It depends on how you use it. A viaduct is technically a bridge that crosses a valley or other roads or railroads, consisting of several spans. A viaduct can certainly be made of steel; one that comes to my mind is the rather long steel and stone arched viaduct that carried the PRR’s freight line through Philadelphia, allowing it to cross Penn Coach Yards at 30th Street Station.
Viaducts can be small by definition, but I tend to think of the giant concrete muti-arched spans that the DL&W built in eastern PA and western NJ: