On the one hand, you don't have gravitational loading for the most part (there's some because of various forces that get applied to a space station during operation, but not at the level you'd have on the ground). However, you can get structural oscillations which behave somewhat differently than they do on the ground, as well as various torques that get applied throughout operation. And because of weight considerations during launch, space structures are often made of aluminum, which doesn't have a fatigue limit, so it will weaken over time even for minor stresses. You hear about fractures being found in the walls of Russian ISS modules because of this. These effects get worse as you scale up a structure.