|Quoting Zvezda (Reply 13):|
In both circular and double-bubble (assuming the floor attaches exactly where the two bubbles meet) designs, the skin is simply in tension where metals are strongest. In other shapes (including the WhaleJet's ovoid), the pressure loads change the shape (not just the size) of the fuselage. Resisting such deformations requires extra strength which adds extra weight.
The A380 is a triple bubble design. It uses both floors as cross beams - one floor exactly at each junction between the three bubbles.
Neither double bubble nor triple bubble designs require extra strength, except that the floors must be designed to take the tension at the bubble junctions.
The A380 triple bubble design makes it a lausy outsize cargo plane. It needs the two floors to keep the three bubbles in perfect bubble shape. If anyone of the floors were removed the fuselage would break apart when pressurized.
The mostly circular fuselage on the 747 (especially with front loading door) is in a totally different league concerning outsize cargo. Even if the floor on the upper deck also is a cross beam on the forward double bubble part. The "problems" on the 747 is where the double bubble part blends gradually into the circular (single bubble) part. That is one very complicated piece of structure.
The British military Nimrod is also a very obvious double bubble, but different. The upper bubble is an ordinary Comet circular single bubble, and the lower bubble is not pressurized. No cross beam needed.
And the A300 Beluga: Not pressurized at all. Only the flight deck is pressurized.