When used as a building material, which material is stronger, pound for pound - balsa wood or steel?
Balsa wood is so light, that I could believe that it is a stronger building material pound for pound…however, I know that steel is also very strong, hence my question.
There’s a certain amount of “it depends” as there are many grades of balsa wood and of steel. However using some common numbers here you go:
Balsa wood ranges from about 4 to 20 pounds per cubic foot. The following site (http://www.balsasales.co.uk/technical_info.html) gives a nice middling number for select, kiln-dried balsa as:
9.5 pcf, tensile strength 1910 psi (that’s pretty strong stuff, a 1" square piece of balsa wood holding one ton of weight)
Standard Grade 60 structural steel is about 500 pcf and has a tensile strength of about 60,000psi (thus the name).
Tensile strength/density ratios are thus:
1910/9.5 = 201
60000/500 = 120
So yes, balsa wood can be stronger than steel on a per-weight basis. This doesn’t mean we’ll be building skyscrapers out of the stuff and going with the new M1A3 Abrams “Woody Wagon” any time soon but wood is definitely amazing stuff.
Ultimate tensile strength doesn’t tell the whole story, different loading situations will change things, I think it’s easier to produce uniform steel beams than to make same out of wood, etc etc and yet more etc.
Not to mention the fact that lots of otherwise rentable floor space would be taken up by walls 15’ thick.
My degree is in civil engineering, not architecture. Can you tell? 
Also, your cite doesn’t mention whether the wood is loaded with the grain or across it. Wood, like any fibrous material, is stronger when loaded along the grain. However, in the other direction, the grains can easily separate and you lose all your tensile strength. The term for a material like this is “orthotropic”, where the properties change with direction, as opposed to “isotropic”. Steel and most metals are considered homogeneous and isotropic.
That’s an important observation. Usually, materials with orthotropic strength are used so that the stress is taken in the high-strength direction (e.g. wooden beams are cut with the grain running lengthwise instead of crosswise.)
Quite true. I intentionally did not go into anything beyond simple tensile loading to ultimate strength.
not to mention the problem that the balsa building would sag badly in wet weather
But on the up side, once dried it would hold its new shape and you could get some cool Dali style architecture on the cheap.
[QUOTE=Valgard…wood is definitely amazing stuff.[/QUOTE]
I have a book called *Understanding Wood * by R. Bruce Hoadley and in that book he is describing the structural strengths of wood and if one could keep the grains from buckling laterally, I (weighing around 245 lbs) would be supported by a 1/8 inch column of ash. The reason we need 1 inch columns on four corners of our chairs is to keep the wood from buckling laterally.
Wood is amazing.
I read this thread last night and was wondering how more common wood varieties stacked up against balsa but I had to wait til I got to the office this morning and could pull out my copy of the LRFD Manual for Engineered Wood Design.
Anyway, here (this is a site promoting Bald cypress but it’s got some handy values) are some typical tensile strengths and densities:
Bald Cypress 8500 psi & 32.2 pcf
Western Red Cedar 6600 psi & 22.4 pcf
Old Regrowth Redwood 9400 psi & 28.0 pcf
So the tensile strength/density ratios are:
Bald Cypress 263.9
Western Red Cedar 294.6
Old Regrowth Redwood 335.7
So you can see that more common woods beat out both steel and balsa by a fair amount.
Woods a pretty neat building material, stong and light. It also tends to hold its strength pretty well in fire situations unless it’s completely consumed. But design is pretty complicated 'cause you have to factor in a lot of things like moisture content, grade (how many knots and imperfections are present), duration of loading (wood is much stronger in the short term and tends to sag with long term constant loading), etc. And, as av8rmike noted you have to be careful how it’s loaded.
I think this is true of columns in general. The way I remember it is that the straightness of a column is one of the really important factors in the ability of the column to support the load. Slight initial bending, i.e. crookedness, in the column produces a lateral moment which tends to increase the bending which increases the lateral moment which increases the bending, etc.
It might be more important in wood than in steel because of the difference in modulus of elasticity. Wood’s is much lower than steel which means more bending for the same load and so puts a premium on column straightness.
Just for fun, here’s a nice interactive chart which shows the relative strength and density for a range of different materials, all on a 2D chart (note the difference between balsa with the grain and balsa across the grain, by the way). Materials with the same specific strength (which is defined as the ratio of strength to density) will lie on the same diagonal line on this chart. Specific strength also shows up on this chart in comparison to specific stiffness.
I just stumbled across this old thread about strength of wood vs. Steel…sorry about that - if there is interest
in my new question, I could make a new thread
What about plywood?
I see lots of things made of wood, and it seems to me
plywood is stronger than regular. The alternating
grains overcomes the shearing along the grain.
Does the glue become the weak point?
As for my project -
I have seen plans for building a backhoe out of steel…
but I don’t have tools for working with it.
I do have tools for working with wood.
I’m not trying to build a whole house…
just dig some trenches. (deep ones, for
geothermal ground loops).
My friend who is a MechE said NO WAY to wood.
I did not pursue the plywood question with him.
Thanks
pachai
Not addressed directly by any poster above is that the term “strength” has a lot of different meanings. There’s tensile strength, shear strength, compressive strength, torsional strength just to start. And then there’s the various flavors of hardness, which a lot of non-technical folks conflate with “strength”.
Which material attribute you care about depends on what you’re trying to make it do.
As one example, for a digging machine the edges of the shovel need hardness greater then the rocks & dirt they intend to displace. Typical wood species won’t have that.
I was wondering if wood could be used for the boom and the stick.
What would make me think such a silly thing?
My new deck is supported by a girder made of 5 2x10’s.
I bet it could be made with 1" CDXs…
Unless, of course, I want to use this as an excuse
to buy a machine shop…
Thanks
You could certainly make a backhoe-like device out of wood. The practicality of it, though, and the exact design, would depend on exactly what you wanted to use it for, and you probably couldn’t use the same design as for a metal one. Your options are to either do a lot of engineering calculations for a new design to know in advance that it’ll work, to eyeball the design but overengineer it enough that the errors in your eyeballing are within tolerances, or to eyeball the design and then test it out empirically to see if it’ll work.
You could use wood to make most of a small backhoe. The bucket wouldn’t be practical if it wasn’t made of steel. You would need metal for the bearings, and to reinforce the joints. You could also use cable and pulleys as early excavators did. Metal would be needed to reinforce the area around bearings and the ends of beams to prevent splitting. The pins the joints rotate on would have to be made of steel, or become very large. Considering the price of hardwood, you may not save any money in the end.
One important advantage of steel is joining. A weld is basically as strong as the plate or beam you are joining.
And todays glues and expoxies are stronger than the wood they glue together. Of course welding is generally a bit less of pain than glueing stuff together.
A homebuilt plane or boat built out of wood can give ones made out of metal or composites a run for the money weight wise.