The high-rise building located at 111 W. Wacker Drive in downtown Chicago was formerly known as the ‘Waterview.’ The structure was originally designed as an 89-story luxury hotel located along the Chicago River. The construction of the conventionally-reinforced building progressed until the 27th floor when the 2007 economic recession hit the U.S. Ultimately, the original project was shelved due to financing issues. The unfinished building became a visible eye-sore in the heart of Chicago for several years. Fortunately, a consortium of creditors and lienholders (Clark Wacker LLC) took ownership of the project and formed a Joint Venture with Related Midwest in 2010.
In 2012, the hotel was redesigned as a 60-story residential apartment building and renamed ‘OneEleven.’ There were several major design issues with converting a hotel into a 500-unit apartment with a smaller floor-plate and different column layout. Since the column were poured until the 28th floor, this floor was built as originally designed. The 29th floor was constructed with a 12-inch post-tensioned slab and functioned as an amenity level.
What separates this high-rise tower from others is the 60-inch transfer deck (located on the 30th floor). The transfer deck was designed to be a 72-inch conventionally-reinforced slab with the post-tensioned floors above. The PT Supplier and Concrete Contractor redesigned it into a 60-inch thick slab using post-tensioned to replace roughly 372 tons of rebar. The redesign saved around $380,000 from the cost of the transfer deck (excluding costs of other vertical elements). The reduced dead load of the slab also helped eliminate the reshoring requirements. The 12-inch building height reduction funneled into lower costs for all vertical elements and lower operational energy costs. A flat-plate was used in lieu of large beams and transfer girders in order to reduce forming costs.
The installation of the transfer slab and coordination of trades was extremely complex. Additionally, the coordination was fast-paced since the transfer slab was the second floor of the new construction. Construction trades and engineer spent almost one month on coordinating PT tendons, rebar and MEP piping/penetrations.
The transfer mat was poured in two lifts (20-inch and 40-inch lifts) for a total of roughly 2,250 cubic yards of 6000 psi concrete. One of the pours lasted 17 hours – and had an increased degree of complexity due to being poured mid-air. In total, there was 60 tons of unbonded-post-tensioning (gross weight) used in the transfer deck.
The use of unbonded post-tensioning in the original design was also used to reduce the high-rise building height by at least 1” per floor as compared to mildly-reinforced concrete. PT helped make a stalled project more economical due to the reduction in material costs (concrete columns/walls, MEP piping, elevators/stairs and curtain-wall).
Furthermore, vertical unbonded post-tensioning tendons were used in the shear walls from level 29 to level 40. The vertical tendons were stressed in two sections. Due to the lack of decking, the two sections of tendons were purposely spliced. Extra “standby” tendons were added for insurance in case the elongations of the other tendons fell short. According to the concrete contractor, “Utilizing vertical post-tensioning through the core walls proved to be a cost effective solution to meeting the structural engineer’s lateral stability requirements. While certainly a unique application in a vertical structural element, the system was straightforward to install and helped the concrete contractor meet the design and budget constraints.”
This project helped the developer win the NAIOP Developer of the Year and was featured on the cover of ‘ENR Midwest’ magazine. The developer eventually sold the property for $333 million (or $661,000 per unit) in late-2014. In total, over 170 tons of 0.5” unbonded post-tensioning tendons were installed within the 29th to 60th floors. Although not required by the Building Code or ACI Code, an encapsulated PT system was utilized in order to comply with the PTI revision to the Unbonded PT Specification (Addendum #3 PTI M10.2-00).
Copyright © 2015 by AMSYSCO, Inc. All rights reserved.