Why HL23 Leans Over the High Line

At West 23rd Street, the High Line passes a building that seems to reach toward its visitors. Glass walls angle outward. White steel diagonals cross behind the windows. Above a constricted lower section, the apartments spread into space that appears to have no support beneath it.

This is HL23, architect Neil Denari’s residential tower beside the elevated park. Its unusual silhouette answers a very practical question: how do you make usable apartments on a narrow Manhattan parcel already complicated by a railway structure? The answer involved expanding upward, securing special planning approvals, and making the supporting frame part of the architecture.

Where it is

HL23, 515–517 West 23rd Street, New York, NY, beside the High Line near Tenth Avenue.

The address and the building’s contracting and expanding profile are documented in Architectural Record’s project account.

A railway in the way

The site measures approximately 40 by 99 feet at ground level. That would be a tight starting point even without the High Line. Here, a spur of the elevated railway also intrudes into the site, leaving the architect to work around infrastructure that predates the apartments by decades. Denari’s practice describes the project as an effort to increase usable floor area while finding a form that could sit comfortably beside a busy public park. Its Mies Crown Hall Americas Prize submission records both the dimensions and the railway conflict.

Developer Alf Naman commissioned Denari, whose practice was based in Los Angeles. Construction was announced in March 2008, while the railway’s conversion into a park was still underway. HL23 was therefore being designed alongside a changing public landscape, with future park visitors becoming an audience for the building’s architecture. Architectural Record’s contemporary report identifies the developer and describes the project at the start of construction.

That timing matters. The High Line’s first section opened in June 2009. Its second section, extending from West 20th to West 30th Street, followed in June 2011, bringing the park past HL23. A former freight route was becoming a place where people stopped, looked into the neighborhood, and encountered buildings at an unusual height. The city’s announcement of the second section describes that new relationship between pedestrians and the surrounding architecture.

Making room above the tracks

The familiar Manhattan setback makes a building smaller as it rises. HL23 reverses that expectation, gaining width above its pinched lower section before its surfaces change direction again. The effect is less a single leaning wall than a sequence of carefully adjusted planes.

Early published dimensions help explain the visual trick. Architectural Record described a lower midsection approximately 32 feet wide and an apex approximately 45 feet wide. That is an increase of roughly 41 percent between those two sections. The same account described a ground-level base that widened to 40 feet, so the dramatic percentage applies to the narrower section above it, rather than the entire footprint. The 2008 report also identifies a waiver from the usual 15-foot setback beside the High Line.

The shape required a negotiated approval. According to the architect’s prize submission, City Planning granted seven zoning waivers. That distinction is essential to understanding the building: Denari developed an alternative volume that needed explicit permission. The approval allowed a larger building to occupy this difficult site while giving the park a conspicuously sculpted neighbor. The project submission sets out that design objective and approval history.

The steel behind the apparent imbalance

Moving floor area outward creates a structural problem as well as an architectural opportunity. Loads from projecting floors need a continuous route through the frame to the foundations. An asymmetric arrangement also creates overturning effects that the structure must resist.

Structural engineer Stephen DeSimone discussed that challenge in Architectural Record’s April 2009 technology coverage. A steel frame reduced the weight associated with the building’s expanding upper portion. Canted columns and diagonal bracing made the unusual geometry possible, with some columns inclined as much as 24 degrees and the braces formed from eight-inch pipes. Those are substantial working members behind the apparent lightness of the glass. Architectural Record’s engineering coverage explains the structural approach.

The diagonals are especially useful because they turn portions of the frame into triangles. A rectangular frame can distort sideways unless its joints or other elements resist that movement; triangulation supplies another means of maintaining its shape. At HL23, the visible braces help make the engineering legible from outside. The building’s apparent imbalance is supported by an organized system of connected members.

Three faces for a public setting

The building also changes character as you move around it. North and south, large glazed surfaces expose the diagonal steel. Facing the High Line, stamped stainless steel and smaller openings create a more enclosed surface. That difference responds to a peculiar condition of elevated parks: passersby occupy space beside the building well above the sidewalk.

The Council on Tall Buildings and Urban Habitat’s project account describes HL23 as both an infill building and a freestanding object, experienced through near and distant views. It also explains how extensive off-site fabrication supported the complex geometry. Large curtain-wall assemblies arrived as prefabricated panels, making precision across the folded surfaces a central construction concern.

Inside, the residential arrangement is more varied than a simple apartment on every story. Architectural Record documented nine full-floor homes, a duplex penthouse, and a ground-floor maisonette with a private garden. The outward expansion translates into usable interiors, while the changing perimeter gives the homes different relationships to the park. Its 2011 account describes the residential program.

HL23 makes the pressures of its site visible. The old railway occupies valuable space; the apartments seek more room; the frame carries that room outward. Seen from the High Line, the result turns a difficult property boundary into something pedestrians can read in glass, steel, and changing angles.

Sources

• Architectural Record: HL23, April 2008
• Mies Crown Hall Americas Prize: HL23 project submission
• New York City: Opening of High Line Section Two, June 2011
• Architectural Record: Architectural Technology, April 2009, page 120
• Architectural Record: HL23, September 2011
• Council on Tall Buildings and Urban Habitat: HL23