# wood tensile strength

• posted on December 10, 2004, 10:34 pm
Does anyone have formulas for calculating wood strength?
I am making a table lift, consisting of three pieces of wood/steel, first piece with bushing t in the middle of it, position toward one end of a long threaded rod. Second piece, with threaded nut wielded/epoxied, etc sits only 2" above third piece. Third piece has casters. Seond and third piece will stay in line using two rods vertically between the pieces. Three pieces extend across table width on the inside of the table. Point is no wheels seen, mobility when necessary, and no casters when doing actual work.
Goal is to turn threaded rod, which extends from above first piece, through second, and then simply sits in a bushing drilled in the third piece. Turning rod raises or lowers third cross piece, which has the casters, due to presence of nut on second piece.
Question is whether a hardwood, like OAK, would support the weight (500 lbs or so) or do I need to fashion a metal solution. There would be one of these 3-piece lifts at each end of the table, meaning each would need to support 240-300 lbs of weight.
Hope this is clear enough for an answer...thanks...
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• posted on December 10, 2004, 11:11 pm

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Your table weighs 500 pounds? Wood can handle it probably, but it depends on how much wood and how the load is applied. My house is made of wood and it is strong. A toothpick is made of wood too. You don't give nearly enough information to get a reasonable answer. Try it out. Wood is cheap.
-j
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• posted on December 10, 2004, 11:47 pm
Chris Carruth wrote:

They're the same as the formulas for calculating the strength of metal, plastic, or any other material, at least to an engineering approximation.

The USS Constitution is built mainly of OAK, and can support the weight of on the order of 100,000 pounds of crew, plus supplies, cannons, wind loads, sea loads, combat damage, etc up to a total displacement of around 2000 tons. So, yes, OAK, properly configured, can support 500 pounds.
The problem is how to configure it to do that.

--
--John
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• posted on December 11, 2004, 1:59 am

I'm not sure if it'll solve your problem, but take a look at http://www.woodweb.com/knowledge_base/Wood_Handbook.html
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• posted on December 11, 2004, 3:09 am

There is no formula for calculation the tensile strength of any material. You find values in a book somewhere. In addition, it is very ill advised to let the load become so high that the internal stress approaches the tensile strength.
You also must consider the buckling load for columns. Jim
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• posted on December 11, 2004, 2:58 pm
I would definitely avoid putting wood in tension. I've made clamps that put the stuff in tension that easily pulled themselves apart. X-dowels, bolts, and screws can also pull the fibers away from each other. Much easier and far more positive to use mending plates, dovetails (tho clearly in tension), corner braces etc.; stuff that exploits the material's best modes of strength. http://www.patwarner.com (Routers)
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• posted on December 12, 2004, 12:33 am

Wood is put in tension every day all over the world. Every single floor joist and roof rafter is in both tension and in compression. The top is in compression and the bottom is in tension.
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-Mike-
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• posted on December 20, 2004, 8:21 am

yes, but the joist would be considered to be stressed in "bending", pure tnesion in wood is generally avoided.
OP needs a designer or engineer
cheers Bob
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• posted on December 20, 2004, 12:00 pm

in
Agreed. I was just responding to a post that said wood should never be put in tension.
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-Mike-
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• posted on December 20, 2004, 5:25 pm

Bob,
No offense meant, but what you state is absolutely false. Look up the definition of a truss. And, yes, I am an engineer and a licensed architect.
-j
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• posted on December 11, 2004, 2:58 pm
I would definitely avoid putting wood in tension. I've made clamps that put the stuff in tension that easily pulled themselves apart. X-dowels, bolts, and screws can also pull the fibers away from each other. Much easier and far more positive to use mending plates, dovetails (tho clearly in tension), corner braces etc.; stuff that exploits the material's best modes of strength. http://www.patwarner.com (Routers)
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• posted on December 11, 2004, 3:55 pm
snipped-for-privacy@patwarner.com wrote:

Well, now, every time a Mosquito with a full bomb load made a maximum performance turn it put quite a lot of wood in quite a lot tension with no difficulties at all. The USS Constitution any time she was running before a good wind did the same (backstays were a new innovation that came along after she was designed).
If you're applying a tensile load perpendicular to the grain then no, it's not very strong--neither is graphite reinforced epoxy or any other "oriented" material. The solution to that is to design so that tensile loads are parallel to the grain, not perpendicular.
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--John
Reply to jclarke at ae tee tee global dot net
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• posted on December 11, 2004, 3:42 pm

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Well, it isn't nearly enough for an answer, but I'll give you some things to think about. First, if you're supporting a table top that sounds like will have something like 500-600 lbs sitting on it, my first guess is that your wood pieces are mostly going to be loaded in compression, not tension. If that's the case, depending on the design of the individual members, you might have to worry more about buckling than compressive failure. For wood, anything with a slendeness ratio of more than about 11 and you are likely to have buckling as a primary mode of failure. But unless you know how to calculate effective length of a column and radii of gyration, this is definitely a case where a test will be more informative than attempting to work out the calculations (just in case you didn't take four years of engineering mechanics and strength of materials). Even for those of us that did, it gets hard to remember. Consider that a piece of oak with a cross-sectional area of 1 in^2 has a compressive strength of approximately 6000 lbs (and a tensile strength of approximately twice that), and I think it's very unlikely that you are going to have a problem strength-wise in strict tension or compression with a decent design.
todd
todd