After reclaiming an long-abandoned elevated railway on Manhattan’s West Side and transforming it into a high-design, 1.45-mile-long urban linear park, The High Line has become an icon of contemporary landscape architecture and design. Seamlessly blending the built environment with the natural world, the pedestrian greenway fully restored the original steel infrastructure and tracks before adding precast concrete paver pathways interwoven with lush green landscapes.
Multiple entrances to the pedestrian park carry the design elements upwards. At its 30th Street West entrance, right outside Hudson Yards, McLaren’s facade team provided design and structural engineering for a free-standing monumental stair that seamlessly melds steel tube stringers, laminated glass guardrails, and precast concrete treads into a thin and narrow profile.
To create the long span stair, a portion of the existing High Line track was removed to provide an opening from which the structure could flow, column free, to the street level with minimum bracing. The team took into account loads, deflection, and vibration criteria, to create custom design solutions that would achieve invisible connections between elements.
In order to limit the deflection to acceptable limits without deviating from the architectural vision, a fixed connection between the stair treads and the stringer was used. This resulted in the stringers and treads acting as a Vierendeel truss, significantly reducing the out-of-plane deflections. As such, the members are joined together rigidly and each is required to transmit bending, shear, and direct stress. The design choice serves to protect the glass and specialty work from any risks associated with the flexibility of the long, unbraced span. To help achieve the needed strength and connection support, precast concrete treads were custom designed with fully engaging, low-profile rebar inside.
1 inch thick, laminated, heat-strengthened glass guardrails run along each side of the staircase from street level up to the bottom flange of the High Line framing at the midspan of the staircase. Two of the sloped glass guardrails are supported by stainless steel stanchions at each end, which frame into support beams below. These support beams span across both stringers and are attached via welded connections.
At the base of the monumental stair, the sloped glass guardrail is cantilevered off the stanchion on one side. At the ‘upper’ glass lite, the glass guardrail is supported by a stanchion at one end, by two button fittings and a glazing shoe at the other end. The glass supports are adhered to the face of the glass using Transparent Structural Silicone Adhesive (TSSA), a product that is not widely used in the US, in order to avoid penetrating the glass at any point and allowing for a low-profile attachment that does not protrude from the exterior face of the glass. These button fittings are custom designed to hide within the stair treads. The fittings allow for flexibility and rotation of the glass to prevent the fragile material from becoming over-constrained by its attachments to the stair system at the bottom and to the rigid High Line framing above.
To achieve a column-less, freestanding stair design, The High Line stair is uniquely connected to the street level below and the elevated rail system above. At the base of the stair, the stringers are supported by a roller pin connection, which allows for lateral displacement due to loading and thermal movements. At the top of the stair, the stringers are reinforced by a bolted connection that ties directly to the concrete slab at the High Line using post-installed anchors. At the lower landing, the stair system is hung by stainless steel hanger rods, which tie into the High Line framing above, adjacent to an expansion joint in the High Line system. Careful consideration of differential movements between the High Line framing and stair supports was incorporated into the design, to ensure the systems are able to expand and contract without impacting one another.