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Large Sign Pole Design
2

Large Sign Pole Design

Large Sign Pole Design

(OP)
I have been asked to design a 65ft tall pole for a sign. See attached dimensions. They want a break at 25ft, and then another pole break at the sign which is supposed to have a HSS Tube behind the sign for attachment. Does anyone have any ideas on how to attach these together besides a plate splice with bolts? Is there a way to do some sort of sleeve at the first break from the ground so it looks a little bit cleaner?

I imagine there will be some sort of frequencies that need to be considered with this as well.

Thanks

RE: Large Sign Pole Design

Keep an eye on fatigue issues.

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.

RE: Large Sign Pole Design

(OP)
Thank you! By the way, what book is that taken out of?

RE: Large Sign Pole Design

What book? Totally made it up myself. Send $15 CAD.

I bought it a decade ago after my first sign and it's been shelfware ever since. Six homes and two countries later, somebody finally asked the right question. I pretty much trampled my wife on the way to the basement to fetch the detail.



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.

RE: Large Sign Pole Design

(OP)
Awesome! Thank you for the reference. I'm going to buy this book, and it will probably sit on my shelf for the next 10 years as well :) BUT, you can never have too many references!

RE: Large Sign Pole Design

Ask your wife about that. The book is actually pretty mediocre beyond that one detail. Caveat emptor.

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.

RE: Large Sign Pole Design

(OP)
Does it at least have a design example for that one PDF? I can recreate it, but sometimes it's nice to see something to reference it against.

RE: Large Sign Pole Design

(OP)
My wife will probably roll her eyes at me and then tell me to go clean the dust off the rest of my books/manuals.

RE: Large Sign Pole Design

Quote (CBSE)

Does it at least have a design example for that one PDF?

Nope.

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.

RE: Large Sign Pole Design

An interesting feature of the detail is that it uses fillet weld pairs in bending to resist gravity loads. We usually try to avoid using welds that way. Small loads I suppose.

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.

RE: Large Sign Pole Design

(OP)
Yeah, I was noticing that and trying to figure out if something was missing like a knife plate thru the pipe section welded to the top ring.

RE: Large Sign Pole Design

(OP)
Or even gusset plates

RE: Large Sign Pole Design

There's an alternate mechanism other than the cantilevering plate. A little weirder to evaluate though.



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.

RE: Large Sign Pole Design

the plug weld on the bottom ring may be tricky and expensive.....one could eliminate the plug welds (but still have a snug fit)and have the top ring take all the axial loads either by increasing it's thickeness or adding gusset plates....

RE: Large Sign Pole Design

on further thought...in resisting the applied moment the lateral load in the bottom ring will have to be resisted by the wall of the pipe.....the plug welds distribute this lateral load more evenly...without the plug welds the bottom ring would just bear on the wall of the pipe so I might add an external ring to the pipe at that location rather than trying to analyze the wall of the pipe for that loading.....

RE: Large Sign Pole Design

(OP)
KootK: you just made me dizzy and almost hypnotized.

Sail3: my first inclination before KootK sent the detail was to let the pipe slide down into the larger pipe 3-4ft, but then realized it probably would be unstable. The welds certainly will be expensive, but so far, I haven't seen anything different or another method. The forces are to great to do a standard plate splice with gussets, so the sleeev solution seems to be it.

RE: Large Sign Pole Design

Yeah, it always seems as though there's just gotta be an easier way.

How many separate pieces do you expect this thing will be in when it ships?

Also, do you know the minimum access hole size that will be required for wiring etc?

When they step ski lift and utility poles, there's sometimes a truly tapered bit at the transition. I could see that being expensive for a shop not set up for that though.

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.

RE: Large Sign Pole Design

Don't forget to check vortex shedding.

BA

RE: Large Sign Pole Design

(OP)
KootK: I'm anticipating (3) sticks.

(1) Large pole for the base...probably a 32 Std pipe
(1) Smaller pipe for the piece between bottom pole and bottom of sign
(1) HSS from the mid pipe for sign attachment that I will do a typical plate splice with on top of the pipe. I think the HSS will provide for more reliable connection of the sign.

Not sure on access hole sizes.

RE: Large Sign Pole Design

Is there a reason that a dodecagonal tapered shaft hasn't been considered? Large A/F at bottom tapering down to a minimal A/F at top. Depending on the thickness required, you would need to check with a typical pole fabricator (like Valmont) to see the minimum A/F required to bend the steel sheet. You can likely just get away with slip joints. Would be more expensive to fab then using standard pipe I guess but would look a lot better than having the sudden reductions in sizes. As KootK said, make sure you double check all your details for fatigue whatever design you end up using...

RE: Large Sign Pole Design

(OP)
damorim: I haven't thought about that type of pole. I suppose it wouldn't hurt to ask the client if it is something they would consider. They are quite common now.

RE: Large Sign Pole Design

Check out Valmont's website. I think it would be a solution that should definitely be considered. Design would be according to AASHTO's LRFD Specs for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. Monotubular structures like these are quite common and the slip joint provides a nice connection.


RE: Large Sign Pole Design

FWIW (which ain't much) I don't like the weld either. you could put a sleeve under the top plate (that rests on the lower tube) and bolt on or weld vertically (welds in shear).

the lateral loads coming into the lower tube at a line of welds ... do you like that ? I understand it is a difficult design to link the two tubes. You could use a bolt on sleeve ... fits inside the lower tube, outside the upper, several cross bolts into each tube ...

another day in paradise, or is paradise one day closer ?

RE: Large Sign Pole Design

Quote (KootK)

An interesting feature of the detail is that it uses fillet weld pairs in bending to resist gravity loads.

Care to explain this comment Koot? I only see the fillet welds resisting shear, but I am sure there is something I am missing.

Thanks!

RE: Large Sign Pole Design

Koot:
Actually, if I’m reading you comment about the pairs of fillet welds taking the bending action correctly (as you intended it), those welds really aren’t that unusual. Take a unit width (unit circumferential length) of the ring pl. at those welds; with a canti. length of [(D1 - D2)/2, +/-], the radial space btwn. the pipes, +/-; the gravity and lateral loads cause bending on the double fillets, plus shear perpendicular to and in the plane of the ring pl. This doesn’t really load or stress the welds appreciably differently than on a “T” joint with a tension/compression load on the “T” web, plus a bending force on the web. And, we see this detail quite often in everyday service. The important thing about the double fillets, and bending, is that they tend to protect the fillet roots, they prevent prying tension across the roots. Then, the resisting moment is the ring pl. thickness, plus, times the fillet weld cap’y. per inch of circumferential length.

RE: Large Sign Pole Design

I think I see it now. Needed to draw the FBD of the plate. Let me know if this is what you were referencing.

RE: Large Sign Pole Design

That's exactly it SNT. Perhaps without the moments at the ends depending on one's perspective.

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.

RE: Large Sign Pole Design

We build a lot of these. Most sign people who fabricate these will be familiar with that telescoped/slotted connection. The slot welds will be expected so don't worry about cost. Although that's the first time I've seen that detail with the slot beveled as shown. I'd recommend against that and just use a square sided slot sized with a min slot height of outer pipe thickness + 5/16" (AISC J2.3b). That bevel would add significant shop time.

I'd also recommend against 32" pipe and use either 30" or 36". Much more common and you can get them in almost any thickness from .312" and above(.375/438/500 etc.) 20" and 24" are also very available for the second stage. Most people in this business will have these pipes in stock.

Personally, I like the 'optional' guide ring shown in that detail and usually use something thicker. I have seen an instance where the field welds were done improperly (horribly) and that guide ring would have saved a failure that couldn't be repaired.

RE: Large Sign Pole Design

Good to have an expert in the room. This may be a silly question but will the pieces delivered to site be assembled on the ground or in the air?

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.

RE: Large Sign Pole Design

It depends. If the site (and budget) can handle a long load of freight we'll fully assemble them in the shop. But if you start crossing more than one or two state lines that gets prohibitive depending on length.

Looking at this one, I'd expect it to be shipped with the top two stages shop welded together. They'll plant the lower column in the ground then drop in the top piece and field weld it at the 25' level. So field welded in the air. It's difficult to manage pipes like this horizontally. We have rollers in the shop to help...but even then it can be a pain. Gravity is a big assistant. That being said, that one field fillet will be an overhead weld so the welders need to be qualified. That's much easier in the shop with the powered rollers in a horizontal position.

Biggest problem we see (fabricators perspective) is when a pipe is assumed to be perfectly round and it's not, the nice circular rings won't fit. And you can't assume a factory end on pipe is always square. We test fit connections to be field welded.

RE: Large Sign Pole Design

(OP)
azcats: that's fantastic information. A couple of questions:

1: is it common to use a HSS at the top for the sign to hook to, or do another pipe and sleeve?
2: for foundations, it's pretty big. Like 5' diameter X 16' deep. Is it more common to go shallower or a balance?

RE: Large Sign Pole Design

1. I see more pipe than HSS for that top piece. The cabinet manufacturers usually want that piece as small as possible because it keeps the size (thickness) of their cabinet down and helps with the internal lighting and minimization of shadows. You could check to see if your client (which might be the cabinet manufacturer) has a preference. Looks like 12" nominal (12.75" OD) STD Pipe would be a good size for that cabinet.

2. That's not terribly deep from my perspective. Obviously, I don't know your site conditions. Personally, when possible I usually opt for smaller diameter and a bit more depth. For a 30" pipe I like a 4' diameter pier which would add a couple feet to your depth. Water table or other considerations also come into play there. I also try to avoid reinforcing the annular space and consider the concrete 'soil improvement.'

RE: Large Sign Pole Design

(OP)
azcats: That's really good info. I have already submitted "Bidding Only" documents and left the HSS at the top. I will look at changing it to a pipe though. You certainly have been helpful in this. Thank you, much appreciated!

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