Robert Silman Associates were in charge of stabilization, repairs and other engineering services for the original structure, including a survey of the High Line’s existing condition pre-park. Having endured decades of aging and abandonment, the elevated rail was in bad shape; significant portions of the steel structure had rusted away, and some of the concrete had crumbled off.
To repair the riveted structure, doubler plates have been bolted with 7/8” diameter bolts over deteriorated sections of the web. Where extensive damage limited the stability of the beam flanges, the sections were cut back, and new double angles bolted on. In cases where stairs cut through existing girders and stringers, engineers had to keep the ends of cut beams in order to preserve the riveted connection with other members; this resulted in an aesthetic that showcases the sawed-through structure as one ascends to the park, playfully highlighting the structural heritage preserved on site.
Starting from the rehabilitated structure of the repaired High Line, Buro Happold’s role focused on structural engineering supporting new landscaping and architectural features. The most challenging aspect of the design is the difference in thermal expansion and contraction of existing and new structures, and possible deflections due to uneven loading conditions. A rigorous system of expansion joints at every 138 feet is installed, falling in line with the original structure’s repaired joints. At these locations, 12 foot long precast concrete paving planks atop sleeper beams 6 feet apart create a saw-tooth joint, where each paver moves to the opposite side of the joint as its neighbors. Each joint is covered with a thin aluminum plate, concealing the structural gymnastics from visitors.
Stairs and elevator shafts are accommodated by cutting existing steel girders, slabs and beams, requiring a high level of coordination between Robert Silman and Buro Happold. In most cases, main structural girders are preserved and only minor beams removed. But on 14th Street, stairs cut through critical girders and an elevator sits along an expansion joint. The team resolves this by building the elevator completely on one side of the joint, using its framing to transfer loads from the cut girder to the ground.
Some key features of the High Line’s first section required all of the engineers’ ingenuity. Tenth Avenue Square, which descends as a bleacher through original girders, requires extensive reinforcement and the establishment of a new framing system. The transformation of this space into a viewing platform is made possible by cutting out web openings in an existing 10-ft deep girder. Remaining steel is reinforced by box beams, mimicking a Verendiel truss to transfer necessary loads and allow views of the street.