Design of bolted joint for wood 2

[engtiuser2]

Wood shrinks and expands. When wood (e.g, spruce or fur) are used underground (lawn where it can be wet and dry), how to design the bolted joint to allow shrinkage and swelling of the wood. with the given clamp load of the bolt, my concern is whether the wood will be cracked if it swell too much (that implied lot of compressive load at the bolt joint. The design involves embedding a stud inside the concrete and drill a hole in the lumber for the stud to pass through, then clamp the wood firming with the nut and washer.

 

[jayrod12]

I don't think I've ever seen a piece of wood damaged at a bolt from swelling. The pressures, and magnitude of the swelling are not that great. Further, typically treated lumber (as you would use for situations like this) are generally still fairly damp when purchased. If anything the bigger concern would be shrinkage and will loose bolt holes have an effect on the design.

 

[hokie66]

Agree with jayrod that the wood will shrink, and the bolts will loosen as a result. Underground, not much way you can then tighten them...but maybe the nut and washer are exposed? Now, when the steel corrodes, that is when the wood will crack, as corrosion means massive expansion of the steel.

 

[engtiuser2]

I can put a belleville washer under the nut to take up the space and the bolt will not be loosen. Is there any simple way to determine the maximum and minimum dimension of the 6x6 wood (spur or spruce)? That can help me to size the belleville washer.

 

[SlideRuleEra]

Is there any simple way to determine the maximum and minimum dimension of the 6x6 wood (fir or spruce)?

Yes:

You will need to know:

1. Which wood will be used. Spruce ≠ Fir... and there are several species of each.

2. Initial dimensions of the 6 x 6 and moisture content while at those dimensions.

3. Maximum moisture content of the 6 x 6.

4. Minimum moisture content of the 6 x 6.

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[engtiuser2]

The maximum content is easier, it can be soaking wet as it is underground by about 1 ft deep. The minimum moisture content is harder to get, in a very dry month, may be 5 percent. The wood can either be spruce or fur, no choice really. The wood is treated and from lumber yard, don't really know how dry they are.

 

[jayrod12]

If it is underground, you will never get below 15 percent. Air dry is 15 percent, you need to provide supplementary heat and lack of humidity to get below that. The interior of most houses have their wood moisture content running between 10-15%.

Honestly, I feel you are making a mountain out of a mole hill. What is the application? Do you really need to provide belleville washers? Can the structure tolerate slightly loose bolt holes?

 

[engtiuser2]

I like to keep the joint somehow tight. I like to know the ball park for the maximum shrinkage and swelling possible for 6"x 6"x 6 ft long wood.

 

[jayrod12]

The shrinkage in length is negligible. Max expected shrinkage in the 6 dimension (I assume that's a 6x6 nominal so 5 1/2") would be maximum 0.22" total. And that assumes a worst case of 30% moisture content at time of install (highly unlikely) and an a lowest moisture content of 15% (also unlikely in this case being below ground). Like I said, mountain out of a molehill. I would expect the actual installation moisture content to be between 15-20% for treated material (as when I buy it, it always seems rather wet) and an average operating moisture content similarly between 15-20%, therefore the expected shrinkage would be near zero.

 

[SlideRuleEra]

OP - Pay attention to jayrod12's comments. Shrinkage / swelling is at the very bottom of the priority list for wood used underground.

The most important is the wood preservative used and it's level of treatment. A typical lumber yard will NOT have lumber treated for in-ground use.

A close second is the hardware; stainless steel (best) or hot dip galvanized (bare minimum).

I am in a wet, highly corrosive area. Plenty of wood, concrete, and steel structures have literally fallen apart prematurely, because no attention was paid to the "important" details. I have worked on the recovery for quite a number of these.

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[jayrod12]

SRE has the right of it. Worry about the more crucial items, like level of treatment and fasteners.

 

[engtiuser2]

Thanks you all for the advice. The information is very helpful.

 

[Ron]

Agree that the focus should be more on protection of the wood and its metal connections from rot and corrosion.

For a good treatise on moisture control and dimensional changes in wood, read Chapter 12 of the Wood Handbook produced by the Forest Products Laboratory.

 

[Tmoose]

I'm unclear if the bolted joint is actually underground or embedded in concrete.
And, if either of those is true, why the design might require that.

Based on a few replies by others, my confusion is not unique.
The solution, as always, is pictures and drawings, as might be provided to the folks actually building this.

http://www.awc.org/pdf/codes-standards/publications/pwf/AWC-PWF2015-Commentary-ViewOnly-1411.pdf

 

[engtiuser2]

The 6x6 is rested on top of a concrete where a threaded rod will be embedded within. A hole will be drilled through the wood for the threaded rod to pass through. The whole thing (wood, concrete and threaded rod) are located underground.