I have a round tank model with bottom reinforced structural underfloor support welded onto the outside surface. The model is at the assembly level with two parts (tank and support). The tank is fixed at the bottom edge of the strake. The tank is constructed with the surface for FEA purpose (real model is not surface), bottom support is with real weldment. It is global bonded and all the relevant constraints have been applied. After evaluating the model stability, the program told me that one rotation constraint about the center axis is missing on the group of support and bottom floor. I have applied a rotation constraint on it, but it brings more missing constraints instead of fixing it. I am currently confused that why introducing one more constraint can make the model more unstable?
Can anyone please instruct? Or how to fix this issue. The model is attached for reference.