FOLLOWUP: mechanica beam question - distributed load acting strange

[lylebeidler]

I discovered that my message below was way off base: the reason the stress
was so low was that the section of the beam was sketched (it's a composite
beam, with a bar welded below a W-beam, and I modeled as a sketched beam)
and for some reason, it calculated that the section modulus was off the
charts, so the stress approached zero. I'm not sure how that happened, but
as soon as I changed the beam x-section from a sketched section to a
standard w-flange beam, it gave realistic numbers . . .



I'm not sure what's going on here - is this a known glitch? In order to get
the stress level Mechanical calculated for this beam with the known moment,
the section modulus would have been on the order of 1.1 x10^16!!!!! The
actual value is in the neighborhood of 60



--



Lyle Beidler
MGS Inc
178 Muddy Creek Church Rd
Denver PA 17517
717-336-7528
Fax 717-336-0514
<">mailto:-> -
<">http://www.mgsincorporated.com>

[GavinBRumble]

Lyle,



I noticed (I think) two things for you to consider. It appears that your
end conditions are FIXED which could very well create higher stresses than
you expect in that vicinity, and may not represent the correct boundary
conditions. Also (and this is in response to your follow-up), when you
sketch a section you have to tell Mechanica WHERE on the sketched section to
evaluate the stresses. It seems like you do this in the sketcher.



Hope this helps,



Gavin B. Rumble, PE

Solid Engineering

336-224-2312

[ptcuser11]

Lyle,

It is not so much a known glitch ... more so known source of error. Sketched beams are so error prone I try to avoid at all costs. The error come from the polar moment of inertia calculation J. The polar moment of inertia J is known for simple shapes such as circles and squares. But for complex geometry this value is not known and is refered to as the "K" factor (see Roark's Formulas for Stress and Strain). It is approximated by Mechanica during the sketching phase, but this approximate can be way off. Just as an exercise look at Roarks and try to determine the torsional stiffness of an open C shape. It is brutal. If you beam is subject to any amount of torsion it can cause bad results to creep into your solution. Refer to the PTC knowlege base article for more information: http://www.ptc.com/cs/cs_24/howto/mst1359/mst1359.htm.

Secondly, Gavin also makes a point regarding where the stresses are calcuated for sketched beams. Mechanica referes to these points as recovery points. You can have up to 9 I believe. Obviously, depending on where the stress is being measured can have a huge impact on the results you seek. For standard shapes, these recovery points are shown in the simple crossection in Mechanica and they are typically located at sharp corners or on the outer most fibers of the crossection.

I fought this problem for weeks...that is how I found out about all this. My main take-away was to simply not used sketched beams if I didn't have to.

Good Luck

Steve