The fuselage will experience a wide range of loads from a variety of sources.
The weight of both structure of the fuselage and payload will cause the fuselage to bend downwards from its support at the wing, putting the top in tension and the bottom in compression. In maneuvering flight, the loads on the fuselage will usually be greater than for steady flight. Also landing loads may be significant. The structure must be designed to withstand all loads cases in all circumstances, in particular in critical situations.
Most of the fuselage of typical commercial aircraft is usually pressurized (this also applies for other types of aircraft). The pressure inside the cabin corresponds, during the cruise phase, to that at an altitude of 2000-2500 [m] (when climbing/descending below/above that altitude, it is usually changed slowly to adapt it to terrain pressure). Internal pressure will generate large bending loads in fuselage frames. The structure in these areas must be reinforced to withstand these loads. Also, for safety, the designer must consider what would happen if the pressurization is lost. The damage due to depressurization depends on the rate of pressure loss. For very high rates, far higher loads would occur than during normal operation.
Doors and hatches are a major challenge when designing an aircraft. Depending on their design, doors will or will not carry some of the load of the fuselage structure. Windows, since they are very small, do not create a severe problem. On the floor of the fuselage also very high localized loads can occur, especially from small-heeled shoes. Therefore floors need a strong upper surface to withstand high local stresses.