Wright was a great designer, but wasn't a very practical builder or engineer. His masterpiece "Falling Water" was plagued by foundation problems even shortly after it was built. Many Wright by buildings had problematic construction or later problems with engineering issues.
I have heard a lot of descriptions of FLW's designs which seem to indicate that he had no concept of floor loadings.
Sounds like he may have have allowed nepotism to override common sense. From the article:
"Wesley Peters, FLW's son in law and first apprentice, who did the calculations for his designs of Fallingwater "