Complex sentence: Nordhagen designed a number of industrial structures, notably Såheim in Rjukan (with Thorvald Astrup); designed several churches; and completed considerable research around Norwegian traditional architecture, but he is probably most famous for the restoration designs of the Nidaros Cathedral.
Split into simpler sentences: Nordhagen designed a number of industrial structures, notably Såheim in Rjukan (with Thorvald Astrup); designed several churches; and completed considerable research around Norwegian traditional architecture. Olaf Nordhagen is probably most famous for the restoration designs of the Nidaros Cathedral.