Sum of Forces Do Not Equal Zero?

Hi,

I have a problem with my forces not summing to zero in my output file. I understand that the y-axis won't due to gravity acting on the system. But the x and z axis (horizontal) should be zero or at least close to it. Can anyone explain this?

Thanks!

I assume you are looking at a restraint report.

Do you have any CNode's in your report? Don't include those.

Dave,

Yes, I do have CNodes. I have several cases to design for and some come close to zero (several hundered lbs), but others will have a net force of several thousand lbs. Will removing the CNodes fix this?

Thanks!

Force equilibrium will not in general happen if your system is having non linearity like friction.

Regards

You need not remove the CNodes to see what you want to see.

In the output processor, click on Filters, then click on the Restraints tab, then click on the Radio Button labeled "None on CNodes" (or some such phrase). Now your restraint report will not display the "other side" of the CNode restraints.

Even nonlinear conditions satisfy equilibrium in C2. C2 iterates until it reaches convergence on a common solution satisfying all nonlinear conditions. It stops when things are in (very close to?) equilibrium.

Dave,

I disagree. I had problem ( approx. two years back ) with non satisfaction of equilibrium in a problem where Friction effects were there and yes, the exceedance was substantial . I discussed this issue with Thomas J Van Laan and his reply was that it will not be possible to achieve 100 % equilibrium but he was also surprised at the level of exceedance.

Regards

I've split my model and filtered the CNodes. I also removed the friction and my forces are summing to zero, or at least close to it. I still have to see how the other cases turn out.

Thanks,

Dallas

You need to also understand that CNODES are internal restraints, and as such you wouldn't want to include them in an external load summation. You only want to include external boundary conditions in your summation.

You guys are correct. I forgot to look at the friction as being non-linear.

Thanks for you help!

Generally speaking, ANY (I do mean general here) apparent static imbalance from a C2 solution is associated with tolerances used in seeking convergence to a solution involving nonlinear conditions.

For that reason, if I had a job that does not converge, I would reduce the input complexity rather than open those tolerances. I think they are loose enough now. I would reduce complexity by converting certain nonlinear conditions to linear conditions.

If it don't equal zero, find out why.

Dave

"You guys are correct. " Everybody gets lucky now and then!!!