hi! I hold a USA passport! do I need a visa if I have a 12 hour layover in Shanghai?

Well, you can enjoy 24-hour free policy, so you don’t need a visa.
I’d like to recommend you with an independent tour with Shanghai Sightseeing Bus. There are mainly 4 routes of this sightseeing bus in total, which will pass through the most attractions like Madame Tussaud's, Garden Bridge on the Bund, Huangpu River Cruise Terminal on the Bund, Old City God Temple, Yuyuan Garden, Shanghai Museum and Postal Museum…
You may choose one you like to follow when you arrive in Shanghai. You can take these sightseeing buses at People’s Square. Have a nice trip.