Reply by sras

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Posted on Expansion and Contraction..Am I thinking correctly

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5289 posts in 3738 days

#1 posted 02-10-2017 06:31 PM

Hi Charles, I’m a little late to this one – it looks like you have a pretty good set of inputs. Since I like these kind of problems, I’ll try to go a little deeper and see if I can add some value.

Given that the “L” shape will have movement between somewhat less than 90 degrees to somewhat more than 90 degrees, it seems like the key question is “how much angle change will there be?”

Using this table we see that flatsawn walnut will move 0.0027 inches/per inch/per % of moisture change.

This article suggests that the wood could see up to moisture shifts of up to 9% (from 25% to 75% RH). If you seal it well, you should see less than that. Given that this is a countertop, I would suggest that it will be well sealed and you could cut that range in half. I’m also going to assume that the counter will be assembled at 50% RH and the movement will be +or- 2.25% of MC. I did not find any reference to back this assumption up. You can double my results if you have less confidence in how well the wood is sealed.

First Conclusion: I’ll assume a 30 inch wide countertop. I’ll also assume that you have a rigid joint along the miter. With the above data and assumptions we get that your miter will move +or- (0.0027×30x2.25) or 0.182 inches or about +or- 3/16ths of an inch. Unsealed wood would move +or- 3/8ths of an inch.

The next question is “how much will this change move the ends of the counter?”. I had to draw this out to make sure I got it right… (off to the CAD program—- I’m back) The angle will shift by +or- .174 degrees or 0.003 inches per inch.

Second Conclusion: As the humidity changes, each leg of the “L” will swing back and forth by +or- 0.003 inches per inch. A five foot (60 inch) long leg of the counter counter would move +or- 0.180 inches. (Coincidentally, about 3/16ths of an inch). Again double this for unsealed wood. A backsplash should conceal this much movement.

Final Thought: If the counter is longer and the movement becomes more than desired, I would suggest that you anchor the counter along the midpoint of each leg of the “L” and let the counter rotate ever so slightly about those two anchors. That will cut the movement in half.

This got a little long, but as I said, I like these kind of problems!

-- Steve - Impatience is Expensive

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