maybe research material within your field, maybe someone more knowledgeable will chime in; until then ...
1) you need to appreciate the fatigue life of the structure. It is possible it is "infinite". Look into the design code.
2) you can't say that just because a building has survived 15 years that it is good for 25 years more. If you're doing it analytically you need to determine teh safe life is 40 years.
3) fatigue is one thing, corrosion another.
4) possibly look into how it's done for bridges. I think they have inspections based on fatigue loads.
"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.