A Structural Habitat in Seismic Seattle

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To meet the complex design requirements of the newest addition to the Seattle Aquarium campus, a two-story, 50,000-square-foot “Ocean Pavilion,” structural engineers at Magnusson Klemencic Associates (MKA) designed a massive concrete marine habitat at the heart of the project that sets new benchmarks for what is possible in concrete design.

The new Reef habitat at Seattle Aquarium’s Ocean Pavilion (SAOP) contains nearly 500,000 gallons of water in a 3D shape optimized for animal welfare. Unlike aquariums with straight walls or smooth curves, the mathematically generated yet free-form geometry dreamed up by LMN Architects and Thinc Design considered the individual hideaway preferences and swimming heights and radii of each species. When it comes to SAOP, form follows function—not the other way around.

The Seattle Aquarium’s addition is more than an exhibit within a building—the marine habitat is the building. Human-occupied floor spaces hang from the main Reef habitat, enveloping it like a wrapper. The building also includes an additional seismically isolated concrete “Archipelago” habitat, various “jewel” habitats, interactive exhibits, circular seating for educational programming, and the Aquarium staff’s animal care and water quality lab. The structural design utilizes the tank’s walls as the building’s primary lateral-force-resisting system during earthquakes or high-wind events—critical in seismic Seattle—resisting the sloshing force of four million pounds of saltwater inside. In an earthquake, the weight and stiffness of the tank provide safety for the entire building. Seismic design considered the tank’s unique geometry, evaluating potential earthquakes from 12 directions—every 30 degrees of compass headings.

The tank’s walls were extended higher to support the roof, eliminating the need for a separate structural system and avoiding myriad unsightly columns that would have extended from the floor to the roof, thereby obstructing public access outside and throughout the exhibit space. Saltwater corrodes metal, so prestressed hollow-core concrete planks were used over the habitat where it connects to the roof, instead of conventional metal decking, which would have been costly due to the vast amount of high-performance coating required. Eleven specially shaped roof beams, each 40 inches deep and up to 80 feet long, support the public rooftop park above the tank. MKA customized these beams to shape the public tiered seating, so the beams served double duty, capping the building and eliminating the added structure (and weight) that would have been needed to form the tiered seating.

Inside, the Reef features five viewing windows, including a massive, two-story-tall, 30-foot-wide window that resists 1.1 million pounds of water pressure and a dramatic, 50-foot cantilever that stretches over SAOP’s south entrance. This cantilever features an 18-foot-diameter oculus viewing window, allowing passersby to look up and see the marine creatures swimming within. More than 2,000 tons of concrete, steel, and saltwater are supported above the heads of pedestrians, allowing the building to occupy a smaller footprint and preserve valuable waterfront public space.

While SAOP was designed to awe visitors, the multi-function scope of the concrete Reef habitat added an order of magnitude to the difficulty of execution.

Seattle’s Central waterfront has long been the bustling center of one of North America’s major ports, home to mills, wharves, piers, docks, and feats of waterfront engineering. SAOP is adjacent to the 48-year-old existing Seattle Aquarium and at the intersection of the new Seattle Waterfront Park to the south, the Olympic Sculpture Park to the north, and the famous Pike Place Market to the east. As the centerpiece of these projects, SAOP makes possible the ADA connection linking Seattle’s iconic Pike Place Market to the Puget Sound waterfront for the first time in the Market’s history. This connection provides a multi-modal, ADA-accessible public pedestrian pathway as it descends the 110-foot-high bluff to the waterfront, spanning the landscaped SAOP roof and offering breathtaking views of Puget Sound. However, poorly consolidated, earthquake-liquefiable soils pocked by century-old fill materials and buried remnants of old piers and railroad tracks, a high water table, and space limitations due to constraints from other projects on all sides make it the “absolutely wrong location” to build a highly settlement-sensitive concrete habitat.

Differential settlement due to poor soil can lead to cracking of the tank and issues with waterproofing. To address this, MKA, alongside Turner Construction, departed from typical, expensive pile foundations, instead opting for 1,000 Deep-Soil-Mixed (DSM) columns that use augers to mix grout with soil to create underground “columns” down to solid bearing layers, improving the stiffness and strength of the existing soil to support the building and meeting all functional and cost requirements. The concrete for the 22- to 28-foot-long underground DSM columns was mixed at a batch plant on site.

MKA collaborated with geotechnical engineers at Shannon & Wilson to prepare for subsurface debris during the installation of the DSM columns. The excavation uncovered remnants of docks—even a railroad trestle three or four tracks wide—requiring the relocation of many planned DSM columns to avoid the existing wood piles, which were impenetrable by standard equipment. The DSM installation was a collaborative process that quickly mitigated the challenging subgrade conditions through the strategic relocation of DSM columns. MKA’s tank design met restrictive criteria for differential foundation movement (less than 1/8 inch over 20 feet) due to the liquefiable soils on which the habitat is built and relied on tighter bar spacing requirements to minimize cracking. The reinforcement typically consisted of two bundled #6 bars at 4-1/2-inch spacing each way on each face, with increased clear cover on the water side.
Meanwhile, fitting SAOP’s 50,000 square feet of structure onto a 38,000-square-foot site in the public right-of-way required an optimized design capitalizing on efficiencies. MKA designed the structure to minimize the number of points of support at ground level. The tank’s role as both the lateral and gravity systems eliminated the need for secondary structures, and a dramatic 50-foot cantilever extends the tank-supported building to open up more site. More than 100 feet of SAOP’s south face is artfully balanced using the weight of the tank.

Using LMN’s 3D geometry model, MKA created its structural analytical model to analyze and design the concrete shear walls. At all stages, the habitat was built from a 3D model—not 2D drawings. Pushing the boundaries of virtual design and construction, the project team’s 3D model was imported into SketchUp to design the custom formwork and create instructions for machining foam inserts into the required shapes.

The main Reef habitat used built-to-curve formwork created by a maker of ship hulls. Typical plywood or other reusable straight panel formwork was impossible for SAOP since the form shape needed to be custom-made for the tank’s unique geometry. Instead, formwork was created using data-controlled 3D milling machines to cut the unique shape.

A Seattle fabricator that makes parts, tools, and molds for airplanes, boats, wind turbines, and ship hulls created foam inserts to produce the shape for the formwork based on the architectural model, sized up to 20 feet by 8 feet, and no two alike. Expanded polystyrene panels made up of large blocks of foam inserts were glued together, secured to the traditional formwork structure, sculpted to shape, and inscribed with easy-to-read piece numbers assigning them to their unique spots, creating the shape for the final geometry of the habitat.

Turner Construction used the same 3D model to establish the amount of concrete required in each portion of the structure, allowing workers to know how much concrete had been placed at any time and ensuring that the formwork pressure was within acceptable limits.

Where most aquarium habitats have planar and singly curved walls, the Reef habitat also includes doubly curved walls that seamlessly integrate into the overall shape, requiring a complex latticework of rebar.

The Reef, functioning as SAOP’s main lateral force-resisting system with constantly varying curvature, required 355 tons of rebar—more than three times the quantity of rebar per cubic yard of concrete than in the typical core of a high-rise building in earthquake-prone Seattle.
No two pieces of reinforcement are fabricated to the same length due to the challenging curvature of the structure. Every piece was bent on a prescribed curvature to fit the formwork and hand-threaded—one bar at a time—to create the dense latticework required to strengthen the 41-foot-tall main Reef habitat. The curving nature of the tank did not allow for rebar cages to be dropped in. Rebar was installed from the dry face and built inward.

Working with the contractor, Turner Construction, smaller diameter, more easily field-bendable bars were bundled and spaced—in lieu of larger diameter bars that could only be shop bent—in order to meet design requirements. Small bars were required for bendability, as spacing would have been too tight for individual bars. MKA’s design also allowed the rebar installation to be tilted in any direction, provided that, in the rebar grid, the vertical bars remained perpendicular to their cross bars, giving Turner the flexibility to orient bars to better suit construction sequencing and preferences. With the constantly changing geometry, the contractor elected to computer model every single piece of rebar.

The tank’s structure, eight layers of rebar deep in some locations, was needed to support the main tank’s hydrostatic, earthquake, wind, and gravity forces. The tank’s unique geometry showcases how complex concrete shapes can be used as structural shear walls in high-seismic zones.

A special concrete mix with small aggregate and specialized admixtures was developed to ensure that concrete could be properly consolidated, meet required high strength criteria, and provide some corrosion resistance (resistance to chloride ions). To avoid construction joints that could compromise integrity, 680 cubic yards were placed continuously over 23 hours.

The Archipelago habitat is seismically separated from the rest of the building. Due to its central location in the building, MKA’s initial computer analysis showed that the tank’s walls attracted a significant portion of the earthquake force, meaning they needed to be made much thicker—and more unsightly. MKA isolated the Archipelago to restrict the seismic forces of the surrounding floors from being resisted by the Archipelago walls. While this required slide bearing connections at the incoming steel beams, the added complexity at the connections paid dividends in achieving the architect’s vision. To achieve the curved shape, the contractor elected to shotcrete the 14-inch-thick walls.

In 2019, the elevated, double-deck Alaskan Way Viaduct was demolished. Today, 20 acres of new public space and a new multi-modal, ADA-accessible pedestrian pathway on the Ocean Pavilion’s roof enable easy movement between Pike Place Market, the new Overlook Walk, and the new waterfront park—complete with ramps and a public elevator—for the first time.
Conquering immense sequencing/phasing challenges, SAOP celebrated its grand opening in August 2024 as a masterful accompanying puzzle piece in the city’s complex urban planning efforts, successfully completing Seattle’s waterfront redevelopment vision. The innovative tank structure that made SAOP possible does more than anchor a waterfront redevelopment plan decades in the making—it redefines the future of aquariums and concrete design for generations to come.

Seattle Aquarium’s Ocean Pavilion proves that virtually any concrete shape can be built using digital tools and collaborating with capable contractors and that complex concrete geometry can be used to build structural concrete shear walls in high-seismic locations. These special concrete shear walls set new benchmarks for what’s possible in concrete design. ■

About the Author

Hannah (Bonotto) Walters, PE, SE, is a Senior Associate at Magnusson Klemencic Associates.