You have misunderstood Jouko's question. He asked how big is the lift-off (in mm) in the middle of 20m span, not the difference in spans from 20-33m. If lift-off is less then say 5mm I would not worry about it on such a large diameter duct. I would add a thin "imaginary" packing under +y to see what happens.
Have you created a node in the middle of the span to check the highest stress?
Is your duct D/t>100?
Local stress has to be considered in any case, 20 or 33m span.
Regards
Ranka