WSP USA was in for quite a challenge when it signed on as the structural engineer for the renovation of 441 Ninth Avenue, but not an insurmountable one.
The building, originally a warehouse that opened in 1962 and stands eight stories tall, had recently been acquired by Cove Property Group, which planned to vertically expand the property. CPG wanted to do so while maintaining a maximum rentable area, which required precise and strategic reuse and rehabilitation of the existing 55-year-old building.
This isn’t your average upward build; WSP was tasked with constructing a modern tower atop the eight-story concrete structure, a project that required not only the merging of old and new architecture, but also the mixing of steel and concrete, all while satisfying the client’s desires.
“The primary challenge is, how do you take a building that was constructed a while ago and add another 17 floors to the existing eight?” said Jeffrey Smilow, executive vice president and managing director of U.S. property and building structures at WSP. “That’s more than double the number of floors—how do we do that when we know it might not have the capacity to hold that up?”
As a result, a number of challenges arose in the construction process, mainly concerning the upgrade of the building’s columns for maximum support.
“A 125-foot-by-25-foot area inside the existing building was demolished from the ground floor to the roof to allow for the installation of a new reinforced concrete core, which provided adequate lateral stability and stiffness for the new, taller building,” according to a statement from WSP. “To achieve this, the existing floor slabs around the new core were reinforced with steel members above and below the slab, some of which remained in place as part of the new core.”
In addition to this, the construction team removed two existing columns from the building’s interior.
Because the existing structure would now be supporting an additional 17 floors, the most important part of the mission was providing increased support to the existing columns so they would be able to hold the new weight. To do this, the columns were retrofitted with shotcrete jacketing and higher capacity steel bars, to ensure that the column jackets did not get to be too large.
“With high pressure application of the shotcrete, we got in very tight crevices and spaces,” Smilow added. “It performed very well, and aesthetically it looked as good as regular concrete.”
Because the steel tower sits on top of the existing concrete tower’s roof, the column grids for each structure had to be meticulously in line with one another. This made it so that no heavy transfers were required, and it let the builders avoid elevation differences.
In addition to the interior changes, another feature included the addition of outdoor green spaces for tenants to enjoy. The ninth and 25th floor now house landscapes of grass, trees and outdoor seating to give the building a little something exciting and new. On top of this, almost every level of the updated structure is fitted with terraces or balconies to give tenants floor-to-ceiling views of the city and river, opening the space and providing natural light.
Though the project was a difficult one to complete, the end result was a satisfying mix of history captured by the classic, original concrete structure and modernity brought in with the steel upper levels and green areas.