The weight of wood...

[Jules]

Are there tables anywhere for the dry weights of timber of different species by volume? I realise it's all likely to be somewhat hand-wavy...

I'm going to need to jack up parts of our old timber-framed barn as I work on the end wall*, and being able to estimate the size of floor jacks needed would be useful (and I'll add on a wide safety margin, of course).

• for some very strange reason they ran poured concrete round three walls (up to about 3' high), but only did half of the fourth. Of course 60 years of moisture hasn't been kind to the 'missing' section where the wood extends down as far as ground level - the framework's gradually rotted from the ground up, dropping about 3" in the process along that section. Thankfully it's an end wall, not side, so doesn't have all of the roof weight on it...

It's a bit of a longer-term project, but figured I'd ask and then I could keep an eye out for the necessary jacks in the meantime.

cheers

Jules

[R]

[Peter Crosland]

Sounds like you need the services of qualified structural engineer to advise you.

Peter Crosland

[Airsource Ltd]

Oak 8 Iroko, Teak 7 Mahogany 6

kN/m3, from the structural engineer's pocket book p34

[The Natural Philosopher]

You need 'understanding wood' by Hoadley..

US book, and the very best there is.

However if you use a 'same density as water, i.e. a cubic meter weighs a tonne' you will be slightly out on the safe side.

I.e. a block of oak floats..just. A block of lignum vitae sinks, but who uses it?

a block of softwood like pine, probably floats half out of the water, so is half the weight of the water.

If in doubt, cut a piece off and float it, and measure the waterline..

[Roger Mills]

Google is your friend! Typing "specific gravity of wood" into the search box gives lots of hits, including this one:

most wood floats in water, if you assume it has an SG of 1 (1 Tonne per M^3) you will build in a reasonable safety margin.

[Jules]

Quite possibly - although what would they do (purely out of curiosity)? Would they cook up some 3D model of the structure, take samples of materials and weigh, and work it out from that? I'm intrigued by what the exact process would be... (mainly because I'm surprised anyone can give an exact answer without a truly enormous - and perhaps impractical - amount of work, although I'm sure a reasonable estimate based on a few calculations and a lot of experience could be attaioned)

cheers!

J.

[Jules]

That relies on me bothering to use it ;) I didn't realise it'd be that easy - I figured it'd be one of those things that was either buried in a textbook or within the heads of more clueful folk...

That works. Initial estimate then, and given that the roof structure transmits most of its load to other walls, I think that 3 tonnes is probably a reasonable number - but I'll do some exact measurements and take it from there! (it's high enough that I think that 2-tonne jacks might be pushing it a little, at any rate)

cheers

Jules

[Peter Crosland]

The problem is that there are a lot of variables to consider and the job is not all calculation. I only suggest it because of the potential for disaster if you try and DIY it.

Peter Crosland

[The Natural Philosopher]

Ive seen substantial stuff jacked up on Acrow props.

The rule seems to be 'if you cant get enough force on the Acrow to jack it, its too heavy'

I do remember the 6x4 beam across an old shed we used to put a 5 tonne yacht pulley and hoist on to lift a Morris 1500 engine out..only when the 5 tonne pulley bent, and the beam deflected about 6 inches, did we realise that the whole front of the car was 3 " off the deck.. 'er I think the engine has caught on something chaps'

[pete]

Well, you don't need a precise answer - what you need is a "not less than X" answer and then to get provision to support "X" (taking into account how the load is distributed, how to go about raising / supporting it without adding undue stresses) and all the elfin malarkey, too. I'd suggest that the calculations are quite a small part of the issue. What you'd be paying for is the experience of the S/E to know how to go about the job. Plus of course, having someone to sue if it all goes wrong :-)

[Mungo "Two Sheds" Toadfoot]

Weigh more than 5 tons, that Morris, did it?

Si

[Jules]

Thankfully no elfin bollocks where I am, but yes, I do need to worry about exactly where to jack / support. Currently the weight for the damaged section is distributed between the wall's framework (which has rotted at the bottom, but still appears to be resting "solidly" on the ground) and on one of the primary beams which runs the length of the barn.

I suspect I'll end up supporting that beam (just to be on the safe side) then jacking it as close to the end wall as I can*. I'm actually tempted to use a 3 ton truck jack, purely because I can then run the handle through a hole in the wall (the cladding will all be replaced anyway) and jack it from outside. Being right next to a floor jack whilst jacking it up makes me a little jittery :-)

• it's a 6x6" beam made from three 6x2" boards - but whoever made it put the overlaps all within a couple of feet of each other. It's bent around this point as the wall's sunk, and remains to be seen whether it'll bend back the other way when things are jacked up.

For a wooden structure that's 60 years old, with little weather protection, and which has apparently seen very little maintenance during its life, I suppose it's amazing that it's still standing at all. The fact that it's such a wreck make it more of an interesting DIY project, though! :-)

cheers

Jules

[The Natural Philosopher]

No. What we discovered was that a '5 ton breaking strain pulley' breaks at 5 tons.

It gets totally buggered at about 1.5 tho ;-)

It just didn't actually break..

[Mungo "Two Sheds" Toadfoot]

That's an excellent get-out they had there! :)

"Didn't break, did it?"

Si