A question that then comes to mind, what exactly was the structure like in the case of the comet aircraft in the early 1950’s and what is it like in today’s airliners?
The Comets structure was called the stressed-skin fuselage structure because the Aircraft skin took the primary structural loads in flight. This type of structure is technically called the monocoque fuselage construction. We saw how this created fatigue for an aircraft fuselage that was constantly flexing under pressurization and depressurization loads.
To overcome this problem, designers came up with a new structural design which distributed the flight loads to stronger lateral(vertical structures such as bulkheads) and longitudinal (horizontal structures such as stringers and longerons) members that ran all along the length of the fuselage. The picture above shows an intersection of the inner wing with the fuselage. Notice how the fuselage is made of frames (vertical members)and horizontal reinforcing members - apologies for the quality of the picture. More pictures here..
So how did these structures help?
Well, essentially, flight loads were systematically distributed amongst vertical and horizontal structural members, easing the overall load on any one single structure. This dramatically reduced the fatigue factor on any single structure of the fuselage adding longevity as well as ensuring that if one, or a set/series of structural members were damaged (due to fatigue or even overloading in flight due to turbulent weather, aerodynamic overloading, foreign object damage etc.), the other members would be able to carry the loads atleast until the aircraft was able to descend and land safely. That is, in the event of damage to part of the aircraft structure, the overall structural integrity of the aircraft would not be compromised. This is what designers and engineers refer to as Failsafe construction, sometimes even referred to as Damage Tolerant structures.
This type of fuselage design came to be called the Semi-monocoque construction design. Today, every pressurized aircraft involved in commercial operations, is built with a failsafe design philosophy.