There is a new beacon of light in the Hell’s Kitchen area of Manhattan’s West Side, bringing together residents and tourists alike. In the midst of the recently renovated Jacob Javits Convention Center and ongoing construction of the Hudson Yards development project stands a new iconic “light column” within the recently opened Hudson Park. The park, which is open to the public and built at an estimated cost of $27 million, stretches three city blocks from 33rd through 36th Street between 10th and 11th Avenues. Among the various amenities within the space, the light column itself stands on the north end of the site as a piece of public art used to stimulate excitement and interest in the park and the surrounding area.
The light column is a custom, 50-foot-tall, free-standing steel structure that is 3 feet in diameter. The structure of the light column is composed of 52, 1/8-inch-thick vertical plates equally spaced along the circumference. These plates were specified with customized profile depths, varying from 3.60 inches at their peak to 1.85 inches at their valley. This customization gives the light column an undulating/wavy profile per the architectural design.
The structure was detailed and fabricated as four separate sections that were welded together. The main reason for this was for fabrication logistics, but this also gave flexibility in fine tuning the stiffness properties of the structure. The plates themselves are held together with a proprietary V-shaped steel wire that is helically wrapped around the perimeter and welded to each of the vertical plates where they cross. The wires, which were developed for use in the screen and well industry, were wrapped with a tight spacing, which averaged a clearance of 0.033 inch between wires. The wire spacing was controlled to give the desired openness in the column to allow light to shine through, but was tight enough to provide the necessary strength and stiffness to resist the shear flow between the 52 plates, allowing them to act compositely as one section. Due to the fact that the light column is exposed to the elements, all the plates and wire were specified to be stainless steel.
As is typical with many large-scale exterior art pieces, the light column was fully engineered as a non-building structure and designed to withstand all the typical code-prescribed ice, wind, seismic, etc. loads. However, the controlling limit in this case was the resulting stresses from vortex shedding-induced harmonic resonance.
During construction it was anticipated that the light column would be wrapped for protection; more importantly, during its service life, ice could build up between the wires, given their extremely close spacing, and render the section completely closed. In either case, there was a concern that a closed circular section in an open area could be susceptible to vortex shedding. This is a phenomenon whereby bluff bodies are subjected to periodic across-wind normal forces due to the shedding of alternating vortices as the wind flows around them. For these vortices to form and shed in regular patterns, the flow tends to be laminar, which occurs at relatively low wind speeds. In most cases the wind speed where vortex shedding is possible is well below the code-prescribed 3-second gust speed, and therefore, there is a higher likelihood of the structure being subject to this type of loading during its service life. When the shedding of these vortices approaches the natural frequency of the structure, harmonic resonance is likely to happen, whereby large deflections and stresses occur.
For the design of the light column, the procedures developed by AASHTO for the design of highway signs, luminaires, and traffic signals was used, in which an equivalent static pressure is determined to mimic the vortex shedding-induced load effects. Various plate sizes and openness characteristics of the four column sections were studied to fine tune the natural frequency of the light column such that the equivalent static pressure from vortex shedding approximately equaled that which was induced by the standard code-prescribed 3-second gust. The final geometry, including variation in plate depth and wire spacing, resulted in a global natural frequency that successfully achieved this goal while still adhering to the desired architectural aesthetics.
Fabrication of the light column was no small task. A custom jig was fabricated to hold all 52 plates in the correct location and radial orientation. The jig was then placed into a screen fabrication machine, which is similar to a lathe, where the wire is continuously wrapped in a helical fashion around the section. During this process, the wires are resistance welded to the outside of the plates. The resistance weld was tested to confirm it had the capability to achieve the plastic capacity of the wire in bending as needed for the shear flow in the cross-section. Once completed, each section was joined together from the inside of the column in the shop by manually welding the plates together with complete joint penetration welds.
The light column was shipped as one piece and erected in place within a couple of hours. As expected, once it was installed it was protected in plastic wrapping until the opening of the park a few months later. In all, the park with its iconic light column has been well received by the community. The light column, which acts as a functional piece of public art, has achieved the goal of contributing to the identity of the area for residents and visitors alike.
Joseph Yamin, P.E., is an associate at Leslie E. Robertson Associates (LERA; www.lera.com) in New York City.