Brian Whatcott wrote:
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
Brian,
Please re-read my post and do these calcs by hand! Your stress results
for the wood beam are off by at least a factor of two. Here is my
previous post:
"Actually, Brian, 5 1/2x11 1/4 is the finshed dimensions of a 6x12. You
may want to do the calc by hand to verify the results from the program.
I imagine you have an input wrong. Your deflections, however, seem to be
correct for the steel beam. For 0.1" deflection and an E of 1.5, the
moment of inertia of your wood section is 856 in4 which is the moment of
inertia of a true 6x12 (6"x12") The section modulus of this beam is
144in3. For a 4225ft-lb moment, the bending stress is 352 psi!"
Based on the deflections you gave, the section properties compute to a
"true" 6x12, but your bending stress doesn't even copme close to this
section. You have an error!
--
Rob Munach, PE
Excel Engineering
PO Box 1264
Carrboro, NC 27510
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