By Joseph Provenza, AIA, P.E., LEED AP BD+C; Jeffrey Smilow, P.E., F. ASCE; Yujia Zhai, P.E., Motaz Elfahal, Ph.D., P.E.; and Gerardo Aguilar, Ph.D.
What began in 1962 as a functional-but-unassuming eight-story warehouse on Manhattan’s West Side has become the foundation of one of the most unexpected and fascinating projects in New York City today – the 26-floor Hudson Commons building.
Over the years, the cast-in-place concrete building at 441 Ninth Avenue has been reimagined; notably in the 1980s when it was converted into an office building that retained most of the architectural character from its original era.
But it never experienced anything quite like its most recent transformation.
When Cove Property Group acquired the commercial property in 2016, they envisioned a vertical expansion of the site to capitalize on the 2005 rezoning of Manhattan’s west side. That left the developer with a decision to make on the fate of the former warehouse: Would they demolish the old building and rebuild from a clean slate, or would they find a way to reposition the existing structure to meet their needs?
Early site investigations showed that the robust structure was in good structural condition making it suitable for reuse. Thus, after weighing their options, Cove selected the most cost-efficient solution that would achieve the maximum rentable area: the rehabilitation, retrofitting and reuse of the existing structure. The existing building would add 17 floors and 300,000 square feet of commercial space, creating a sleek new steel and concrete office tower.
Now that its transformation is complete, Hudson Commons emerges as a 26-floor, LEED Platinum Class A property that provides 700,000 square feet of rentable office space, topping out at 421 feet.
The architect of the renovation and addition is Kohn Pederson Fox Associates (KPF), with WSP USA serving as the engineer of record. Mueser Rutledge Consulting Engineers (MRCE) served as the geotechnical consultant. The construction manager is Pavarini McGovern (PMG).
Best of Both Worlds
For the project to succeed, several challenges had to be addressed, from precise demolition procedures without the use of interior shoring, and retrofitting of the existing columns, slabs and foundation systems; to the articulation of the new building core.
Shoring can be costly, so it was important to the owner that the project included a plan for shoring-free demolition of a 125- by 25-foot area throughout the existing building to accommodate the new core. In lieu of shoring, the existing slabs were reinforced with a combination of temporary and permanent steel members that provided the required support of the structure.
The original structure is comprised of two-way concrete slabs on a 24- by 28-foot grid with drop panels, “mushroom” capitals, and a masonry core providing lateral stability. The low-rise, massive building is representative of the 1960s.
The addition takes the form of a sleek modern office tower, which speaks to the 14-acre Hudson Yards megaproject development rising to the west of Hudson Commons. By contrast, the subtle renovation of the original building, which reuses the existing wrap-around brick façade while upgrading the structure to meet current design and construction codes, will keep the project grounded in the context of the neighborhood and its 1960s roots.
WSP faced several unprecedented challenges that demanded highly innovative structural solutions if the vision developed by Cove and KPF was to be realized. The existing cast-in-place columns and footings required sizeable retrofits for the gravity loads imposed by the new tower above, and the existing roof slab required extensive retrofitting to accommodate a heavily landscaped amenity space.
The most invasive feature, however, was the addition of a new reinforced concrete core linking existing and new construction that would provide the required lateral stability and stiffness for the new taller building.
A Solid Foundation
Although the existing structure was bearing on the good-quality substrate 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 constraints of working within the confines of an existing structure were immediately evident, particularly for the use of deep foundation elements 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 location 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.
One major challenge was the original building columns insufficient 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 concrete 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 bearing 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 constrained 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.
The final layer of concrete was applied by a skilled technician using a hand trowel, an aesthetic treatment that left the final appearance indistinguishable from traditional methods. This application by the construction team was a genuinely innovative feature of Hudson Commons.
To provide stability to both the existing structure and the new tower, a full upgrade of the Hudson Commons lateral force resisting system was required. WSP collaborated closely with KPF on an architectural design that included a new circulation and mechanical core eccentrically placed along the north side of the property that would maximize the building’s rentable area.
One notable feature of the building’s core is its prominence in the architectural expression of Hudson Commons as it rises above the existing building. The new spine of the building uses exposed architectural concrete as a tribute to the historic architecture of the neighborhood.
The new reinforced concrete core runs from foundation to the top of the building and is comprised of 10,000-psi concrete shear walls ranging from 12 to 24 inches in thickness. WSP envisioned a box-like configuration wrapping the entire mechanical and circulation program that would counteract the torsional effects associated with an eccentric core. In this closed-box layout, the core is placed along existing column lines, and the existing columns create breaks in the shear walls analyzed by considering individual piers at the base building.
One intrinsic benefit of this approach was the absence of link beams in the base building, which allowed maximum flexibility with regards to routing services out of the core. Above the existing roof, the walls were connected through reinforced concrete link beams to provide adequate lateral stiffness. Per industry standards, three-dimensional finite element analysis software was used to model both the existing and new structure, allowing for an optimized and efficient structural design.
WSP’s retrofitting approach for the gravity system included the reinforcement of existing concrete slabs with steel members around the perimeter of the area to be demolished for the new core. Steel members were installed above and below the slab following the bending moment demand associated with gravity loads. Top steel members were removed after casting the core walls, while steel members below the slab were fitted with studs to trigger composite action and remained as permanent bracket connections between the new core walls and the existing slab.
This innovative solution resulted in significant savings in terms of both cost and schedule.
Another feature included in the project is the potential to turn outdoor open areas on the ninth and 25th floors into green spaces for the tenants.
The existing structure has been reinforced and designed to allow for landscaping that could include grass, trees and other green uses. It will be the decision of the tenants on those floors, but the structure has been designed to allow for the creation of something quite elegant on those spaces.
WSP worked closely with PMG (Construction Management) to determine the most cost-effective approach to accommodate green spaces on the existing roof. The final structural solution was a secondary support system of steel beams installed under the existing slab to support the significant load of a fully landscaped area on the original building’s roof. The structural solution was detailed in such a way as to not penetrate the existing roof, which would have added significant cost to the project.
New steel members were designed as non-composite sections to eliminate the need to perforate the roof for stud installation, while clips were installed along the beams to prevent lateral-torsional buckling.
Nearly every level of the new building features terraces or balconies providing remarkable views of the city and river. Vertical integration between floors is promoted by the exposed stairway, which also allows for an exceptional naturally-illuminated interior. Floor-to-ceiling windows further welcomes daylight coverage on each floor.
The green roof, daylighting architecture and plans for efficient energy and water systems are features that lead to Leadership in Energy and Environmental Design (LEED) Platinum certification from the U.S. Green Building Council, and Wired Platinum certification.
Hudson Commons was completed in March of 2020 and tenants are now occupying a building that challenged conventional engineering and construction practice. The building has become one of the most innovative adaptive-reuse projects in New York City.
This reimagined commercial building demonstrates the potential for large-scale vertical expansion when innovative solutions are applied. With developers and cities striving towards sustainable solutions, the relevance of projects like Hudson Commons is clearly recognized.
One rarely comes across a project like Hudson Commons, mixing steel and concrete in a way that perfectly blends the two, taking advantage of the benefits each material offers economically and in an engineering sense. This project provides developers with a methodology and alternative cost-effective, sustainable solutions that could change the way existing structures are upgraded for future use.
Joseph Provenza is a senior associate at WSP USA Buildings Inc. and served as the firm’s project manager for Hudson Commons.
Jeffrey Smilow served as the principal-in-charge.
Yujia Zhai was the project director.
Motaz Elfahal served as the structural analysis manager.
This article was originally published in January 2021.