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I am thinking of constructing a wooden bed/desk/roof/shed/ship/bridge/rocket

  • How can I calculate the maximum load a rectangular horizontal wooden beam of dimensions L x H x W can safely support if the beam is adequately supported at both ends?
    • assuming worst case - load concentrated at center
    • for commonly available types of wood (e.g. Spruce)
RedGrittyBrick
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3 Answers3

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There are lots of span calculators available online, which help you determine what size lumber to use in home or deck construction. For example

http://www.awc.org/codes-standards/calculators-software/reversecalc

and

http://www.awc.org/codes-standards/calculators-software/spancalc

You could try to figure out what the live loads and dead loads for the bed are and go from there.

Shortcut - I might try to get by with 2x4's spaced 2' or less apart if they run side to side across a single mattress, but I'd want 2x6's if they run long ways, or for a full or larger.

batsplatsterson
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5

Your question referred to a simple central load. So the formula here seems useful:

https://en.wikipedia.org/wiki/Flexural_strength

Looking up the bending strength of Spruce here:

http://workshopcompanion.com/KnowHow/Design/Nature_of_Wood/3_Wood_Strength/3_Wood_Strength.htm

We get 10,200 psi.

Assuming a 6ft length of 4x4 that actually measures 3.5 inches square, and plugging these numbers into the formula, we get:

10,200 psi * (2 * 3.5in * 3.5in^2) / (3 * 72in) = 4049.306 pounds

This appears to be the point at which your beam will deform.

LRU
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3

Here are two documents I've found helpful, giving specs for southern yellow pine, which is the wood typically used in treated lumber for its added strength compared to SPF pines.

wallyk
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Barn
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