A cast stainless steel "puck", swaged onto pairs of 1/4-inch diameter stainless steel cables, was the pivotal component in the design of the cable-net structure which supports the Sky Reflector-Net. This unique solution, perfected exclusively for the project, brought the artwork to life.
Cold-Forming and Net Fabrication for Sky Reflector-Net
The Sky Reflector-Net (2013) is a cable-net supported sculpture designed in collaboration by James Carpenter Design Associates (JCDA), Grimshaw Architects, and the engineering firm ARUP. It is intended to make light and daylight an integral feature of the Transit Center’s identity. Inspired by sky-lit classical structures like the Pantheon, JCDA proposed a complex, three-dimensional cable structure to support perforated metal panels carefully treated with reflective coating in order to re-direct sunlight through the Transit Center’s entry sequence all the way to the subterranean subway platforms. 1
A multi-facetted design team including Grimshaw Architects, Arup (structural engineering) and Enclos Corp (glazing and specialty metals subcontractor) analyzed the concept for structural specifications, created preliminary connection hardware designs, and developed an easily managed digital model. Through completion of this preliminary work, the team determined that the net support-system should consist of pairs of 1/4-inch diameter stainless steel cables, joined at each crossing location (“node”) by stainless-steel hardware designed to lock the cable pairs into a precise geometry. The hardware also needed to provide a seamless connection for the aluminum reflector panels to make the project a 79-foot tall reality.
TriPyramid Structures tested two concepts for this “node” hardware—a clamp-style fitting and a custom swaged fitting.
Using empirically established coefficients of friction for the cable, the clamp was designed for expected loading conditions and optimized for minimal diameter and thickness. Destructive testing confirmed the analysis, but also revealed a surprising disadvantage for this model: Positioning the clamp fittings introduced human error which could easily exceed the 1/16 inch tolerance required to ensure a proper fit of the aluminum panels. Although the clamp fitting could be adjusted on site, the infinite loop of fine-tuning 1,680 clamps would be a significant challenge, given that the complex geometry of the net can only be verified with tension applied to the structure.
Swaging is a common application of cold working, a method of plastically deforming metal in a controlled fashion at room temperature to create new geometry and stronger, harder material in the affected area. When used to apply fittings to stranded cables, the cable end is inserted into the swage shank. The shank is then cold-formed around the cable. In the swaging process, the material of the swage fitting literally flows into the cable interstices, forming a mechanical lock between the two components. This fabrication method is used to connect many of our wire rope and structural strand end fittings.
Military specifications used for standard swaging applications of cylindrical components are designed to ensure that the cable breaks before the swaged fitting fails (MIL-DTL-781, MS21259J). A standard swage-stud length is designed accordingly, but was too long for the aesthetic intent of this project. Through destructive testing of modified studs, Tripyramid established that the swage length could be cut in half and still withstand the expected loads with appropriate factors of safety.
Using the test results, TriPyramid created geometry for a cast stainless steel “puck” and a custom die-block to compress the puck to a prescribed thickness. This unique solution, perfected exclusively for the Sky Reflector-Net, brought the artwork to life.
To successfully fabricate each of the 112 cable pairs used to form the net, TriPyramid first designed and fabricated custom equipment on which to proof-load and pre-tension the cables and set the pucks. This machinery used a laser-guided, hydraulic assembly to position and swage each puck. The final quality control verification was completed with a secondary set of laser measuring equipment while the cable assembly was still under tension on the rig. All 15 pucks per cable pair were located within a 1/16 inch of ideal geometry (provided by Enclos Corp), with no fear of parts shifting during assembly or transit.
When all the cable pairs were complete, the TriPyramid team assembled the net in flat form, creating the 150-foot-long and over 70-foot-tall skeleton for the finished artwork. By carefully rotating each cable pair about the central bolt at the crossing nodes, the team was able to “bundle” the massive structure into a manageable form. All 5,500 lbs of American-made stainless steel were manipulated such that the ends could be laced together to complete the tube shape before shipping. The finished product was loaded onto custom rolling carts and moved outside for a test lift, orchestrated by Enclos Corp.
To prove the installation method planned for the job site, the Enclos crew, assisted by TriPyramid employees, connected the ends of each cable pair to an aluminum lifting ring (by Enclos). The ring was center-picked by a rigging crane and hoisted, using great caution, until the entire net was off the ground
The extreme weather conditions which were endured during the lift more than confirmed the sequence and technique. Finally, the net was “re-bundled”, wrapped and transported to site.