Hudson Commons is a 700,000 square feet renovation/addition project situated on 34th Street and 9th Avenue in New York City being developed by the Cove group. KPF serves as the design and executive architect. The existing 8-story cast-in-place concrete building is to be renovated and retrofitted with 17 additional floors of steel construction on top of the existing building. A new cast-in-place concrete core is threaded through the existing structure extending from the foundation to the new roof to serve as the lateral load-carrying system of the new building. The project is planning to qualify for LEED Platinum. This presentation will address the challenges and constraints of working within an existing building from demolition to retrofitting and construction of the new overbuild.
Notable features of the project:
1. Demolition: A 125 feet by 25 feet area of the existing building was demolished from ground floor to roof to allow for a new concrete core. The demolition was done without the use of interior shoring. To achieve this, the existing floor slab around the new core was reinforced with steel members above and below the slab.
2. Column Retrofitting: To support the increased load of the new tower above, existing concrete columns were retrofitted with cast-in-place column jackets with large diameter bars continuous through the existing floor slabs. Shotcrete was used to place the column jackets. Existing column capitals were rebuilt to mimic the aesthetic of the original columns.
3. Foundation Retrofitting: Existing column foundations (pile caps) were retrofitted to support the new loads of the larger building. Column foundations near to or within the footprint of the new core were articulated with a new cast-in-place foundation mat bearing on rock, which included rock anchors to control uplift.
4. New Superstructure: The new steel tower sits on the top of the existing roof. The steel tower above has a different column layout from the existing structure, optimized for the tower footprint. To maintain a continuous load path, steel columns are sloped from level 9 to level11 avoiding large transfers. Built-up beams on the 9th floor transmit the horizontal forces from the sloped columns to the core and reconcile elevation differences from the existing/new building.
5. New Green Roof: A new landscaped green roof which will serve as amenity space on levels 9 and 25. To support the associated load of a fully landscaped area on the existing roof at the 9th floor, the existing slab was retrofitted with a new steel sub-structure below the existing slab and detailed to avoid the penetration of the existing roof.
6. Lowered Ground Floor: The ground floor was partially lowered to allow for new programmatic spaces with taller ceilings (lobby, amenity areas, etc.). The existing slab was reinforced with steel below, and a new steel structure was installed for the new lowered ground floor area.