Kl/r preferably <200 2

[smokiibear]

I provide engineering for light guage steel covers with Solar atop. They are light gauge purlins, light gauged metal roof. When columns reach over 14.75', I've typically followed the recommendation to not exceed Kl/r < 200, and consequently, forced to go to 6" std pipe. I'm curious if you could comment. I don't think it's warranted on these tiny structures, but don't want to make a poor choice. Could you comment.

Here is a sample of the product.

 

[jayrod12]

6" feels right for a 15 foot tall cantilever column structure.

 

[dik]

Rather than STN pipe, can you go to a thinner wall HSS section?

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

-Dik

 

[MIStructE_IRE]

It looks right to me. I wouldn’t go much smaller.

We used to have a Kl/r of 180 minimum to ensure robust structures. Eurocode has since gotten rid of it, but i still use it as a good rule.

 

[dik]

I was thinking of HSS because of the much greater strength, less material, and the r value decreases with thickness... albeit maybe more costly.

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

-Dik

 

[SlideRuleEra]

...light guage steel covers with Solar atop.
...forced to go to 6" std pipe. I'm curious if you could comment.
I don't think it's warranted on these tiny structures, but don't want to make a poor choice.

How does the value of the solar installation (which could be made worthless if the structure fails) compare with the differential cost of going with larger pipe columns?

http://www.SlideRuleEra.net

 

[JoshPlumSE]

I like SlideRule's line of thinking. So many times we think we're doing the client a favor by trimming down every last penny on structural materials when the real cost of the project is in what we're supporting / protecting. And, the labor costs.

Those columns look okay to me, but only because they support so little weight. Also, if this were a car port, I'd want them to be bigger or at least protected at the base where a car could hit them.

 

[JLNJ]

I'd be less worried about kl/r and more worried about the amount of wind grabbed by this sail.

These are the first things to fail during a wind event. (Well, second, behind wood power poles.)

 

[glass99]

If this is a 6" tube with a 15ft cantilever, the kl/r is closer to 125 isn't it?

We have exceeded 200 a few times. If there is no snow load on the roof, why not? As JLNJ says, it's probably limited by lateral load any way.

trimming down every last penny on structural materials when the real cost of the project is in what we're supporting / protecting. And, the labor costs.

True but for a solar panel installation, carbon footprint might matter as much as construction cost.

 

[Shanman_]

I worked at a firm that did a substantial amount of solar. Much like the two-post ground mount structures you pictured but also single post carports. These solar structures are effectively wind sails as someone pointed out and can see heavy compression loads due to wind. The single posts suffer from the same but also see a moment applied at the end of the cantilevered column - leaving the slenderness ratio even more of a concern for those. We always strayed away from breaking the KL/r < 200 rule. If you are the stamping engineer, consider the risk to life safety and the net gain (how much steel do you propose to save?) Most will probably find it's not worth the liability.

 

[Teguci]

What happens if your slender columns are installed significantly out of plumb or the foundations apply some moment to the columns when they settle a little bit? I believe that some of the rationale for limiting KL/r to less than 200 is to allow for normal construction tolerances that aren't necessarily spelled out in the calculations. Not your problem here, but I've also heard the horror story about a construction worker cutting out a round "rain gutter" pipe (next to the actual larger rain gutter pipe) that kept on binding on his saw.

 

[XR250]

What value are you using for K?

 

[Colostruct]

Isn't Column KL/r only part of the issue here? Those columns will be in both axial loading AND flexural loading due to wind loading. I would advocate for performing a unity check on those columns. I would run a number of iterations of the design using different slenderness ratios just to see what difference it makes.

For the maximum slenderness ratio, I am old school and use a maximum of 180.

As a general question, are you providing weep holes in those vertical columns? We've started doing that with HSS and pipe sections here due to frost expansion on directly embedded columns.

 

[1503-44]

I'd tend to use ONE 16"? pipe column at the center. Lot less holes to dig, less number of rust prone points, cleaner look and feel, easier to mow, easier to orient and easier mount trackers.

 

[smokiibear]

Thanks, all. I'm using K=2.1

Wind...sure, on paper. But after 1000's of these structural around california, not a single one has had wind issues. The issue has never been about spending more money on materials, but the extra weight of a 6 is much harder for the installers to handle. I think this relates how those solar contractors are set up.

 

[dik]

another reason for using a thinner wall HSS.

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

-Dik

 

[Shanman_]

Be careful with that logic.. when we justify leaner seismic/wind designs simply because "they haven't had any issues yet" ..

The same was said for solar canopies I've seen designed which utilize aluminum clamps as opposed to bolts to secure the panels. The clamps calc'd, sure, but their deflection under even 50mph gusts allowed for the solar panels to dislodge and go flying like a kite down the street.

 

[smokiibear]

Here's another issue, why we go to the std pipe. The galvanization matches the rest of the structure. When going the HSS, we have to get HDG, which looks like junk.

 

[phamENG]

The traditional upper limit of 200 was based on professional judgment and practical construction economics, ease of handling, and care required to minimize inadvertent damage during fabrication, transport and erection. These criteria are still valid and it is not recommended to exceed this limit for compression members except for cases where special care is exercised by the fabricator and erector.

but the extra weight of a 6 is much harder for the installers to handle. I think this relates how those solar contractors are set up.

Sounds to me like the folks building these are not going to be able to take the necessary care required to avoid damaging a smaller, thinner, less robust column since, by the sound of it, they're just showing up in a pickup truck and tossing them in a hole. Anything bigger than a pair of laborers manhandling a 16' pipe should be able to put that up without noticing it. Stick to 200 as a hard limit.

 

[KootK]

1) Given that these columns will likely arrive in a pickup or small flatbed, and they'll never be handled by crane, I feel that the kL/r < 200 requirement is superfluous when it comes to the handling and erection argument. The small scale of the erection operation strikes me as just the antidote for such worries.

2) Given my expectations for the loads on these posts, I feel that they will be almost more like beams than columns. As such, if designed properly for lateral, I also struggle to support KL/r < 200 on any kind of design / accidental eccentricity basis. The dominant load on these posts may well be applied moment.

I vote for disregarding the compression slenderness in this instance.