On Mon, 21 Mar 2005 15:39:10 GMT, Bob Morrison
wrote:

In a previous post Brian Whatcott says...
If I say that a rough sawn 6 X 12 inch measures 6 X 12 inch, (duh)
and provide the Code approved results for such a beam, given the
Youngs and stress limit I specify, then I have provided an engineering
calculation which is explicitly correct.


Brian:

The calculation may be correct, but the answer is wrong. What is the
point of doing a calculation on a product that one cannot readily
purchase? That is not an "engineering" solution, but simply a
mathematical exercise.

What Rob is trying to tell you is that if you are going to make a
recommendation then base your recommendation on real lumbers sizes with
real lumber material properties. It does no one any good to specify
some material that cannot be purchased.

Ah, now we are getting to the nub, I see.

Can you purchase a 13 ft beam in the US?
Can you purchase a (metric) 13 ft beam elsewhere?

Can you ask a mill to resaw used timber?
Can you have beams rough-cut to specified dimensions?
Does the allowance for timber "finished four sides"
allow a wastage off the rough-cut (nominal) dimensions?

We are (presumably) not talking about dashing down to
Lowes with a bill of materials, but talking engineering stress.

The answers to these questions may help one
be better prepared NOT necessarily to suppose a weak
hypothetical wood beam, and speculate that it is Southern yellow
pine finished four sides, rather than rough cut Douglas Fir on nominal
dimension, and so to avoid converting it to a steel beam that is
lighter than a conservative conversion would provide.

If a home-owner asks for advice, it is far, FAR better to err on the
conservative side, wouldn't you say? To put it another way:
whose advice would YOU take, if you were not in the field:
mine or Rob's?

Still, I will keep batting this one around with you, by all means, at
least until the ad hominem approach supervenes.

This can be educational, after all.

Have a happy day!

Brian Whatcott, Altus, OK