Complex sentence: In 1904, Barnett served on the Commission of Architects for the Louisiana Purchase Exposition (better known as the St. Louis World's Fair) and personally designed the Palace of Liberal Arts for which he earned the fair's "Gold Medal for Architecture".
Split into simpler sentences: In 1904, Barnett served on the Commission of Architects for the Louisiana Purchase Exposition (better known as the St. Louis World's Fair) and personally designed the Palace of Liberal Arts. American Art Annual; 1903 for which he earned the fair's "Gold Medal for Architecture".