New York City’s Little Island project shows precast is an innovative and flexible solution.
By Kirk Stelsel
The iconic children’s story “The Little Engine That Could” extolls the virtues of bravery, perseverance, optimism and dreaming big. The little engine takes on the seemingly impossible task at hand by staying focused and repeatedly building self-confidence one “I think I can” at a time.
The project team for New York City’s Little Island project has demonstrated those same virtues in order to get over its proverbial mountain. The journey has not been easy. It required everyone involved to dream big and believe they could succeed in the face of seemingly endless challenges. Now, they have crested the hill and are ready to enjoy the journey down the other side.
Barry Diller may be best known for founding the Fox Broadcasting Company, but he also is no stranger to philanthropic endeavors. He and wife Diane von Furstenberg’s work took center stage in 2013 through a bold commitment to the Hudson River Park Trust. Their gift kicked off the effort to create a beautiful park out of thin air over the Hudson River.
The area of Manhattan the park serves has an extreme shortage of parks per capita. In fact, it’s the second worst “park desert” in the city. Hudson River Park Trust has been working to address this, and Little Island is its most ambitious project to date. It is a space where people can pause to play. In addition, visitors can take advantage of park’s extensive gardens and amphitheater.
Diller sought the advice of leaders in art and engaged internationally renowned architects Heatherwick Studio and MNLA to dream big for what the space could be.
“This is an intricate garden that is an oasis where you step away from the city, because you cross over the water to get into it,” said Celine Armstrong, ASLA, LEED AP, project executive with Little Island. “The design is very meticulous. Everything was thought out so carefully to create an experience for the visitor that is unlike anywhere else.”
The journey from concept to completion was not easy. The project found itself in limbo numerous times, but in the end the momentum and desire were simply too great to let it go.
One of the major design focuses was the support structures that are known as the “pots.” These pots not only are the most powerful aesthetic component but an engineering marvel brought to life by Arup, a global design, planning and engineering firm, and Fort Miller, a precast concrete manufacturer in Schuylerville, N.Y.
David Farnsworth, P.E., a principal with Arup’s New York office, said Arup has worked with Hudson River Park Trust for the past 20 years to redevelop the waterfront and create lasting solutions.
“We all wanted to focus on finding good, durable solutions to the challenges of building economically in the Hudson River environment, which is a saline environment,” he said. “The solution we all arrived at after these 20 years is precast concrete.”
Precast concrete piles have been a go-to for Arup, but the volume and complexity of the precast in Little Island far surpasses previous projects. The designers knew the pots would require a completely custom product. The island, while appearing to be square from above, undulates from 15 feet at its lowest point to 65 feet at its highest. This design protects against flooding – which was fresh on everyone’s mind at the outset thanks to Hurricane Sandy – keeps the ampitheater’s sound in, allows natural light to reach the marine life below, and lets visitors enter under pot structures on the southern accessway bridge.
Fort Miller was chosen to manufacture the precast concrete pots, column heads and the pie-shaped planks that act as a stay-in-place form for the cast-in-place beams and mat. Coastal Precast in Chesapeake, Va., manufactured the precast/prestressed piles.
Joe O’Malley, sales engineer with Fort Miller, knew creating the pots would be a tough task filled with company firsts, but Fort Miller felt confident it could complete the job.
Since Fort Miller’s production facility is not on the river, Arup broke the pots into petals, sized to ensure they could be transported on a flatbed to the Port of Coeymans without police escort. There, Weeks Marine assembled the petals into completed pots and barged them to the job site for placement.
Arup and Fort Miller worked together to create the complex shapes with the desired durability, color and finish. Arup served as the structural engineering expert, and Fort Miller was the concrete expert. Fort Miller sent many different mix designs out for testing. There was a coulomb test for the saltwater environment, a freeze-thaw test and other tests for durability and serviceability. In addition, Diller personally visited Fort Miller’s plant to see two full-scale mock-ups in its yard, where each petal showed him a different finish and color. In the end, the team chose a mix design consisting of white cement, slag and a corrosion inhibitor, among other components.
Arup’s 3D models were sent to Fort Miller, where each file was translated onto high-density foam using a hot wire machine to cut out the basic shape and a robotic, multi-access milling machine to create the complex, 3D curved shape. Each form was made of multiple foam pieces that were assembled and sprayed with a polyurea coating, which provided the smooth finish and enough durability for multiple uses.
Farnsworth said the engineering team used parametric software to generate the internal shapes. This work included not only the pots, but every single piece of rebar and stainless steel embed, which were all clickable in the software to determine bends and cuts.
“Every bar had a tag on it, and we’d take all the rebar and explode it from the 3D model and lay it flat in a virtual yard with its own unique identifier,” Farnsworth said. “They could go into a 2D CAD drawing that had every single bar laid flat and they could print off and measure to do the bends. They used RFID tags for every single piece of rebar that goes into the project so they could click on the bar in the 3D model or scan the tag on any of the rebar that had been built and search for that in the model.”
Arup’s designers grouped the pots into 39 categories, each with subcategories, to create as much repeatability as possible.
“We didn’t want to have a bunch of square precast pieces, and we realized we could plan repetition by utilizing something called a Cairo pentagonal tiling pattern,” Farnsworth said. “You could create this whole shape with effectively six pieces of tiles. But, of course, there’s the elevational variation as well and the other tricky parts that end up being created around the unique geometry of the amphitheater and the fact that the southern accessway bridge comes under the pots.”
By adjusting the petal form closure points, Fort Miller was able to cast the various wall heights to create the desired elevation change. While Arup made every effort to minimize the customization, in the end each petal had some unique factor.
“It was 655 petals, 132 pots and 132 column heads,” O’Malley said. “Within those 39 different types, you might have six of a type-six pot, where they are all different but the basic structure is the same. Truly, there’s not one petal that is identical to another.”
Despite the complexity and being a first for everyone involved, including site contractors, the project ran smoothly.
“This was a true partnership between Fort Miller and Arup,” O’Malley said. “They worked with us every single day when we got into the production.”
The feeling was mutual across the entire team.
“We found a phenomenal partner in Fort Miller,” Farnsworth said. “They were up for trying new things and brought a lot of detailed precast concrete expertise to the project. The integration and collaboration between the design engineering and the fabricators and even the erectors was really fantastic.”
“I am so thankful we had Fort Miller as a partner because we needed someone who could create large elements that were beautiful,” Armstrong added. “Contracts are important, but you build a job on relationships, and you need a team that understands that you will be fair, but you also expect them to step in and help out regardless of what phase you are at in the project. One of the biggest joys of being on this project was seeing all the egos checked at the door, and we all rolled up our sleeves and figured it out together.”
Once the piles and pots were set, the onsite contractors tied the pots back laterally to each other and then cast and installed the beams and rebar for the top layer of concrete. Landscaping, the amphitheater and other flourishes added the proverbial icing on the cake.
Armstrong, who has been a part of the project since the start and has been at the site for every milestone, could not be more excited.
“It will be a space where people will love to come together once it’s safe to do so and enjoy art in the neighborhood,” she said. “It’s incredible to see the final product look like the rendering because in my experience that isn’t always the case. To see it come together is spectacular.”
Kirk Stelsel is the former vice president of communications and public affairs at NPCA.