Nucor Corp., Charlotte, NC, has agreed to buy Magnatrax Corporation, a provider of metal building systems in the North American non-residential market, for $280 million.   ; Magnatrax has seven fabricating plants, as well as service centers and transport facilities, throughout the U.S. It operates through its American Buildings Company, Kirby Building Systems, Gulf States Manufacturers and CBC Steel Buildings subsidiaries and divisions.   ; The supplier sells, engineers and fabricates custom metal roofing systems for new and retrofit construction and sells third-party metal building components.   ; The acquisition of Magnatrax is an important growth opportunity for one of our core downstream businesses," said Dan DiMicco, Nucor's president and CEO. "Although Nucor is already a major national player in the metal buildings business, the addition of the Magnatrax brands, facilities and most importantly people significantly enhances Nucor's market position."   ; Nucor and its affiliates are manufacturers of steel products in the U.S. and Canada.