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Brian Whatcott
 
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On Mon, 21 Mar 2005 10:44:00 GMT, Rob Munach
wrote:

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 This has a
safety rating of X2 to failure for reasonable assumptions:
Youngs 1.5E6 psi limit stress 1.2 kips

A steel I beam: W8X15 deflects 0.09 inches for this load,
a W8X13 deflects 0.11 inches.
Youngs 29E6 yield stress 36 ksi

Respectfully

Brian Whatcott Altus, OK

Brian,
You may want to re-check your calcs.

6x12 (assuming 5 1/2 x 11 1/4 - which is larger than the (3)2x12
specified) S = 116


I made explicit my inputs, as to Youngs, limit stress and dimensions
for both the wood beam and the steel beam.
You note that you used commercial "finished" dimensions rather than
gross rough-sawn, and that's a not unreasonable basis for the
difference in your wood beam calcs, I certainly agree.

Respectfully

Brian Whatcott Altus, OK


Actually, Brian, 5 1/2x11 1/4 is the finshed dimensions of a 6x12.



Let me say this again, slowly:

a finished (planed) 6X12 may measure about 5 3/4 X 11 3/4, I believe
depending on its moisture content.
A finished composition of 3 off 2 X 12 may measure about 5 1/4 X 11
3/4,I believe. These figures may or may not represent your
experience, probably not. Still, I could care less, because it is
not material to my response.

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.

If you wish to say that your planed, finished beams are 5 1/2 X 11
1/4 that's just fine by me too. And your figures may be accurate for
these dimensions. Or not. I haven't checked.

Wouldn't it be better to ask the questioner what are the ACTUAL
dimensions of the beam he has in mind, rather than playing "I'm
smarter than you"?
[People don't usually win these games with me, Rob!]
:-)

Sincerely

Brian Whatcott Altus, OK