Why an armored flight deck must also carry structural loads

Can a warship place all its deck armor on an upper deck while retaining a second deck below?

In theory, but it risks top weight and duplicated structural work; for a carrier, making the armored flight deck the strength deck is usually more efficient.

The strength deck resists the hull's tendency to flex apart, so an upper deck that must land aircraft needs substantial structure whether or not it is armored. An armored flight deck can be efficient when it is also the strength deck, whereas separating the two adds heavy reinforcement higher in the ship. A second heavy flight deck above an armored hangar deck can duplicate structure and worsen topweight. An unconventional upper armor deck is possible, but its value depends on ballast, stability, and whether the protected volume justifies the extra weight.