Hemmed in by a combination of barriers including the city’s set-back zoning requirements along the Chicago River, and a bustling, seven-line Amtrak right of way spanning more than 140-ft., the developable parcel offered only a small sliver of land just 55-ft. wide upon which to build. “Meeting the challenge of building a cost-effective high-rise on this site came down to delivering the required floorplate area with a 45-ft. lease span supported by four-story trusses on either side of the 39-ft.-wide core,” says Erik Harris, Associate Principal with Goettsch Partners.
“From the hard edge of the building, we were able to secure the Air Rights over the Amtrak right of way,” says Harris. “We decked over it to create 2.5 acres of public greenspace that conceals the parking structure, lobby area and loading dock enclosing about 28% of the site. Though the building is extremely vertical, the site is quite horizontal—both presented equations to solve.”
The investment was significant and stretching every dollar to improve pro forma is rarely a waste of time. Goettsch Partners originally planned to employ hollowed slab-on void construction to build-up the site topography, but when value engineering analysis revealed the complexities of using that much site concrete, it was too cost-prohibitive. Wolff Landscape Architecture’s experience with an alternative, lightweight, structural-fill was extensive, and Soncrant proposed Geofoam as a very workable surface substrate substitute.
Working with Geofoam enabled designers to create a custom contour of substrate material in the exact depth need below specific panting areas. Since a tree may need a soil depth of several feet, a shrub some 18 in., and grass just 6 in., building a Geofoam base that accommodates appropriate soil depths decreased the overall dead load on the structure and supported controlled, positive drainage across the site.