September 12, 2007

Engineering a sea change at the aquarium

  • The facility won ACEC’s Grand Conceptor Award in 1977. Thirty years later, it won ACEC’s Engineering Excellence Gold Award for a remodel and pier restoration project.

    Photo courtesy of Seattle Structural
    The Seattle Aquarium is housed in a century-old building on Pier 59.

    Today’s internationally acclaimed Seattle Aquarium began as an expression of public will in the 1960s. At the time, Seattle citizens enthusiastically endorsed the idea of building a new aquarium. The consensus was that they “wanted something great” designed and built at a cost of around $10 million.

    Defining something great and raising the necessary funds, as well as long deliberation over the site, took until the early 1970s.

    When Pier 59 was chosen as the site, Tetra Tech/KCM (at the time Kramer, Chin and Mayo) and Bassetti Architects (then Fred Bassetti and Associates) were selected to engineer and design the new facility, including placement of the offices, gift shop and some exhibit spaces in the adjacent nearly century-old historic pier. They were charged with providing a view of Puget Sound marine life, perhaps from an underwater tube, and developing more than the traditional series of marine exhibits by using a story line that would take visitors on a voyage through Northwest waters.

    There were many engineering challenges, including an all-saltwater fish ladder for returning salmon. Some of the difficulties of building over the water included a severely constrained site, seismic requirements, a 17-foot tide differential, wave action, building the “ground” floor below the waterline, water filtration, cleaning up pollution in the existing waters and devising an innovative undersea experience.

    Precedent-setting design

    Photo courtesy of Tetra Tech/KCM
    The aquarium’s dome room is not in Puget Sound, but contained within a large tank above the sound.

    Building the ground floor below the water line called for using prestressed concrete piles driven into the floor of Elliott Bay with precast pile caps on top. The 13,500-ton, pile-supported structure was then built on top of the caps. Lateral forces are resisted by a system of batter piles, with a test conducted during construction to verify the pull-out resistance of the piles.

    Seismic analysis had to take into account the complicated geometry of the structure, but calculation of the building’s center of gravity showed it was less than 2 feet horizontally from the center of rotational resistance.

    An underwater dome, the first of its kind, replaced the idea of a viewing tube. The observation room is actually a 1,700-square-foot tank with a wall area of 2,700 square feet with a glass dome inside of it. Viewers inside the dome look through 25 feet of water with walls and floors painted to effectively make them disappear, giving the appearance of open water. The controlled environment allows the water to be filtered for clarity and ensures that the constant variety of marine life is maintained.

    Placing the dome room within a large tank also allowed for accurately factoring water pressure, density and depth into the structural needs to ensure the safety of the dome environment. Engineers worked with an East Coast firm that was doing pioneering work with thick glass that could withstand the weight and pressure of the water.

    A few years after the aquarium opened, acrylics advanced to the point where they could support the necessary water pressure. Glass panels in the dome room were then replaced with acrylic to avoid the distortion that comes with viewing water through glass. The coefficient of refraction of glass and water are so different that the passage of light makes it appear that moving animals vanish for a time, which made a very odd viewing experience for the visitor.

    The water filtration system, needed to clean water for the animals and ensure visitors could see them, draws water from Puget Sound through vertical turbine water pumps located at the end of the pier. Water is pumped up through pressure sand filters to clean out particulates. Three filters operate in rotation: one is filtering, one is being cleaned and the third is used to backwash the most recently filled and take the particulate to a sanitary drain.

    There are no specific building codes for aquariums, so they must meet standards set for buildings that don’t have major water features and other structural differences. For example, codes require that sprinkler systems cover a high percentage of an entire floor area, which meant sprinklers had to be installed over open water tanks in the aquarium.

    The street-side window showing wave action was the brainchild of a Tetra Tech/KCM mechanical designer who had conceived the system as his senior project at the University of Washington. He donated the design that used a Scotch yoke and a drive motor with an adjustable belt to change the amplitude and frequency of the waves. He also helped with drawings for its installation.

    On April 1, 1977, ACEC awarded the soon-to-be-opened Seattle Aquarium the Grand Conceptor Award for the highest achievement in engineering excellence. Shortly after that, a team from the Walt Disney Co. reviewed the design and patterned the underwater dome at Disney World in Florida after it.

    Continued innovation

    Photo courtesy of Seattle Structural
    During the recent renovation, the aquarium entrance was moved to the north to allow it to remain open.

    For nearly three decades, the aquarium stood as a precedent-setting project. In 2003, officials discovered that accelerated decay of the all-timber piling system threatened the facility’s existence. The shoreward end of the pier was in an advanced stage of decay and it was not practical to repair the rotted timber deck. The area occupied by Steamers cafe and the Omnidome Theater needed to be completely replaced and aquarium officials decided to undertake a modest expansion of the exhibit space to take advantage of required reconstruction.

    Studies showed that it would be less expensive to complete the renovation while the facility remained open, so the challenge was how to replace the pilings while the building was occupied. Seattle Structural PS was hired as the structural engineer and prime consultant, along with Miller|Hull as architect, to develop solutions to the unusual problems that took into consideration both the structural integrity of the building and economic issues.

    New concrete aprons were built around the exhibit portion of the pier with 30-inch concrete-filled steel piles. The aprons provided a “doughnut” of lateral stability around the precarious structure, but because it was not possible to drive new piles underneath the occupied exhibit space, new steel pipes were installed to replace the rotted timber piles in these areas. Replacement steel supports used existing timber stubs below the seabed by using an inverted “can” at the base of the replacement piles to seal the existing stubs against future decay. That saved $500,000 and eliminated the potentially polluting excavation of the silt layer below the pier. It also enabled more than 1,000 creosote-treated timber piles to be removed from critical salmon habitat.

    The new foundation replaced 12,000 square feet of exterior timber aprons with a concrete lateral frame structure. In addition, the eastern 40 percent of this Seattle Historic Landmark building was reconstructed with 18,000 square feet of exhibit expansion space. The centerpiece of this two-story build out is the 120,000-gallon Window on Washington Waters exhibit with a 17-by-39-foot viewing window.

    To maintain the integrity of the 1894 structure, the design called for removing, restoring and replacing the historic façade. The façade was picked off the building by a crane and carefully transported for restoration on Piers 62/63 just to the north. Nine months later, it was reinstalled as a non-structural façade in its original location.

    One of the biggest challenges was timing. All pile driving had to occur between July 15, 2005, and Feb. 15, 2006 — a seven-month fish window mandated by state and federal law. Adding to that challenge were erratic winter conditions, including extreme tides, short daylight hours and heavy winds.

    Construction phasing minimized the aquarium’s closure to one week when the primary utilities were transferred and the major demolition of the east-end shell occurred. That saved the aquarium $100,000 per week in lost revenue.

    Construction impacts to aquarium operations were mitigated by reversing the visitor circulation and temporarily relocating the entrance to Pier 60. During construction, the aquarium continued its trend of increasing attendance, posting record highs in 2005 and 2006.

    Prior to the renovation, the aquarium was under live-load restrictions and directed to not allow visitors when wind speeds were above 50 mph due to concern for the overall stability of the pier. Today, the city’s decision to save Pier 59 maintains the connection between the Seattle Aquarium, the waterfront and downtown. The renovation also sets the course for keeping the aquarium in its home at Pier 59 with the 50-year replacement life for the building.

    ACEC Washington last January awarded the project its Engineering Excellence Gold Award.

    This article was compiled using information from engineers and architects, as well as city and aquarium staff.

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