The office building at 300 East Randolph has been expanded from its original height of 30 stories to 54 stories, making it Chicago’s most extensive addition to an office building, according to architectural historians. To carry out the vertical expansion, the contractor, Walsh Construction, ripped off the building’s roof and extended its steel frame upward.   Once the frame was complete, new cooling towers were built atop the high-rise, replacing those on the 30th floor. The exterior of the building was clad in the same materials as the original—stainless steel, granite and glass.