Design of a Residential Tower on Top of an Existing Parking Garage

Constructing a new building on top of existing structure can sometimes feel like trying to fit a square peg into a round hole. Several years after construction had stalled on a prominent development, Thornton Tomasetti was asked to re-engineer the unfinished plans for a mid-rise building near Chicago's Magnificent Mile. A new development team purchased the property and commissioned an architectural design that varied greatly from the original plans. The key change was from a 13-story square donut shaped tower to a 25-story tower with an elliptical floor plan. The new tower would be centered above a previously designed light core. This loaded the interior columns and foundations beyond their designed capacity. A 6'-0" deep transfer mat and a 16'-0" deep transfer girder were required to spread the load to columns with adequate capacity. Overcoming these challenges also influenced many other aspects of the building's design. The construction team likewise faced challenges with the site. A parking garage and grocery store were located in the existing podium. The need to keep these facilities open during construction greatly affected construction sequencing. This paper will explore the design and construction processes that facilitated the development of a 25-story composite tower on top of an existing 8-story concrete garage. Project milestones, creative design solutions, and construction challenges will be outlined in an attempt to present a set of best practices for similar projects.