Being based on beam elements, I expect Caesar is not the best program for this application, but it is the tool I have available.

Here's the scenario - I am trying to make a recommendation for some field routed tubing that is going to have to be flexible enough for a large displacement between the end points. Using the tubing vendor's minimum bend radius recommendation, I am trying to figure out how many coils of tubing will be required to absorb the deflection. I have made a model that essentially looks like a spring coil with a continuous series of bends with offsets that give a 2" pitch between coils. I have model each end with a 2.1 SIF as I know there will be socket weld fittings near the tubing connections that will likely be the limiting factors on stress.

At one end of the coil there is a full anchor and at the other end I have input a lateral displacement that is parallel to a line runing down the middle of the "spring".

So, essentially, all of the piping (tubing, actually) is perpendicular to the line of displacement and should participate in providing flexibility.

Now, when I model two complete coils, the stresses are pretty low. However, if I model a straight section tangentially out from the end of the last coil, the stresses start going up at the end of the straight portion (where the SIF is applied). The really nasty part is that the longer I make it, the higher the stress goes.

What I can see from the displacements is that, as the straight section gets longer, the displacement shifts from the coil to the straight leg. That much makes perfect sense. What doesn't make sense is that the shift seems to be putting so much displacement into the straight section that it overstresses the end point.

So, what I'm having trouble resolving is, how/why is this happening? And, am I really getting any kind of valid information trying to run a model like this in a program like Caesar?