The new string after it't up to pitch, wouldn't put any more stress on the piano than the old string did-- aditionally, the piano has a cast iron "harp" whose purpose is to transfer the tension and would keep the piano from exploding---however, you brig up another point and would/should be a consideration in restoring an old piano-has the piano ever been mis-handled where that "harp" has been broken-- cast iron is brittle-- lots of people I delt with years ago assumed(in error) that the "harp" because it was brass colored, was brass--not so--also the "upright grand"-- that was an advertising ploy several piano manufacturing companies used to use ,to "sell" their product over competition--- there are upright pianos with longer string legenths than some small grands, and many of them when properly restored, sound better than some of those grands--