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Odd response running load combinations...

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Boothby171

Mechanical
Aug 27, 2001
72
I have a relatively simple model in RISA-3D. No "tension only" members. No "Euler Buckling" members. Spring constraints and some RIGID members, though.

It's got a few HUNDRED load combinations, and I'm trying to run ONE, using the "Solve Current Combination" command.

So far, so good.

Here's where it gets weird:

Load Combo 213 runs the DL, scaled by 2.0, plus BLC8, scaled by 2.0
Load Combo 214 runs LC213, scaled by 1.2

BOTH LC213 and LC214 run without problem.

Load Combo 219 runs the DL, scaled by 0.9, but NO OTHER load cases added
Load Combo 220 runs LC219, scaled by 1.2

NEITHER LC219 NOR LC220 runs!! After a few minute's iteration, I get the following error message:

"Solution could not converge while solving: [Load Combination Name]
Please review your model. Solution stopped!
Error code = 1162"

Why does the SAME MODEL work for a MORE COMPLICATED Load Combo, but not the simpler one???!?

Thanks.



--Boothby

"The biggest cause of trouble in the world today is that the stupid people are so sure about things and the intelligent folks are so full of doubts." - Bertrand Russell
 
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Add to that: The model DOES have Tension-Only Springs and Compression-Only Springs. I'm focusing my attention there...

--Boothby

"The biggest cause of trouble in the world today is that the stupid people are so sure about things and the intelligent folks are so full of doubts." - Bertrand Russell
 
Have you verified that the dead load value it sees from your input is not zero?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
Mike,

Fair question ("Make sure that the computer is plugged in?" and "First, can you make sure your friend is really dead") that I have asked people myself many times before!

Yes; the DL is non-zero.

I have since upped the TS and CS values by a factor of 5, and I got the Load Combination (LC220) to converge. Now I just have to make sure it makes sense!

--Boothby

"The biggest cause of trouble in the world today is that the stupid people are so sure about things and the intelligent folks are so full of doubts." - Bertrand Russell
 
Sometimes models will not converge with different load combinations due to the stiffness (or lack of) in the model.

I just did a quick model study of a thin member in tension with self weight (sag) and multiple nodes. With PDelta turned on the axial load had to be in a certain range for the model to run.

With tension-only and perhaps PDelta this could apply to your model.

It might suggest you have a less than stable model.
 
JAE,

Thanks for taking a look!

It's definitely a load-sensitive instability, but I don't think I have any slenderness issues with my elements. I DO think it's related to relative spring stiffness issues, since increasing the spring rate of the tension and compression springs seems to have done the trick.

--Boothby

"The biggest cause of trouble in the world today is that the stupid people are so sure about things and the intelligent folks are so full of doubts." - Bertrand Russell
 
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