This is a fascinating structure, which justifies a far better photograph. Unfortunately the circumstances at the time prevented me from taking that better picture!
Anyway, this is the White Cart Bridge, on the A8 between Renfrew and Inchinnan.
The bridge can lift to allow marine traffic to pass under it. According to Secret Scotland (www.secretscotland.org.uk/index.php/Secrets/WhiteCartBridge), the bridge is a Scherzer Rolling Lift Bascule Bridge. "Bascule" means that there is a counterweight to make the bridge easier to lift, and the elegant curve you can see in the centre of the frame is the surface on which the bridge’s platform rolls. It appears that Scherzer is the name of the inventor – the American William Scherzer, who invented this form in 1893.
This bridge is a Grade A listed building, renovated in 2005 by Historic Scotland