I was surprised to learn that a prototype bridge constructed during World War II is still standing - the oldest example of its type in the world.
Donald Bailey designed his world-famous Bailey Bridge in 1940, using the back of an envelope for the initial sketch. After development during 1941, it entered service during 1942. Millions of linear feet of bridge structure were manufactured in the UK, Canada and the USA during the war. It remains such a simple, effective, easy-to-use design, even compared to more recent developments, that the Bailey bridge is still widely used to this day (and a modernized version is still in production). General Eisenhower called it "One of the three pieces of equipment that most contributed to our victory in Festung Europa." (You can read a 1945 article about the effectiveness of the bridge here, and see many photographs of wartime bridges - some of them spectacularly large and/or long - in this article.)
It all started with a prototype installation across Mother Sillers Channel at Stanpit Marsh in southern England in 1940. That first-ever Bailey Bridge is still standing. Here's a video description of it, and the development of the bridge. (Don't let the detour into Middle Eastern history fool you - it's all relevant to the story.)
I have warm fuzzy feelings about the Bailey Bridge, because I drove over several of them in South Africa's Border War operational area. They were the standard portable bridge used by SA engineers. They were very effective, and got us into (and out of) our combat zones with no trouble.
I'm pleased to know that the original Bailey Bridge is still standing, and still in use, 79 years after it was erected. I think that somewhere, Sir Donald Bailey (knighted for his wartime efforts) is smiling down on it with fond affection.