Photo: Copyright Neoscape, Inc. 2017
capacity to accommodate 17 additional floors. To achieve expansive column-free areas, large spans in the office tower – some reaching 48 feet – amplified the demands at the base of the building where higher loads go to fewer columns. Conversely, a smaller number of original columns required retrofitting for the project. The solution implemented by WSP was the retrofitting of existing con - crete columns employing new reinforced concrete jackets. The first task was the assessment of existing conditions and the study of available construction documents. Although some original drawings were available, core samples were extracted from various locations to determine the in-situ compressive strength of concrete. WSP provided 3D laser scanning services for the entire structure, yielding an accurate representation of as-built column dimensions and locations, the latter being critical to tie in the column grid of the tower above. To minimize potential compatibility problems, concrete compressive strength for the retrofitting jackets matched the one determined through the coring campaign (approximately 5,000 psi). Furthermore, to minimize the increase in column size, large diameter high-strength rebar was used. Reinforcement continuity was critical to maintaining a consistent load path. Ground Penetrating Radar scanning was performed around each column, allowing WSP to map and coordinate locations for holes to be drilled for reinforcement to pass through the existing slabs. Large diameter (#14 bars) and high-strength (Gr 75) reinforcement were used to maintain jacket thicknesses under 12 inches, resulting in 60-inch-maximum-diameter columns. Column capitals were removed to achieve the desired reinforcement continuity and full column bear - ing but were rebuilt during the cast operation to maintain the original aesthetic, which was important to KPF’s vision. The forming and casting of circular columns in the building’s con - strained environment presented a potentially costly and troublesome prospect, so shotcrete was used for columns and capitals. Concrete was placed, compacted and consolidated all at the same time due to the design pressure with which the shotcrete was sprayed.
A Solid Foundation Although the existing structure was bearing on the good-quality sub - strate typical of Midtown Manhattan, the geotechnical composition of the site and its existing foundation elements presented a significant engineering challenge. The bearing capacity of the structure ranged from 20 to 40 ton/ft2, and there was a steep drop-off throughout the site. In addition, the con - straints of working within the confines of an existing structure were immediately evident, particularly for the use of deep foundation ele - ments and the equipment required for their construction. The reinforced concrete core is supported by a new 48- to 72-inch- thick mat foundation bearing directly on sound bedrock. With the core walls in-line with existing columns, consideration of existing column foundations added another layer of complexity. While some column foundations were narrow piers reaching the bedrock, a few columns were supported by larger pile caps. The latter were treated as local interruptions in an otherwise continuous mat foundation with cold joints only. The small footprint of the new core in combination with its eccentric lo - cation translated to large overturning moments, which were addressed by providing 45 600-kip anchors socketed 45 feet into rock. MRCE specified 450-ton micropiles socketed 15 feet into sound bedrock to achieve the required load demand. From these constraints emerged the development of three types of foundation retrofits: • piers-to-rock encapsulating and tied into existing piers-to-rock, • new caisson caps encompassing existing piers-to-rock, and • enlarged caisson caps articulating existing pile caps. Concrete Jackets One major challenge was the original building columns insufficient The addition of a 17-story tower to the original building has resulted in Hudson Commons emerging as a 26-floor building with 700,000 square feet of rentable office space in Manhattan. Photo: ©2017 NEOSCAPE, INC.
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