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Creating a moment connection for timber post base?

Creating a moment connection for timber post base?

(OP)
Working on a free standing trellis design where the client is wanting no visual connections of the framing over the top of the trellis posts. No knee-bracing either, so no clear way to create a moment connection at top of post to create frame action. Anyone looked into creating a moment connection at the base of a timber post or know of a good document to use for guidance?

RE: Creating a moment connection for timber post base?

That is a tough task. remember the fixity of a base connection is also related to the foundation/soil interaction. It may force you to have a large footing to minimize rotations.

I dont know of a reference, and maybe you could have it embedded in concret, but I don't really like that idea.

RE: Creating a moment connection for timber post base?

(OP)
They are not afraid of the footing size which is good. We discussed embedding into the concrete, and I though we were going to proceed this way, but they are rightfully concerned that they can't easily replace posts in that scenario. This is going into a city park, so longevity/maintenance is an issue. The loads are minimal, but I am more concerned about shrinkage around bolts and slop in a connection over time...

RE: Creating a moment connection for timber post base?

Sounds like a square HSS section would make more sense.

RE: Creating a moment connection for timber post base?

(OP)
I am dealing with a landscape architect, steel is not an option (at this point).

RE: Creating a moment connection for timber post base?

To get base fixity for a trellis, just use a PT 4X4 post embedded in a concrete footing embedded a minimum of two feet into the ground.

Mike McCann
MMC Engineering

RE: Creating a moment connection for timber post base?

(OP)
That was the plan, they said that is not acceptable on a maintenance level due to moisture/rot concerns.

RE: Creating a moment connection for timber post base?

Ask them to suspend gravity, wind, EQ’s and moisture at the next city council meeting. And, to get used to seeing connections, most good timber structures have them. Wood also weathers, and deteriorates over time, unless gov’mt. can suspend that too. Problem solved.

You could cast the verts. in concrete, with a bark finish and color, and a few strategically placed limbs (knee braces?) as part of your connection details. Do em in fiberglas.

RE: Creating a moment connection for timber post base?

Use ASABE EP486 for the required embeddment depth with the post restrained at the surface by the concrete slab. Specify the posts be treated with copper naphthenate to 0.10 pcf or more. The posts will be there long after the maintenance crew retires. I have also wondered if you could use a product such as Adeka P-201 at the interface between the post and the concrete.

RE: Creating a moment connection for timber post base?

If the landscape architect was competent, he could make steel look good.

RE: Creating a moment connection for timber post base?

kbsooner,
I have done a few of these. We have used an HSS base and base plate with anchor bolts that the wood column can insert into after the steel is installed. Usually you can get a pretty decent fit. We designed through bolts in a force couple for the lateral load in each direction. It has worked pretty well. Especially if they are doing stone or some other finish around the base that will cover up the steel boot.

RE: Creating a moment connection for timber post base?

kbsooner:

That is pure bunk with PT members properly set in a dry concrete mix. Mine have been in the ground for over 30 years and I am in the wet Pacific Northwest. You will see rust on steel sooner than that!

Mike McCann
MMC Engineering

RE: Creating a moment connection for timber post base?

You need to specify the Preservative treatment and embed those babies. Else you need dhengr's solution or find a different material.

______________
MAP

RE: Creating a moment connection for timber post base?

As a corollary here, I had placed small PT 2X6 blocks over a SOG as leveling shims under my swim spa about 15 years ago. After moving the spa, I inspected the shims and found most sill in very good shape. For a few though, the interior had begun to deteriorate at the untreated ends. The outside was still intact and functional though.

Personally, for the posts I put 6" of gravel in the bottom of the hole for drainage and backfill with a dry concrete mix. None of my posts have shown any rot yet over the 30+ years of use.

Mike McCann
MMC Engineering

RE: Creating a moment connection for timber post base?

If the landscape architect is partial to seeing steel, you can use a pre-drilled knife plate welded to a base plate which is then bolted to a concrete pier/footing. Hot dip the entire assembly before installing. Slot the bottom of the wood column to fit over the knife plate and use galvanized thru bolts. Be sure to apply preservative to the cut areas. Countersink the bolt and nuts so they don't become a hazard to children and have sharp edges ground. This will also provide your uplift resistance. A wood trellis isn't normally designed as a permanent structure so it is generally understood there will eventually be maintenance or replacement.

RE: Creating a moment connection for timber post base?

Rayengr took the words right out of my mouth.

We've used that exact detail but typically only when dealing with glulam column and beam frames as you generally want a larger number of bolts in a square pattern to ensure fixity and here in Manitoba we can't realistically get PT lumber in anything larger than a 6x6.

RE: Creating a moment connection for timber post base?

I fixed a neighbor's trellis by putting Simpson tie straps on the face of the posts and pouring concrete around the lower, hooked ends. The straps were hidden with a cedar 1x6 plank over them.

Another time I used an embedded steel WT section with a slot cut into the wood post and thru bolted. The WT gives good bending properties in the 2 directions.

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