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#32837 - 03/07/15 05:10 AM Expansion or contraction in large structure design
Drwood Offline
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Registered: 03/12/14
Posts: 6
How does one design around the expected shrinkage with drying green or semi green timbers in a structure that is about 275 feet long? The structure in question is a set of opposing stalls 12x12' with a 12' isle between with hay loft above that serves secondarily as a buttress system on a large 100x275' indoor arena. The arena cover is engineered metal truss with fabric cover. There is a central bump-out aspect that will be about 35' wide. That breaks things up a little...but Yes, a huge project! Verticals are 10x10" with fully housed 8x8" and 8x10"members and 4x6" braces. Roof pitch is 8/12.

So, if there are 2 "stall sides" with 10 sets of the 12x12 stalls each making 120', each side will have 12 bents, and if each of those bents' 10x10" vert beams shrinks 1/4-1/2" then the structure will either bust pegs (I'm guessing) or it will contract by several inches, which concerns me as the roof of the timberframe stalls/hayloft is to be contiguous solar panels. Just to be sure, I know the length of each member is stable...just not the 10" sides.

I considered designing the central bump-out as a separate structure, but that gets complicated and the structure looses some stability as a whole. Buying standing dead timber or dried timber comes to mind, but that is cost prohibitive. Some woods shrink a little less than others such as eastern white pine, so that has been another possible solution. I could also potentially purchase the entirety of 10x10 beams, or all the material, now, over dimensioned at 10.5x10.5", for example, and let them dry in open air (covered) with ends sealed for 2 years before planing to dimension and doing layout and cutting my mortises, etc.

I have done all the reading I can on this, but I am hoping for some experiential anecdotes or steering me towards resources that I could further read and digest to answer this question and allow me to move forward with the design.

Thanks!

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#32839 - 03/07/15 03:20 PM Re: Expansion or contraction in large structure design [Re: Drwood]
Jim Rogers Offline

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Registered: 03/14/02
Posts: 1661
Loc: Georgetown, MA, USA
I believe that eastern white pine only shrinks 1/8" in 12". So I don't think that a 10 x 10 is going to shrink 1/2" especially in an un-heated environment of a horse barn.

Also, as wood only drys, on average of 1" per year letting a 10x10 sit for 2 years isn't going to do much.

Pegs are going to bend a lot before they break.

The engineering council has been doing tests of joints at each conference. At one conference I brought two "tie beam to post" joint connection. Both were made from one timber. All the same age and moisture content. I made all four pegs from one 1x1x4' oak stick. All the same age and moisture content. The only difference in the two joints was where the pegs were placed in the post. One set was traditional like the old fashion way of doing it. One was the modern way an engineer told me to place the pegs. I felt that the modern way was too close to the end of the tenon.

Both joints took over 6,000 lbs of force to get them to fail. Both failed within 200 lbs of each other. Basically almost the same.

Both joints failed by the pegs bending over from straight to a "U" shape. And there were two pegs in each joint.

I was expecting the "too close to the end of the tenon" peg holes to blow out the relish on the end of the tenon. Which neither did blow out.

My advice to you is to have each joint you question properly engineered by an experienced timber frame engineer for the size and location of the pegs.

In order for one joint to fail, others in the line will have to fail as well, at the same time. Unless the structure is exposed to external wind forces or seismic conditions, like earth quakes, the chances of more that one joint failing at the same time in the same way that would cause a catastrophic failure is very small. In my opinion.

Shrinkage failure can be preplanned by making the housing deeper so that there is more "shelf" for a beam is sit on should the post shrink some.

Draw boring the peg holes, which will cause the peg to act like a spring and which could help pull the joint together is a fairly common method to keep joints tight.

Just my ideas on the subject.

Jim Rogers
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Whatever you do, have fun doing it!

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#32840 - 03/07/15 09:24 PM Re: Expansion or contraction in large structure design [Re: Drwood]
Roger Nair Offline
Member

Registered: 10/20/99
Posts: 463
Loc: Bakerton, WV, USA
Drwood, I hope you take your project to a PE for consult. I am sure that you have over estimated the shrinkage accumulation through the succession of posts. Normally, the length of the wall is regulated by the scarfed plates and sills, again shrinkage in the posts does not accumulate into the plates. In the case of wall girts tenoned and pegged to the posts, the relevant shrinkage is between the pegs and not across the entire face of the post, also checking may relieve some accumulation. To further relieve shrinkage accumulation, free tenons can be used instead of common pegged tenons in the wall girt connections. That's a carpenters take, an engineer will have a different point if view. Other means will be needed in such a long building.

The solar panel array will of course need a thermal expansion design that is another design specialty.

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