You have very thin sidewalls, and absolutely no double-bubble discontinuity. This means that your thin sidewalls have to carry immense bending loads, and therefore be very heavy.
However even the 747 cross-section caused all sorts of structural problems its early years and provided oodles of work for MROs to carry our strengthening. Think it was described as "Section 41" work or something similar. Used to be a big advertising point for MROs in the 80s.
http://www.airliners.net/open.file?id=1281454