900 North Michigan Avenue

Benesch provided the structural engineering services required for this complex project.

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. The retail area and most of the hotel function rooms are located within the low-rise structure and the base of tower. In mixed-use buildings, the tower, residential and office space frequently stand independently behind the retail space. In this case, the four functions of 900 North Michigan were placed one atop the other.

The mixed-use nature of the building presented some unusual challenges, requiring the use of an exterior moment frame, or “tube” structure, for the lateral (wind) load resisting systems. By locating the lateral load resisting elements on the perimeter, maximum flexibility was achieved in the location of interior columns and cores for the various building functions.

The most unique aspect of the building’s structure is the stacking of two different structural framing systems in the tower: concrete on top of structural steel. In the first 30 stories, where long spans and small columns were required for retail, office and hotel functions, structural steel was the logical choice. For the upper 36 stories of residences and hotel rooms, a concrete flat plate system was chosen because long spans were not required. This type of framing also allowed the bottom of the floor slab to serve as the ceiling for the floors below. Between the hotel and residential components at the 48th floor, a concrete transfer girder system accommodates the different column locations for each function.