Mechanical properties?

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Forum topic by Chris posted 08-27-2010 12:36 AM 1251 views 0 times favorited 5 replies Add to Favorites Watch
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8 posts in 3093 days

08-27-2010 12:36 AM

Hello all,

I’ve been reading a couple books about furniture design, and while everything I’ve read says that you need to be aware of the strength limitations of your material, I can’t find anything telling how to figure out what those limits are. The books’ focus is solidly on ergonomics and aesthetics rather than structural strength. Is there a good reference for the mechanical properties of common woodworking materials? Maybe a table that shows the elastic or compressive strength of various woods/plywoods so you can calculate an acceptable unsupported span of a cabinet floor or tabletop, for example, or the thickness of a leg needed to support a given weight? Something to help figure out how thick of stock you should use. I couldn’t find anything via Google (at least, not that I could make sense of).

I know there are accepted standards for most furniture dimensions, and I do not currently have any specific project that I’m working on (my first few projects will be from purchased plans anyway), so this is just academic curiosity right now. I suppose it’s possible I’m over-thinking this. I do that sometimes.

I’m really looking forward to getting a workshop set up so I can put the books down for a bit and actually build something. Hopefully just a couple more weeks. So close… :)

5 replies so far

View Tony Strupulis's profile

Tony Strupulis

260 posts in 3364 days

#1 posted 08-27-2010 01:13 AM

The Forest Products Laboratory developed a formula for modulus of rupture (strength at which it breaks) for woods:

For air-dried woods, Modulus of rupture = 26,200 x (density of wood/density of water)

What this means for you is the denser the wood, the stronger it is.

-- Tony -

View Catspaw's profile


236 posts in 4056 days

#2 posted 08-27-2010 11:52 PM

Forget it. Unless you can calculate the forces applied to every joint from each direction, I think it’s an excercise in futility.

Generally speaking, woodworking projects have been tried and trued over the centuries. Poplar, Oak, Hickory, Cherry, etc. all have sufficient strength for most every app. The simple fact of the matter is, you aren’t going to make a chair out of balsa wood unless you’re going to use it in a gag on t.v. or something. The traditional woods are usually strong enough for the standard applications.

So unless you want to figure out mathematically how a brace entering the side of another peice of wood at 32 degs. with an average of 150 lbs. of downward force 13” off center supported by a rocker whose center of support varies as the chair shifts position…..well you get the idea.

Other than that, build it and start stacking concrete on it ‘til it breaks. Then you’ll know how strong it will be when you build it for real.

O.K. so there are some rules of thumb that vary from person to person. Like 3/4” bottoms for cabinets unless you provide some support for 1/2”, say, beyond 12” to 14” span or some things like that. Usually I just go standard and over build a bit so I don’t have to worry about it.

-- arborial reconfiguration specialist

View ChrisForthofer's profile


150 posts in 3308 days

#3 posted 08-28-2010 01:47 AM

I agree with the above folks, I think you are better off looking at existing pieces and gathering general thickness/span info from that. I am of the belief that unless you are constructing something that will be pushed to its mechanical/structural limit do calculations like that need to be made. This would also apply if you are planning on building products to sell, not necessarily doing the calculations but at least building a prototype and having having your fattest friend sit on it :P Seriously though, if you are undertaking woodworking as a hobby, build it, enjoy the journey and learning (read making mistakes) and refine your future designs from that. Also of course if you are doing something out of your comfort zone and want some very good advice there is always this forum :) Folks here are friendly and happy to answer questions from new and veteran wood workers.


-- -Director of slipshod craftsmanship and attention deficit woodworking

View Chris's profile


8 posts in 3093 days

#4 posted 08-28-2010 05:03 PM

Thanks! Like I said, this was mostly just academic curiosity, prompted by the books saying to account for material properties with no indication of how to do that.

View canadianchips's profile


2616 posts in 3238 days

#5 posted 08-28-2010 05:20 PM

Whe nyou are building your project:
Ask yourself.
Which is heavier ?
1 lb of feathers or 1 lb of bricks ?

-- "My mission in life - make everyone smile !"

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