That’s where they were doing the tensioning work on one of the diagonal beams.
It would be nice to see an engineers rendition of how this span was tensioned because it doesn’t make sense that they would do it on the top like that.
OK, so it was a post-tensioned structure.
It's like trying to move a row of books from a desk. If you squeeze the books together with your hands, you can carry the books, no problem. But if you don't exert enough pressure, you are going to have books all over the floor.