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When the developer, Miglin Properties, decided to build a unique residential extended stay project in Chicago, the American Institute of Steel Construction (AISC) was approached to provide suggestions for a structural steel system that would be economical and provide an opportunity for an accelerated construction time. After preliminary discussions with Miglin Properties, the AISC introduced Structural Affiliates International to the developer to help them refine the structural system and obtain preliminary costs. When compared to other systems, the steel staggered truss framing system was selected as the most effective solution for the developer’s needs.

 The resulting project will have 13 residential floors plus penthouse above three levels of parking. The residential floor plate will be generally rectangular 186’ in the east-west direction and 56’ in the north-south direction. The perimeter configuration will be adjusted at some floor locations to vary from a true rectangle.  In addition, the floor plates will be offset at the 9^th through 14^th floors to create a building projection to the south building insets occur on the east and west elevations.

The parking levels will be rectangular in plan with an east-west dimension of 186’ and a north-south dimension of 100’. The first level of the garage will enclose retail space.  The garage area not beneath the residential floors will slope to provide vehicle access from level to level.

Lateral Load Resisting System: In the short direction, a staggered truss framing system will be used for lateral resistance for the fourth floor level and above. The story-high trusses are staggered vertically at alternating grid lines between grids A and H. The configuration of the trusses is similar to that of a Pratt truss pattern with a central Vierendeel opening for corridor circulation. The top and bottom chords are rolled wide flange sections and the web members are square hollow structural sections. The trusses will be cantilevered at locations where the floor projects beyond gridlines 2 and 6.

When the staggered truss framing system terminates at the fifth floor, lateral shear forces will be transmitted to a series of 12-in thick shear walls located directly under the staggered trusses. These shear walls then transfer the lateral forces to the foundations.

In the long direction, a braced frame will be provided along gridline 2 from gridline D to E. Although only one braced frame is provided in this direction, the torsional rotation of the building is resisted by the staggered trusses in the transverse direction.

The braced frame will be terminated at the fourth floor and the lateral forces will be transferred to two 12-in thick shear walls. One shear wall is located along gridline 2 from gridline D to E, directly under the braced frame above. The other wall is located near gridline 6 from D to E.

The floor system will consist of 10-inch-thick precast hollow core planks. The planks will provide a floor diaphragm for lateral load resistance. Joints between the precast slabs in the direction parallel to the span will be grouted to provide a means to transfer the diaphragm shear flow. The top and bottom chords of the trusses will have headed shear studs to transfer the lateral loads from the floor diaphragm into the truss system.

Hollow structural sections will be provided along the building perimeter to resist the diaphragm chord forces. When the building floor notches at the center, structural tees will be provided to transfer chord forces at the notch to the chord members on the perimeter. The structural tees will continue to a sufficient length to provide anchorage and transfer of the forces from the HSS sections.

This project has been featured in both the May 2004 issue of Architectural Record and in the April 2007 issue of Modern Steel Construction.  The 187,000 square foot, 200-room hotel was described as “made of huge, seemingly floating masses resting on a balanced structural skeleton” (Architectural Record).