RE: Units of measurement?

From: Bruce McCrory <kbdmccrory_at_comcast.net>
Date: Sun, 12 Mar 2006 21:29:30 -0800

 

From: Indoor_Construction_at_yahoogroups.com
[mailto:Indoor_Construction_at_yahoogroups.com] On Behalf Of Don Slusarczyk
Sent: Friday, March 10, 2006 6:20 PM
To: Indoor_Construction_at_yahoogroups.com
Subject: Re: [Indoor_Construction] Units of measurement?

 

* SNIP If you are seriously wanting to make a better SC
calculation then I
have a few ideas which I never followed thru on. I was going to test
about several hundred samples of wood in various ranges from 4# to 7#
of similar size. I typically cut sheets into 18 x 1 sheets. Then based
on the buckling load, back out the Elastic modulus. Then calculate the
"beam stiffness" which is E x I . Then plot the elastic modulus vs
density and the "beam stiffness" as a function of density. I then was
going to hope that a nice relationship would fall out where one could
predict estimated "normal" E values based on density. And actually I
would prefer EI function based on density as the amount a beam (spar)
deflects is proportional to EI. So I had hoped that a corrolation could
be made (mathematically) so one could say a .030 x .060 spar of 130 SC
4# wood is equal in strength (meaning deflection in this case) of .030 x
.060 SC100 5# wood. That way you can decide in real world numbers that
if I use average 5# wood, then this piece of 4# wood when cut the same
size will have the same strength but be 80% of the weight. Bernards
calculation does not do that. I also know he tested mostly 1/8 x 1/4
strips as I have a tube with about 20 of them in it he sent me. He was
very fond of his Jim Jones balsa stripper and told me it was a
convenient size to use.
    SNIP

Don,

I am working on a spreadsheet, tailored to my own interests, that can be
analyzed for several mechanical properties; and yield useful information.
Right now, for me, the problem is a few learning curves; but I'm motivated.
There are some concerns I have with the suggested method and you might be
able to clarify.

 

Wouldn't testing in the actual size ranges we deal with be more accurate?
After all, the material is extremely variable, even 1-inch widths, and the
B-E Beams theory excludes anisotropic materials as well as buckling for this
reason. Drela pointed this out earlier. There are a slug of potential
variables when testing slender columns. The only reason the formulas can be
applied to wood is due to the built-in margins for conservative design. We
are looking at actual tests for the material on the limits of the Mo tables.


 

The density range and size of material introduces some other properties,
also. I see a lot of elastic to plastic and back to original deformations.

 

BTW, the H-T buckle test is useful for separating glider sheets, forms, and
other stuff from the handful it's identifying.

 

Thanks,

Bruce in Seattle

 

 

 

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Received on Sun Mar 12 2006 - 22:23:00 CET