View Full Version : Critical wood movement

Sun 11 April 2010, 09:10
I have a project for a spirial staircase. The center pole is 4.125" in diameter and 18' long in 2 pieces and the treads have a matching hole. Both are from fir. Treads are installed by sliding down the center pole. I can hold a tolerance of 20 thousands but here is my problem.
The humity is higher than normal so I expect that the center pole will shrink. Here are my thoughts in instulation. Put center pole in plywood box with several light bulbs at the bottom and air flow restricted at the top. Let set for at least 2 days. Instulation will have to be completed within 8 hours.
MY questio is if I use the same procedure on the treads will the hole enlarge? My feeling is that the hole will shrink. Any sugestions?

Sun 11 April 2010, 12:35
If you are going to stack on the center pole why not use a steel core and let the wood slide over that?


Cool site here that covers what I am talking about.

Sun 11 April 2010, 12:57
That is the way I would have done it. But you know archetects.

Sun 11 April 2010, 13:17
My concern with all wood core is if it will meet code for safety.

Sun 11 April 2010, 20:55
How about using steel & line/cover by non structural wooden cosmatics?

Mon 12 April 2010, 00:36
I have no input for the design. Don't like what they came up with but have to what the customer wants.

Gerald D
Mon 12 April 2010, 01:04
Art, I am not sure that I get your question properly . . .

If a piece of lumber, with a big hole, shrinks (dries) then any hole in that lumber will also shrink. In the perfect theoretical world, the whole system will shrink/expand together. However, the amount of shrinkage/expansion across the grain is greater than in the direction of growth. (just like humans). I suspect your tread grain is at right angles to the pole grain.

Can you run a test with pieces of the actual fir?

Mon 12 April 2010, 07:59
I will deliver the poles today and they PRESENTLY fit the hole in the thread. I am woried that before the stairs are installed the pole will expand because of humity so I will recomend that the poles be heated for 2 days to get any extra shrinkage to ease sliding the threads on pole. Customer will do the treads so I have no control. I thought that the hole in the treads would contract if dried but wanted to be sure. It would be great if I could get the hole to temporarly expand.
I don't like the design of this stairs as it seems structurly weak but then the architect didn't ask me.

Mon 12 April 2010, 09:55

write down your concern about the issue and the structural weakness and send that off signed on paper (postal return signed, not sure what it's called) to the company so you have proof you send that to them in case legals turn up.
I believe you life in a country where people can be sued for millions when slipped over a banana peel in a super marked, so better make sure you have written down your concerns.

Mon 12 April 2010, 11:06
Art are you putting the stairs together or were you contracted just to make the peices.

If you are there just to make the peices I would ask for them to inspect on deliever and sign and after that wash my hands and walk away.

If you have to put them together I would do what was mentioned in the previous post.

Mon 12 April 2010, 13:38
I also fear that if the tread shrinks, the hole will no longer be round (if I am visualising the tread correctly - ring of wood at end of solid wooden tread, varying thickness of material around the hole) the tread ring will also shrink at a different rate to the solid pole so may end up splitting if it shrinks too much??? sounds risky.