At the heart of the $465m extension to the American Museum of Natural History in New York is a five-storey atrium quite unlike anything ever built before.
Designed by Studio Gang, the space rises like a canyon, with bridges, walls and arches all contoured as if hewn by the forces of time and nature – Gang apparently developed initial models using a blow torch and a massive block of ice. Since it opened in May, it has been described variously as an expression of the Earth, a windswept outcrop and a “Flintstone cave”.
But there is more to the Griffin Atrium than aesthetics. Built from shotcrete, this innovative structure borrows techniques from transport infrastructure and theme park design and brings them together with advanced parametric modelling. It’s a spectacular but pragmatic solution, designed to stitch together the institution’s patchwork of existing buildings, built over the past 150 years. In all, the extension includes 33 connections to 10 other buildings within the complex.
“The idea emerged of these flowing walls that guide people and create a sense of flow that had never been there in the past,” says Matt Jackson, associate principal at Arup. The geometry was also influenced by constraints on where foundations could be placed. “There's a lot of existing infrastructure, some of which had to remain operational throughout the project. There's a tonne of stuff buried there, so we knew we had to land in certain places. This led us to the system of arches and walls, forming the canyon look. Once we had that concept, the question was, how do we build it and get the aesthetic we want?”
The team considered various systems for creating the curved forms, including in-situ reinforced concrete, and precast cladding on a steel frame. Both of these were ruled out, says Jackson. “We were very much against cladding. The architect wanted what you see to be the actual structure. So it really is a canyon holding it up – this whole part of the museum serves as part of the lateral system.”
In-situ concrete could have acted as structure and finish, but would have required very complicated formwork: “We looked at CNC-moulded forms, but each piece would have had to be unique, and then thrown away, which would have been incredibly wasteful.” Almost by process of elimination, this led them to shotcrete – a method of applying wet concrete through a hose at high velocity onto a receiving surface, which requires no formwork at all. There was also a pleasing echo of the institution’s history: shotcrete’s inventor, a taxidermist called Carl Akeley, had worked at the American Museum of Natural History in the early 20th century.
But shotcrete has very rarely been used architecturally, and certainly not on anything as high-profile as a half-billion-dollar extension to a much-loved national institution. It is typically used in tunnel linings, which are structural but tend to follow a regular geometry, and theme parks or zoos, for more sculptural but non-structural applications.
To make sure it could do both, Arup used Grasshopper parametric software to determine the forces at play, rationalising the form in a back-and-forth process with Studio Gang. “The architects would define the flows that they wanted and where they wanted the circulation to go, and we would look at it and say, well, can we adjust this arch or lift this up a bit to make it work better?”
Everything in the resulting structure curves in three dimensions, with the walls undulating between 100mm and 600mm in thickness. Grasshopper scripts were also used to generate a 3D model of the reinforcement: “Every bar was mapped in space, which meant the contractor could bend them into the exact shapes and bring them to site tagged and ready for assembly,” says Jackson. “It was so complicated, we realised there was no other way.”
A number of mock-ups were constructed to get a better understanding of the performance characteristics of shotcrete. At first, these were small-scale models, exploring different textures and finishes: “You can float it, you can trowel it, you can make it super-smooth. But we ended up with a rodded finish where it’s just roughly screeded off.” A later full-scale performance mock-up included rebar and service routes, to ensure there would be sufficient consolidation around the reinforcement. A further series of trial sections were set up to check that every “nozzleman” on the project could achieve this in practice.
The more conventional elements of the structural frame were cast in situ. These included shear walls around elevators, perimeter columns on three sides of the 75m x 60m building, and flat slabs, which temporarily spanned from the perimeter structure to falsework until the shotcrete walls were in place. “We were originally thinking that we would shoot floor by floor, casting the slabs as we went,” says Jackson. “But partway through we realised that was not going to work with the schedule and we needed to take the shotcrete off the critical path. It took a lot of coordination to make sure that the slabs were supported until we had the shotcrete in place, and then work out how to transfer the load back to the walls.”
When it came to shooting the walls, the first step was to install a series of vertical pipe forms, following the general contours of the future walls. These pipes spanned from slab to slab, and were spaced at 300-600mm intervals. A flexible dense metal mesh was then attached to the pipes – this effectively serves as the formwork, keeping the shotcrete from spilling out from the back face. The rebar was built out from this mesh, each bar precisely located by the tagging system.
The nozzlemen could then proceed with a horizontal layer of a regular concrete mix, including 20% GGBS. There was no standard length of section, with sprays varying from 1.5m to 6m, depending on the complexity. Wall depth also varied, from a minimum of 130mm. The height of a lift was typically 2.4m. After the initial pours were complete, a second finishing layer, this time containing white cement, was sprayed. This was screeded off to the correct thickness using a system of guide wires.
The resulting atrium, naturally lit from above, is perhaps as organic-looking as 21st-century construction gets. The texture of the walls is rough and sedimentary, with as much of the building services as possible concealed within the structure. The surface texture of the shotcrete also meant that no additional acoustic treatment or sound absorption was required, adds Jackson, confirmed by testing the 3D model in a sound lab.
A lot of different things converge and connect in the Griffin Atrium: ten different museum buildings; a host of facilities including galleries, a shop, restaurant, library and immersive theatre; and the surprisingly overlapping histories of tunnelling, theme-park design and taxidermy. The common thread is a century-old method of spraying concrete. “When you walk in the space, what you see is just structure,” says Jackson. “I just love the way it brings it all together.”