Adding friction to a job makes the job highly non-linear. There is a discussion on the solution to the "friction problem" in the
CAESAR II Technical Reference Manual, P6-16. In general, the steps to achieving convergence in a "friction problem" are:
1) Determine what load case isn't converging and make sure it is realistic. By this, I mean don't setup a load case with only a single load component such as WIN1. Why? Well, wind will never act alone, and for code compliance, you should really obtain the effects of wind by subtracting two operating cases, one with wind and one without - then combine with sustained.
2) Once you have determined that the load cases are ok, for the case that isn't converging, use the [F2] key to see exactly which restraints aren't converging and why. Wait until the solution starts cycling before you do this. The report will show which restraints aren't converged, and the difference between the last two iterations.
3) While you can use the [F3] key to adjust the friction settings during the solution, I don't like doing this because it makes the solution hard to repeat, especially after 6 months or a year. I prefer to change the friction parameters in the configuration file, and then archive this file with the job.
4) Here are my recommendations, based on the [F2] report:
- If the report shows "sliding - not sliding", the best thing to do is reduce the "friction stiffness". The lower you make this value, the easier it is for the job to converge, but you are making the system very loose, in other words introducing some error into the solution.
- If the report shows "err=xxx", the value "xxx" is the percent change in the normal force on the friction restraint. To help convergence, you need to increase this value. Obviously you don't want to go too high here, what ever you're comfortable with.
- If the report shows "angle=xxx", the value "xxx" is the angle variation between iterations. Again you would increase this value to help convergence. Again you don't want to go too high here.
- If the report shows "open - closed", this is not a friction issue. This is something like the pipe bouncing on a +Y or against a gap. Try changing the gap size or moving the restraint.
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