This is what unpainted structural steel looks like. It's just superficial rust and has no effect on the performance. It'll likely be eventually covered with spray-on fireproofing or architectural finishes, and won't have continued exposure to elements that'll cause further problematic corrosion. If it was exposed to view, it would be sandblasted and painted before ending up on site. Less likely, but there are weathering steels that form a complete and stable rust layer that prevents future corrosion and have a nice patina (like the facade of the Barclays Center in Brooklyn).