As per subject, is Birch any good as structural timber?
I've got a large birch that came down in a storm a few weeks ago (waterlogged ground and it was on the edge of our woods) - I was going to chop and dry it for firewood, but then it occured to me that I want to put some more floors in our barn one day and I could get four hefty 8' posts out of it (I'd likely just keep 'em as-is, bark and all).
That's assuming it's not going to rot or distort horribly if I try and store it under cover for a few years, though...
In terms of strength and elasticity it's better than oak, downside is it has no durability or resistance to wood borers. It makes good plywood if kept dry and it also absorbs preservatives well.
Best thing for it
Birch bark is waterproof and it keeps the moisture in, hence it will rot within the bark, rotting produces water so you end up with a bark enclosed mush.
Ta... I'll squirrel that info away in the back of my head :-) I'll have to think if I know any tame sawmill folk who might be able to do something with it for me.
Fairy nuff. Probably back to seeing if the telegraph pole I have is any good, then! Just seemed a shame to chop it as it's a big mature tree with a nice straight trunk to it, but it sounds as though I'd have to at least strip it and bombard it with chemicals first.
Question: is unseasoned timber as strong (or stronger) than seasoned stuff? I do also have a need of some temporary supports / scaffold in the barn while I'm doing various repair work, so maybe that tree would still be useful there...
Yeah, I see that often - those trees come down a lot (but also grow fast and pack themselves in quite densely) and many a time I've walked over a downed one only for my foot to go straight through the bark.
Jules - if you google on timber compressive strength you will find all sorts of hits but the outcome seems to be that wood gets stronger as it dries. I imagine this will be due to the cells no longer having water in them. Certainly dry wood is far harder when turning - turning wet wood produces wonderful great long shavings.
I couldn't get the compressive strength for different timbers but a maximum working figure I got for ship builders using douglas fir is
1100psi which as everything else I was working with was SI units is 70 kg/cm^2. I was interested in what the load capacity of 4 x 4 pine was
- and it far exceeded my load of a bit less than 1000kg.
You might find a derating somewhere for wet wood, but it could be that you have a sufficient safety margin from those figures.
Thanks for that! I suppose another way of looking at it is that any given section of trunk is supporting the weight of the rest of the tree above it - so under compression, when wet, any lower sections of trunk have quite a significant weight upon them.
My gut feeling is that I could put a huge load on four 8' long sections of 'wet' trunk - but as I'd want to be working on top of it all, it'd be nice to run a few numbers :-)
We are talking tons here Jules. For any reasonable length of pole, the dominant failure mode is not compressive strength, but tensile strength combineed with elasticity. Google Euler's slender column theory - essentially if the rate of change or resistive elsatic force for a delta bowing is negative, you go into full bowing and collapse. The dominant bowing eigen(value/Vector) [cant remember]..is the 'first harmonic - i.e. the simple bow.
wood becomes stiffer as it dries, so that improves slender column max weight.
Compressive strength is more or less unaffected.
Depending on how thick those 8' sections are, you will probably (waves wet finger) start to see Euler instability down around 4 " diameter.
And that can be prevented by trussing the stricture with cross braces.
Certainly 8x3 timber is great as vertical studwork, but needs trussing in the thin direction.
Birch is not the greatest structural timber in the world., Its lighter and less strong than typical structural lumber, but its not bad either. Good for plywood anyway. Outside it rots fast..many species of fungi love birch.
Hmm - it looks as if I have some more reading to do ! Thanks for that input, NP - I'm looking at the design for a self build conservatory and have some 4 x 4s I'm looking at using.
I take it that the failure mode is that the column starts to bow rather than crush so the force becomes tensile rather than compressive ?
I'm electronics rather than mechanics so I may put this one back to my mechanical friend who no doubt hasn't used it since university days 40 years ago !
4x4 birch? VERY nice indoors. I like birch. Not outside please, not without treatment.
Yup. EASILY fixed by bracing.
In fairness a bending force is compressive on one side and tensile on the other, but wood usually gives by tensile first. Concrete certainly does, which is why we pre-stress it. The idea being the concrete never is being pulled apart under any load..the steel takes all the tensile forces. You can do that with a steel cable inside a wood beam as well.
Once I did the maths, Now I remember the principle, and use the tables. that most places have. Or consult a specialist.
Any architect certainly will. Any form of structural engineers will advise for a couple of hundred as well.
The best source for wood tables at the raw structural level is 'understanding wood' by Hoadley. THE definitive tome and worth every penny. It's a great read if you are interested on wood anyway. Highly recommended.
My mechanical friend was not impressed about being asked questions on a topic which he didn't really understand over 40 years ago !
Anyway I found this
"For a column of length L and cross section radius of gyration rho, it is possible to distinguish 3 failure regimes under compression:
Long column range: for large values of L/rho (usually above 60) the failure is elastic and Euler=92s equation is valid. Short column range: for intermediate values of L/rho (usually between
20 and 60) the failure is inelastic and Euler=92s equation is no longer valid. Several semi-empirical methods have been developed to describe failure in this region from the engineering point of view, e.g. Johnson=92s equation. The book =93Analysis and Design of Flight Vehicle Structures=94 by E. F. Bruhn has a good compendium of methods and design tables. Block compression range: for small values of L/rho (usually under 20) the failure is completely plastic and can be assumed to be, in absence of cross sectional instabilities, equal to the compressive yield strength of the material."
Having had to google for 'radius if gyration' and then '2nd moment of inertia', I find my column is an intermediate one, but as the load is less than 20% of the recommended maximum working compressive stress I'm comfortable that I don't have a problem, but will include this data in the design record.
I do think somehow, Jules, that are going into this a little deeper than you had intended ! :>)
Heh :-) Not a problem... I've not properly digested it yet, but it's good stuff to learn.
Oh, I just measured the trunk in question - four 8' lengths gives me a minimum diameter of 7", which is roughly equivalent to a 6"x6" post, assuming things can be directly translated like that.
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