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PostPosted: Wed Oct 05, 2016 10:18 pm 
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Does anyone know the formula to find the strength of a stick of wood? It is usually used in calculating beam strength. I had it once, from Hoadley's book, but can not find it now. Horner's Woodworker's Essentials has discussion and the formula for deflection but not the modulus of rupture, the formula needed to determine the actual strength. Forest Product labs has a lengthy and complex discussion but the formula section either doesn't have it or has it in complex terms that I'm missing. Several sources have the "live load" a beam can sustain. What is a live load?

All I want to know how is much weight can a beam support at its mid point. e.g. a 2x8 10' long supported at the ends will break when x pounds is placed on its mid point. I have all the data to put into the formula but I don't have the formula.

Geoff


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PostPosted: Wed Oct 05, 2016 10:40 pm 
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GEOFF K: I'm an OOPS when it comes to math. Go through these and see if any help.

http://engineersedge.com/beam-deflection-menu.htm

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PostPosted: Wed Oct 05, 2016 10:44 pm 
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From Hoadley, page 124

Image

where k is a function of placement of the load. For the center of the beam, k = 1.5
Fb is the allowable bending stress, often stamped on graded lumber.
b is the width of the board, d is the depth. Width and depth are relative to the orientation of the beam. So a 2x10 on edge has width 2, depth 10 (actually 1.5 and 9.25)
l is the length between support points

(carn, it's hard typing math on this forum)

(are we really that fragile that we autocorrect dam?)


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PostPosted: Fri Oct 07, 2016 9:16 pm 
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Thanks. The engineering site had all kinds of formulae but none that determined point of rupture. Even if it did, it would not let me use the formulae unless I became a paid member.

In Hoadley's formula, is Fb the same as modulus of rupture? What is on the other side of the equal sign? This does help a lot. My "Understanding Wood" is at my son in laws and I will have him scan and send me that page. And I'll ask for my book back next time we visit.

Geoff

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PostPosted: Sat Oct 08, 2016 10:56 am 
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Hoadley describes Fb as "allowable bending stress". Elsewhere on the web this does appear to by the same as "modulus of rupture", but this is not my area of expertise.
And as to the left side of the equation - sorry about that. The left side is the allowable load. The k value of 1.5 is for a point load at the center of the beam.


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PostPosted: Sat Oct 08, 2016 12:47 pm 
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Thanks. This helps a lot. I'll play with the numbers and see what I get.

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