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Thread: A wood strength question to ponder............

  1. #1
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    A wood strength question to ponder............

    Does a board of a given dimension and volume have the same compressive/span strentgh as a board of a different dimension but with the same volume? In other words, does a 4 foot 2 x 6 (1 1/2" x 5 1/2"), on edge, have the same strength as a 4 foot 1 x 12 (3/4" by 11")?

    Anyone have any info on this?
    Thanks, Mark.

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  2. #2
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    An unedikated guess, on edge: same.
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  3. #3
    Instinctively, I would say not. I work on the principle that depth (vertical dimension) is the main contributor to resisting deflection across the unsupported span. The proviso being that there is sufficient thickness to the board to resist warping. I would say that a 12*1 on edge would deflect less than a 6*2 on edge. Don't ask me for calculations though.

  4. #4
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    In a word, no.

    I don't have a sample of the calculation handy, but one timberframing book I've read goes into this in depth in one of the appendices. If you really want to see the gritty details, I can find them within a couple of days.

  5. #5
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    I'm with Ian on this one. I think double the width at half the thickness would withstand compression better. I don't think, though, that it would be as stable or stand up to similar lateral forces like Ian mentions.

    Not an expert - but some do tell me I have a spacial brain ... maybe they're just telling me i'm spacey ...
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  6. #6
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    Roll a 1x6 over flat and see if it is as strong as it is on edge.

    Same size and same volume, no where near as strong.
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  7. #7
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    draw a picture mark....i can`t picture the application? a 1x piece would roll over quicker than a 2x piece...but if the board can be held ridgid and force applied straight down my bet would go for the 11" over the 5-1/2" even though the 11" board is half the thickness.....given similar grain and same species.....but i ain`t no engineer
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  8. #8
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    My structural engineering is real rusty, but I believe the both cross sections will have the same compressive strength (psi), but the taller cross section will have a higher higher moment strength (or flexural strength may be a more correct term).

    Compressive:

    Attachment 11604

    Flexural:

    Attachment 11605

    The compressive strength of a material is based on straight force (pounds) over a given area (square inches, for example). So even with equal compressive strengths, the wide, short board offers a larger surface, and thus can support more pounds. More square inches, more pounds.

    The moment strength is resistance to a combination of compressive and tensile forces (pushing and pulling), depending where in the board you're looking. It ths sketch above, the top part of the board is under compression and the lower part is under tension. I believe strength is gained by going to a taller cross section faster than it is by using a wider one. Like I said, I'm real rusty on this stuff, so I could stand corrected.

    In other words, I believe the shorter board (the 6 x 2) would have more strength in the first picture, but the taller board (the 12 x 1) would be stronger in the second example. Of course, this doesn't account for racking and other sideways forces.
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  9. #9
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    Quote Originally Posted by Vaughn McMillan View Post
    My structural engineering is real rusty, but...
    What he said... It's right on, rusty or not.
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  10. #10
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    If a board is twice as thick, it is 8 times as strong in bending, (third power).

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