Comment by WalterBright

The wing spar is dual, too.

The idea is to design the airplane to survive an explosive decompression failure, not pretend that explosive decompression doesn't happen. For example, on the DC-10, the floor collapsed from explosive decompression, jamming the control cables and causing a horrendous crash.

The fix was not preventing explosive decompression. The fix (on the 757) was to locate the redundant set of control cables along the ceiling. Also, blowout panels were put in the floor so the floor wouldn't collapse.

It's not always practical to fix an older design like the 747. When it isn't practical, a stepped-up inspection protocol is added.

P.S. The 747 was designed to survive a decompression. The oversight was nobody realized that a failure of the rear bulkhead could destroy the tail section. Things like that happen in complex systems, and an airliner is incredibly complicated.

P.P.S. When I was a newbie at Boeing, I asked about the wing spar, too. That's how I know it is dual!