so low, you needn't be so concerned with kl/r. With your fa/Fa ratio, this isn't acting as a true compression member (at least not to the point where kl/r limitations would be a concern). I would recalc kl/r, though. The 2.1 you use is for a cantilevered column, that is not your case. You are more like case (f) in the steel manual (page 16.1-240), but that assumes a rigid connection at the top. Your connecting pipe is not infintely rigid and will allow rotation of the column top. Use the equations for a sway frame on pag 16.1-240 in the 13th ed. steel manual along with the nomograph for a sway frame to calc your k. I don't think it will make a difference in your result, but it's nice to have the calcs be right for when someone checks them.

 

[aswierski]

Thanks a bunch, gang. You all get a star for the day... The problem I'm running into is that the pipers (and client for that matter) sort of see this work as "secondary" and thus not critical. I couldn't disagree more, but you know how them pipers are... Thanks again for all of the valuable input, much appreciated!

 

[kslee1000]

One reminder:

Unless it is electrical conduit (which obviously not), you will need to check alternate arrangement of some pipes filled with water (will occur during pipe tightness testing), while other is empty.

 

[JoshPlumSE]

My only contribution above what other people are saying is that you need to make sure that you account for 2nd order effects for this frame.

Anytime your slenderness ratio gets higher, the effects of P-Delta on your frame will increase substantially.

If you don't have software to do this for you automatically for you, then you can get a decent approximate answer using the B1-B2 method from AISC.

Josh

 

[kslee1000]

EIT? It is really a nick name for all practicing engineers (PE,SE ,or not). Since we are constantly trained, and re-trained. When the fun (training) stops, then you may drop it.