Originally Posted by Grunf
...
Some of the upper chords do see compression for the pressure case.
Sure, these are beams after all, and the pax/cargo loading creates bending stress (compression in the top chord, tensile in botton) whether pressurized or not. These stresses add algebraically to the pressurization (tensile) stresses. Whether the summation of compression & tension in the top chord of the floor beam is ultimately positive or negative must depend on the individual design.