Those are special bolts called "structural rib bolts", and one head looks like a rivet. Welding has it's own set of issues, like xraying and stress releiving the welds (not to mention the creation of voids caused by impurities in the steels being welded, and you can bet that isn't US-made steel they're using). The big advantage of hot riveting is that the holes are completely filled by the rivet as it's driven, which is not the case with bolts (unless they're fitted bolts).