On Tue, 22 Mar 2005 13:39:19 GMT, Rob Munach
wrote:

[Days ago, I wrote this - Brian]
A specimen load for a 6X12 in wood beam at 13 feet is 200lb per foot
//
uniform loading, for which the max deflection is 0.1 inch
//

[Rob]
Brian,

What is confusing is this paragraph:
[brian]
uniform loading, for which the max deflection is 0.1 inch This has a
safety rating of X2 to failure for reasonable assumptions:
Youngs 1.5E6 psi limit stress 1.2 kips

Regards,
Rob

Ah, yes: I suspected that this might be the root of the confusion.

1) For any load at all, on any beam at all, there is a place on the
beam that experiences maximal deflection (for that load).

2) For a limit load on a beam, there is a point that shows a maximal
allowable deflection for that beam.

You know that: I know that: and I possibly made it to easy for you to
suppose that the maximal beam deflection of type 1) was referring to a
deflection type 2)

Moreover I suggested that this specimen load might be half of the
load at failure. It probably isn't though, wouldn't you say?

Now for one interesting feature of using beam programs (as every
professional would be well-advised to do

steel beams have pretty uniform parameters, so very much derating is
unnecessary. Wood in contrast, has rather variable parameters and as
an anisotropic material, there are just plain more of them too!

So a program offering wood beams had better provide low-ball values of
important parameters. Nobody will disagree with me on this, I don't
suppose.

Now here's the kicker:
Say we want to replace a wood beam with a steel beam, for strength,
for reduced deflection, or most likely,for reduced depth wasted.

In this situation we DON'T want the low ball safety values, we want
the HIGH ball values for the wood, in order to guarantee the same or
better performance when we match strength or stiffness in steel.

The reason is because the previous (wood) beam may have been a lucky,
strong example at the upper end of the distribution: it is vital not
to degrade values of that rare HIGH end sample.

That has been the other issue: everyone has been so comfortable
with their timber or steel beam program, it was easy to forget that
the brain has to stay engaged, for the safety of the public, whom one
is mandated to keep safe.

This has turned out to be a remarkably civilized thread, in the end.
I salute you all!

Brian Whatcott Altus OK