 The advent of steel frame structural systems allowed completely vertical construction; but as with many skyscrapers of the era, the exterior was designed with multiple setbacks at increasing heights, which served to allow additional light into the ever-deepening concrete valleys in urban cores.At night, the setbacks are upwardly lit by floodlights, further emphasizing the structure's vertical elements.The night illumination design was a common contemporary Chicago architectural theme, seen also in the Wrigley Building, the Jewelers Building, the Palmolive Building, the LaSalle-Wacker Building, and the Tribune Tower.