I'm surprised to see no mention of Calatrava and his beautiful brdiges - part engineering, part sculpture. The classic design is Puente del Alamillo in Seville but as the author of the piece seems to prefer American bridges I can propose the Sundial Bridge at Turtle Bay, California. In the end there are too many beautiful bridges in the world to make a top ten but thanks for the pointers on here - some new ones to me.