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Cbeam in tension and Cquad4 in compression?

Cbeam in tension and Cquad4 in compression?

Cbeam in tension and Cquad4 in compression?

Quick question for anyone that has any insight.  Fuselage stringer is modelled as a beam and the skin as a plate.  The beams have offsets.  When I run a pressure case only, the skin goes into tension and the beam goes into compression.  Anyone have any ideas as to how this can happen?  GPF balance shows a net tension, which is expected as the pressure bulkhead expands and the skin bulges out due to pressure.  I'm trying to explain why the beams see compression.  When I look at displacements, end a of the beam/skin (fwd grid) expands in the hoop direction, but end b (aft grid) contracts - im guessing because the pressure bulkhead is moving aft as this beam element is just fwd of the bulkhead.  However, the net displacement between grids shows tension (moving away from each other), so how can the beam be in compression?

Thanks in advance  

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