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Wood light pole base

Wood light pole base

Wood light pole base

(OP)
Hello,

Has anyone designed a light pole base for a wood poll? I have a 14' wood post (8x8) with a single fixture a the top. The owner wants a concrete footing (drilled pier or square, I don't think that matters to them) and a fabricated steel base connection, that is connected to the wood post with bolts. Anyone had success with this type of connection. Critical areas to consider in the connection design? Thank you and have a great day.

RE: Wood light pole base

More smaller fasteners will perform better long run. The wood will tend to shrink away from both the steel saddle, and the fasteners, and will then be a little wobbly. There's not a good way around it, the demand on the connection can't be enormous, does something like the simpson strong-tie MPBZ work for you?

RE: Wood light pole base

(OP)
The owner wants a fabricated connection to make the base look more attractive.

RE: Wood light pole base

I would model it after their intent. They've got testing backing up what they've done. There's a reason that for all of their other post bases they have warnings, not designed for cantilever poles.

Like I said, it will be loose if they only want to use a couple of bolts. Yes, it won't fall over, but it will be wobbly after a few years as the wood shrinks.

RE: Wood light pole base

Sounds like a round square HSS sleeve that the wood pole will fit inside of welded to a base plate may work. I would have a hole (large enough it won't easily get plugged) in the base plate to allow water to drain. I would also have a few through bolts to prevent the pole from lifting out of the sleeve.

RE: Wood light pole base

Given that the post is expected to shrink, would a base with separate plates up each side of the post be better than a box section, so that the fasteners at the top of the base could be retightened later, after the timber shrinks? Possibly with a pin (or pipe) at the bottom matching a hole in the end of the timber to keep the bottom centered?

What I've seen most often done in those situations is just embedding the post directly in the concrete. Keeping a seal around the base can be a challenge, though.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Wood light pole base

1) I would normally do the embedded solution that BridgeSmith described.

2) This may well be one of those cases where advising your client well will mean telling them that they don't really want what they think they want.

3) The scale of a light pole for a connection like this unnerves me a bit. I've taken liberties with these connections for stupid stuff like small signs and garbage enclosures. A 30' tall light pole that might ice up at etc, and possibly fall on cars, is another kettle of fish in my opinion. How tall will the poles be? Will there be a metal mast arm out to the lamp?

4) I'm okay with a vertical fin plate slotted into the pole and bolted. However, that means that you're relying on the fin plate in weak axis bending in one direction which will seriously limit capacity.

5) Like BridgeSmith's four plate suggestions, I've seen details where this was done with a pair of vertical angles wrapping the corners of the pole and bolted in as a means to provide a way to tighten up shrinkage related slop. Frankly, I think this is pretty ugly though.

6) I might be willing to bend a bit if the client could point to similar detailing, already in place, that had a satisfactory performance history.

RE: Wood light pole base

Any chance you can just run a steel pole up the middle and make the wood and base connection all architectural?

RE: Wood light pole base

Many moons ago I was involved in one of these, we actually had a row of them. We made "H" sections where the center was not welded to the sides, there was actually a small gap. So effectively we had fin plate up the center of the post, and side plates to brace in the other direction. We bolted all of it with carriage bolts and then trimmed the excess bolt off and put domed metal caps over the bolts. It looked like it was riveted in place but it could still be snugged up by removing the caps. We went back at the end of the next summer and re-tightened it. After they were fabed they were galvanized. And the concrete piers came 6-9" out of the ground to try and keep things dry. I don't recall how they were anchored to the pier. Last I saw they were still there after about 35 years.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

RE: Wood light pole base

Quote (KootK)

A 30' tall light pole that might ice up at etc, and possibly fall on cars, is another kettle of fish in my opinion. How tall will the poles be?

The OP stated that it was only 14' tall.


Quote (KootK)

I'm okay with a vertical fin plate slotted into the pole and bolted. However, that means that you're relying on the fin plate in weak axis bending in one direction which will seriously limit capacity.

Why not take this a step further and have a vertical "Plus" Shape to avoid the weak axis bending.

RE: Wood light pole base

Quote (dauwerda)

Why not take this a step further and have a vertical "Plus" Shape to avoid the weak axis bending.

Love it. But then the question become what is that "plus" shape, how does it look, and how does it allow the connection to be drawn in for shrinkage. Obviously, Ed's got on answer to that. What have you got in mind?

RE: Wood light pole base

No doubt I am completely fooling myself in thinking that anyone might find this sufficiently attractive.

RE: Wood light pole base

The problem with tightening bolts in the cruciform configuration would be similar to that of the I that Ed suggested when you look at the bolts going through the "web" of the I. If we assume 6% average shrinkage perpendicular to the grain (radial and tangential) over a 30% moisture swing, and further assume that we're going from 19% as delivered lumber to a 12% dry equilibrium MC in dry months, then a 3.5" block would shrink about 0.05" (figure a 1/4" steel plate and you have 4 3.5" blocks with .25" gap). So we end up with a total of about 0.1" of slop in the joint. Half of that is taken up by the bolts being snugged up to the surface of the wood, the other 0.05" by bending the wood to contact the steel plate. Might be a problem if your connections only an inch long, but if you run your cruciform up, say 8" into the wood, then you're looking at rotation in the wood of about 0.36o. Probably not enough to be worried about.

RE: Wood light pole base

Oh, and don't forget stand-off tabs to keep the wood up off of the concrete/steel so it isn't sitting in or wicking up water.

RE: Wood light pole base

Quote (KootK)

No doubt I am completely fooling myself in thinking that anyone might find this sufficiently attractive.

I don't know - use it the parking lot of a structural engineering office and I'll bet everyone would be pretty pleased.

RE: Wood light pole base

Ah... plus = cruciform. Now I get it. I don't love it though. I think that the post would be too chewed up at the bottom by the time that you got all that slotted in.

RE: Wood light pole base

Not if it's done right. Assuming 1/4" plate works, you essentially have four 4x4's within the connection.

You could also go so far as to reinforce the connection region with a) a thin steel plate or sheet screwed into the post or b)2x12's mitered around the corners and screwed into the post.

RE: Wood light pole base

Can you put the post into an HSS sleeve that is welded to the base plate? KottK's fins look nice but I forsee someone walking into them or tripping over them.

RE: Wood light pole base

Quote (phamENG)

Not if it's done right. Assuming 1/4" plate works, you essentially have four 4x4's within the connection.

I don't buy it. By the time that you get two columns of bolts flying through that, I think that it'll be a dog's breakfast of beat up timber. And each one of those 4x4's needs to be an upside down cantilever of sorts.

RE: Wood light pole base

Anybody think this might work - core and insert a fully grouted pipe into the pole, and embed in the concrete pier.

RE: Wood light pole base

It probably would, but finding a hole saw that deep would be hard enough - making it go in straight would be even harder.

RE: Wood light pole base

Other than difficult to core, any shrinkage concern between the pipe and wood, and the base connection?

RE: Wood light pole base

I have a blast from the past. JAE did some wood moment bases several years ago. I saved the picture he posted, but I do not remember the name of the thread.

He had multiple posts, so was able to create a system of moment resisting columns. Unfortunately, you just have 1 post.

There might be other good info in the thread though. Or JAE might see this post and remember.

RE: Wood light pole base

It's too bad that one can't count on a slotted hole to not bind up. Otherwise, you might slot the anchor bolt holes and provide connection angles that move with the wood shrinkage.

I can never seem to resist trying for a better mousetrap with this connection typology but I really should know better by now. We've probably been through this here half a dozen times since I've been paying attention and we never seem to arrive at a fully awesome connection other than the Simpson gadget.

Super fancy eruo-timber connections aside, I've come to the conclusion that wood just isn't meant to base cantilever unless it's via a root network.

RE: Wood light pole base

If you did decide to just embed the pole in a concrete foundation, any chance you could spec out Creosote treating or even CAC treating for the wood? Not sure if this is residential or commercial.

RE: Wood light pole base

Quote (retired13)

Other than difficult to core, any shrinkage concern between the pipe and wood, and the base connection?

I would think so. The wood is still going to shrink. I guess it comes down to how the wood shrinks. When looking at a cross section, will it always shrink toward the centroid (meaning the exterior diameter will get smaller and the interior diameter will either stay the same or get smaller) in which case you're either ok or the steel post will split the wood as it shrinks onto it or it will shrink toward a center line of the area drawn between midpoints of a series of lines drawn between faces of the member (so the OD will get smaller and the ID will get bigger) in which case you loose your "grip" on the pipe. I'm leaning toward the latter.

RE: Wood light pole base

What about tapering the wood and steel socket like the way a shovel is connected to it handle. It can be a square taper. Tapers are very good at producing tight joints.

RE: Wood light pole base

Yeah, I would agree the latter be the concern, just not sure, so asked.

RE: Wood light pole base

Taper idea is slick. Can leave open at bottom for drainage. Self correcting for shrinkage. Totally flexible on aesthetics.

RE: Wood light pole base

Quote (bridgebuster)

...fins look nice but I forsee someone walking into them or tripping over them.

That's definitely something to consider, depending on the anticipated proximity of foot traffic to the post.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Wood light pole base

Last time I looked at the Simpson moment base they only went up to 6x6 - looks like they added the 8x8 this past summer.

I'd say your best bet is to use the Simpson base and fake the appearance. Just weld 4 steel plates together in a box that'll wrap around the base and have them stand a few inches taller than the Simpson post base so you can put a bolt through to hold it in place. If that's too expensive, use wood trim to hide it (the wood trim can be replaced on the cheap every few years as it gets beat up).

RE: Wood light pole base

I probably would just water/moisture proof the bottom of the pole and insert into the concrete pier. Then provide nice looking removable fittings and sealant to cover the pier-pole interface.

RE: Wood light pole base

I like the pipe embedded in concrete and up into the post. If the pipe is relatively small (say in the 2" range) there should be plenty enough 'meat' to the surrounding timber to keep it from splitting. If the depth of the hole is kept slightly shallower than projection of the pipe, the bottom of the post will be above the foundation, preventing any issues with the post staying wet.

A similar option would employ 4 rods, rather than a single pipe, in a similar way.

Check out this presentation from the American Wood Council, particularly slide 124, may be of interest.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Wood light pole base

BridgeSmith's slide 124.

RE: Wood light pole base

Braves25:
You could dap a couple shear pls. into each face of the 8x8 post, about 18”-24” apart in elevation at the base, with through bolts, depending upon the moments you are dealing with. Then use 4 matching 5.5” wide side pls. welded to a stiff base pl. with A.B’s. into a conc. pier. Drilling a center hole in a long wooden post would seem to be kinda tough to do with any accuracy. Laminating two 4x8’s would allow you to route a center hole for a pipe, and probably laminating the pipe right in place during the post build-up. There are a few wood lam. outfits that laminate posts out of 2x6’s and 2x8’s for post frame building. They can also treat the bottom 6-8’ of the posts for direct embedment.

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