The Bridge Project is a new game, sort of a physics simulator where the point is to build bridges, strong enough for cars, trains, even tanks.
The game costs $24.95. It is not on Steam, but a demo (mac & PC) is available at chroniclogic.com, and Rock, Paper, Shotgun has some videos. Of bridges.
It reminded me of a game I used to have when I was a student, Pontifex. Well that's not surprising, since they are both made by the same company :)