Way up in about the second or third message, Alan asks:

Could someone tell me why a lot of bridges in America have something that resembles a wooden shed over them ?

Wikipedia says

a covered bridge is a timber truss bridge with a roof and siding which, in most covered bridges, create an almost complete enclosure. The purpose of the covering is to protect the wooden structural members from the weather. Uncovered wooden bridges have a life span of only 10 to 15 years because of the effects of rain and sun.

Sounds logical to me, although I know of a few covered bridges where the covering forms part of the superstructure of the bridge. I see that the Bridgeport Covered Bridge in my area (Yuba River in California) is now closed even to pedestrian traffic, so I wonder how long it will survive. If you enlarge the photo, you'll see that a wooden arch is incorporated into the side of the bridge.

Bridges fascinate me, and I've made a hobby out of walking across famous bridges. I've walked the Brooklyn Bridge, the Golden Gate Bridge, the Charles Bridge in Prague, the Key Bridge in DC, the Chain Bridge in Budapest, every bridge I could find in Paris, and of course the Tower Bridge in London. I recently crossed the London Bridge in Lake Havasu City, but I'm not sure I want to admit that.

-Joe-