Brian Whatcott wrote:
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
I use them, however, for simple span beams with uniform or central point
loads, I can calc 'em much faster by hand then using a beam program. I
probably do 20 a day by hand.
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!
True, but if you do this long enough you will have memorized S, E and I
for most of the sections. When I go into the field, I rarely need a
cheat sheet or beam program. I simply use my calculator and my brain.
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.
It shouldn't lo-ball anything. It should use the design values set forth
in the NDS. Believe me, not only is there a huge factor of safety in the
material, there is also a huge factor of safety on the loads. It is
rare, if ever, the structure even comes close to seeing its design
gravity live loads. Additonally, there is no accounting for the large
amount of unintended composite action going on in light frame
structures. Although, I am starting to believe that the NDS modulus of
elasticity numbers are high. I am seeing in the field, members that are
deflecting signifcantly more than they should.
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.
Exactly what you did. While safe and conservative, it won't keep you in
business too long as you will get a reputation for being "too
conservative" However, with all due respect, you probably dont' do a lot
of this type of work anyway.
FWIW, serviceability controls most beams designs in light frame
structures. I have seen plenty of excessively sagged beams, but have
never seen one break.
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!
Agreed.
Brian Whatcott Altus OK
Regards.
--
Rob Munach, PE
Excel Engineering
PO Box 1264
Carrboro, NC 27510