Commissioned in 1965, designed between 1966 and 1968 and completed in late 1971, the Phillips Exeter Academy Library has received the 1997 American Institute of Architects' 25-year award. Renowned architect Louis I. Kahn designed the structure's dramatic architectural-concrete interior. The inner shell of concrete forms a great central hall that rises six floors through the core of the building. Large circular openings in this core allow a glimpse of book stacks on the cantilevered balconies that make up the second concrete layer. The library's exterior shell, made of load-bearing brick masonry to harmonize with other campus buildings, is a series of arcades housing reading and study areas. The building's post-tensioned concrete frame features two deep concrete beams extending diagonally across the top of the atrium and intersecting at its center. Although the beams are 18 inches thick, 16 feet deep and contain 87 tons of concrete, their structural function is relatively limited. The chief purpose of the beams is visual: They act as a baffle to screen and diffuse light entering through clerestory windows on the sides of the central hall.