That’s the underside of a highway overpass. It’s old enough that the built-up plate girders that are the main structural elements are riveted together rather than being bolted or welded. The concrete deck looks very new and is a replacement. Since water and (in the past) road salt attack this structure from the driving surface above, it’s not particularly surprising that the concrete would deteriorate faster than the steel and would need to be replaced first.
The big surprise is the presence of wood planks resting on the bottom flanges of the girders. Wood is not allowed in highway construction and it shouldn’t even be nearby, as the steel is (obviously) not fire-protected. Those planks are a leftover from the deck replacement, almost certainly installed to protect the street below (where I was walking) from falling tools, debris, and wet concrete during the deck replacement. They are protection that was once vitally important and is now unnecessary.
How did they end up there? I can imagine them being installed before the deck demo began, and then being forgotten after the new deck was put in place. All construction projects end with a punchlist, where the built result is compared to the drawings and specifications, and discrepancies that need to be addressed are written down. Sometimes something escapes this process, usually because it’s not very important and it’s one of the last steps. In this case, I’d guess that sooner or later someone will notice and take the planks down.
I had an investigation a few years ago at a low-rise office building. Wood was found in the steel-frame building, and all sorts of panic ensued. It turned out that the wood was shoring for the steel floor deck to help carry the weight of the wet concrete. Once the concrete cured, the wood shoring was meaningless, but it wasn’t removed. Some of it was covered with spray-on fireproofing, disguising it. It was more than ten years old when I saw it.
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