Would a slightly up-arched passenger cabin floor be a practical way to reduce airframe weight?
I may be wrong but from what I recall, metals have greater resistance to compressive than to bending or tensile loads. Conceivably, if an arched cabin floor were to involve mainly compressive loads (or a greater proportion of compressive loads), one could make it lighter for a given floor load than a normal, level floor. The degree of convexity need not be significant and would not be too apparent to passengers, perhaps only resulting in seat sides which are slightly divergent instead of being perfectly parallel to each other.
If the floor's points of attachment to the fuselage can handle the lateral loads resulting from such an arrangement, could this be feasible?