Here's a collateral question . . .
What if the building doesn't get built? How long do you keep your plans and calculations then?
The reason I ask this is the following: Probably 25 years ago I designed three or four, mid-rise residential buildings, with parking under them. The plans were completed, but the project never went ahead.
About 5 to 8 years later, I was threatened to be sued by my original client. When I asked why, he said that they had submitted the plans for construction permit, and that my structural plans were incorrect, and it was costing him a lot of time and money to make changes now. He even threatened to take me before the state engineers' board.
I actually had to go to a pre-hearing to help clear my name, and luckily I still had my calculations in my files. The structural plans had been rejected due to inadequate capacity of primary structural members in the framing above the parking.
I had to prove that I did not make an error, and that the current building code was different from that which it was designed under, primarily with respect to Live Load Reduction. If I did not have my calculations, the suit would probably have gone beyond the pre-hearing level, and I could have been embroiled in a long battle.
I know it sounds ridiculous, but it is true. I found out that the client never had the site approvals and got held up in court for numerous years until he finally received court-ordered site approvals. The client never stayed in touch (he was very unwilling to part with any fees for professionals, as he knows better than them), so I had no idea that the project was ever going to be built. (And by the way, I had a very hard time collecting the final fees way back when I designed the buildings.)
Ah, the great life of a structural engineer !!! Always new ways to be sued that you can never forecast.