Council on Tall Buildings and Urban Habitat

HL23, New York
Posted March 2012
HL23 was recognized as a finalist in the 2011 CTBUH Awards Program.
Other Featured Tall Buildings
“HL23 is a building which celebrates its adjacency to the phenomenally successful High Line park, a stunningly beautiful resolution to the challenges of its compressed urban site.”
Richard Cook, CTBUH 2011 Awards Chair, Cook+Fox Architects

New York
54 m (176 ft)
Primary Use

23 Highline LLC
Alf Naman Developers
Design Architect
Neil M. Denari Architects
Structural Engineer
DeSimone Consulting Engineers
T.G. Nickel and Associates

The building addresses the adjacent High Line in a very successful way, rising from its ultra-compact site to create a very positive relationship with the elevated linear park. The building’s structure, skin and form come together admirably. Neil Denari has beautifully bridged the scale of his work to the level of a tall  building in a dense urban environment.

HL23 is a 14 floor condominium tower that responds to a unique and challenging site directly adjacent to the High Line at 23rd Street in New York’s West Chelsea Arts District, partially impacted by a spur from the elevated tracks that make up the High Line superstructure. 
Figure 1. Site Plan
The site and the developer demanded a specific response, yielding a project that is a natural merger between found and given parameters and architectural ambition. For the client, the question was how to expand the possible built floor area of a restricted zoning envelope. For the site, a solution needed to be found to allow a larger building to stand in very close proximity to the elevated park of the High Line. Together, the demands produced a building with one unit per floor and three distinct yet coherent façades.
With a custom non-spandrel curtain wall on the south and north façades, and a 3D stainless steel panel façade on the east facing the High Line, the project’s geometry is driven by challenges to the zoning envelope on the site and achieving complexity through simple tectonic operations.

The building was constructed on a very compact 12x30 m (40x100 ft) site. The volume of the building increases as it rises up.  The project features expansive column-free zones. Each floor makes up one residential unit, except for two duplexes at the 2nd and 13th floors.  The building also has a single-story basement level, a ground floor gallery space and a double-height lobby with artwork by artist, Joseph Kosuth.  Above the fourth floor, the structure projects outward, creating an undulating façade that cantilevers over the High Line.
Figure 2. Building Approach
The building’s south façade features a structural diagonal that serves to “hang” the cantilevered floors from above via the interior columns. Since the building sits close to the Hudson River, a unique concrete matt foundation with 12 rock anchors was used to counteract the pressures on the soil from such a close relationship to the water bed.  In addition, HL23 is anchored by a steel plate shear wall core with braces that allows for the building to cantilever in two directions.
With a captive audience in mind, the building is designed as a complex prism whose proportions, angles, and profiles continue to shift as one walks past on the elevated park. In this sense, although HL23 contains a private program, the building participates in the public experience of the park. The building thus oscillates between urban infill and object building, mediating the context of near and far views, which reaffirms its optically-complex characteristics. Materially and structurally, the building strives to reflect its prismatic geometry through exposed steel pipe columns and diagonals, and an undulating east façade clad in stamped stainless steel panels. 

Except for the cast concrete substructure and some interior finishes and surfaces, all components of HL23 were fabricated off site.
Figure 3. Stamped Stainless Steel Panels
With more work done off site, tolerances in most areas were reduced to a minimum, challenging each fabricator to work as close as possible to physical limits. Although prefabrication is a long standing ambition of the industry generally, here the efforts to raise the level of precision of the building’s components allowed otherwise complex conditions to be resolved more simply. In particular, the north and south curtain walls (produced in Dongguan Province, China) were shipped in mega-panel assemblies allowing for rapid erection and a uniform appearance across the folded surfaces.
Figure 4. North façade as viewed from the High Line

Related Links
CTBUH Skyscraper Center Profile:
HL 23

HL23 was recognized as a Best Tall Building Finalist in the 2011 CTBUH Awards Program.
Download HL23 2011 CTBUH Awards Book section

2011 CTBUH Awards Book

The CTBUH would like to thank Neil M Denari Architects for their assistance with this article. Photography © Rinze Van Brug Photography, Drawings © Neil M. Denari Architects.