900 North Michigan Avenue is a 66-story, mixed-use development containing retail, office, hotel and residential spaces. The 2.7 million square foot building consists of an eight-story low-rise structure with a 57-story tower rising from it. Benesch provided structural design engineering services required for this complex project. The structural design solution utilized an exterior moment frame for lateral load resisting systems. By locating the lateral load resisting elements on the perimeter, maximum flexibility was achieved in the placement of interior columns and cores.
In the first 30 stories, long spans and small columns were required for retail, office and hotel functions, therefore a structural steel system was the logical solution. Long spans were not required for the upper 36 stories of residences and hotel rooms, so a concrete flat plate system was chosen for those stories. While a steel and concrete structure is not uncommon, it is unusual to set a 36-story concrete structure atop a 30-story steel structure. Between the hotel and residential components at the 48th floor, there is a concrete transfer girder system, which accommodates the different column locations for each function. The transition of 15- by 20-foot bays to 30- by 30-foot bays required a second transfer system at the 30th floor. This structural steel transfer system also provides the transition between the steel and concrete framing.