I have a basic frame model that I am using to test things - two columns connected by a beam in an "H" shape. The connections take the form of springs and connecters, exclusively between the outermost node at either end of the beam and the midpoint of the column closest to that node. This worked fine, and gave physically plausible behavior under loading of beam (bowing under Line Load etc), however I have now tried to add a second (identical but for location) beam between the top of the two columns and the behavior under loading is strange. The connections (springs and connecters) are as far as I can tell identical (but with the nodes in question changed) but a Line Load on the top beam, instead of causing bowing of the beam, causes the lower (unloaded) beam to translate downwards. Under a moment (applied to the left hand side of the upper beam) the lower beam raises at the left side and falls at the right (in both cases described the beam remains straight).

Obviously this is physically somewhat absurd but I have no idea what is causing it. I am assuming that one or more of the upper connections is actually linked to the lower beam somehow, but I've checked all the nodes and they seem correct. Is there some sort of obscure setting in connecters (or springs) that I may be unaware of that could be causing this? Thanks in advance for any help