Engineering Underwater Worlds — Daily Journal of Commerce

Underwater worlds — oceans, seas, lakes and rivers — contain mysteries that exceed our imagination and fuel our human curiosity to explore. Except at aquariums, this spectacle of life is inaccessible to most of the public. Made from enormous concrete vessels, often holding millions of gallons of water, aquariums provide habitats for aquatic creatures and plants from which visitors are separated only by a vast acrylic window. While inspiring awe for the scene within, this experience also grants us a moment to pause and consider the incredible feats of engineering that make the encounter possible on both sides.

CONCRETE IS THE KEY

Large aquarium exhibits present many unique, highly specific design challenges that distinguish them from other industrial-scale projects. This is an opportunity for structural engineers to put a new spin on traditional materials and reveal a story about the versatility of concrete. Developed with meticulous innovation, often through nuance, concrete can become a water-tight vessel under relatively high pressures, able to remain strong and durable throughout its design life.

Photo courtesy of MLA Engineering [enlarge]

The finished tunnel at the New York Aquarium’s Ocean Wonders: Sharks! exhibit welcomes guest into the immersive exhibit space where sea life can be viewed from many perspectives.

Before delving into more detail, the obvious first question is why concrete is the preferred structural material for aquariums. Regardless of the design's size, shape, and budget, the free- form character of concrete makes it a versatile choice. Concrete can be configured on site to match the organic curves of natural habitats, unlike steel or plastic which would require off-site prefabrication and then could be too large to transport or would require costly bonding in place.

WATERPROOFING TECHNOLOGY AND TESTING

The next question is how a porous material like concrete can be made watertight — ensuring the livelihood of the exhibit's occupants, as well as the durability of the structure and safety of the guests and staff. Instead of simply lining containment pools with a fiberglass reinforced polymer (RFP), designing concrete to be leakproof right from the start means lessening the repairs over time that would disrupt the aquarium's function and involve the costly removal of animals and theming elements within the exhibit.

There is no aquarium design book; however, structural engineers follow the standard ACI Code-350 for environmental engineering of concrete structures, such as water treatment facilities and reservoirs, and then significantly expand upon the basics to accommodate the aquarium-specific detailing required for the acrylic windows. All non-pre-stressed concrete develops cracks throughout its life — even concrete used to contain millions of gallons of water —as the cracking mechanism transfers the internal tension stresses from the concrete into the rebar. What makes the concrete for aquariums distinct is being designed to prevent leaks by keeping the cracks microscopic in width.

Engineers accomplish this in several ways, such as by adding more closely spaced rebar, perhaps fifty percent more than for comparable loads that do not involve water. Engineers also tightly control heat gain during cement hydration by substituting cement with pozzolans (fly ash or ground granulated blast furnace slag), using water-reducing admixtures, and specifying wet-curing methods that reduce the rate of water evaporation immediately after pouring — the time when the concrete is gaining significant strength. The key is to maintain the proper balance between all the variables, avoid causing an unwanted reaction between incompatible ingredients, and provide consistent results that allow placing the concrete with excellent consolidation.

Further distinguishing aquarium projects is the repair and testing phase built into their construction schedules. Water proactively finds a vulnerable path, so any defects typically present themselves. Ensuring that a leak test of the pools is conducted at the critical point in the schedule provides the ability to repair any leaks before costly theming is installed and gives the construction team the best chance of meeting the higher performance levels required.

LOCATION, LOCATION, LOCATION

Photo courtesy of MLA Engineering [enlarge]

A reinforced concrete walkway and walls provide structural support for the half-cylinder acrylic window at the New York Aquarium’s Ocean Wonders: Sharks! exhibit.

Large aquariums regularly take specialized designers and engineering consultants far from home, across the country, and anywhere in the world where such projects are constructed. Accordingly, knowing the availability of materials and differences in construction practices nationally and internationally is integral to the work. For example, understanding variations in the availability of local aggregates used in the concrete is critical, as their density, strength, and porosity affect the potential for leakage in the finished vessel. Researching the availability of local materials allows the engineer to specify products that local batch plants can readily procure and, thereby, promote a cost-effective mix design with the durability and water-tightness required for water-holding concrete vessels.

MANAGING DESIGN LOADS

Another critical factor in aquarium construction is that the maximum design load, except for seismic conditions, is there all the time. Unlike most buildings, designed for a transient live load, aquarium water is exerting design-level forces continuously from all directions.

When an aquarium requires large acrylic windows and acrylic tunnels for viewing exhibits, the vessel design must consider the transfer of large forces at the points where the acrylic panels bear against the concrete framing. This is where the structure is most vulnerable to leakage and where the detailing of rebar and placement of the concrete must provide a crack-resistant, well-consolidated, and watertight system.

The windows and tunnels are held in place largely by the pressure of the water itself, although waterproofing sealants can help secure this glazing. Temporary interior clamps are often used to hold the window and tunnel acrylic sections when filling or draining the exhibits. Also, engineers must design for potential buoyancy and uplift forces that can occur when a tunnel cross section has an arc exceeding 180 degrees and is surrounded by water exerting pressure in multiple directions.

TEAM APPROACH TO WATER QUALITY

Photo courtesy of MLA Engineering [enlarge]

The centerpiece multi-story Endless Ocean Pool viewing room at SeaWorld Abu Dhabi includes the “oculus” acrylic panel as a dramatic portal into the 65-foot deep exhibit.

When discussing aquarium construction, the force of millions of gallons of water easily becomes the focus, but containment pools are only part of the equation. Installing the pipes that carry the water relies on a carefully orchestrated team effort. The tanks must be refreshed continually with clear water filtered to remove contaminants that would harm the animals. While similar to the large conveyance requirements of water treatment facilities, the life support systems (LSS) for aquariums are additionally challenged by needing to coordinate with the large viewing panels, skimmers, and pool foundations. In other words, the pipes must be positioned not only to integrate seamlessly with the structure's watertight reinforced concrete walls and foundation, but also with the visitor experience.

Structural engineers can be instrumental in helping assign priorities to these competing factors. The large diameter pipes are expensive, so their configuration — balanced with the flow of guests and location of the LSS pumps, basins and degas towers — is critical to the budget. Working closely with the other disciplines, engineers look for innovative ways to increase the proximity of the pipes to the containment pools and filtration room. Combining everyone's knowledge and experience leads to the best-informed choices and cost-effective solutions.

CONSTRUCTION SEQUENCING

Routing the LSS pipes also involves pressure testing for leaks — an essential project component built into the schedule with a focus on conserving time and protecting the budget. The underground piping beneath the main exhibit tank goes in first, with pipe corridors carefully located to reduce the effects that the LSS pipe construction and related earthwork have on the project's critical construction path. Adhering to a well-planned sequence of events and anticipating scheduling issues reduces the construction timeline, which also results in overall cost savings.

LOOKING FORWARD

Water containment technology has long been in existence for water treatment facilities and reservoirs, but developing its use for aquariums has introduced innovations to better serve the wellbeing of aquatic animals. Understanding that these projects are not just business-as-usual opens new possibilities for engineers to improve the performance of environments specialized to support animal welfare and public awareness of underwater worlds. Every new project is a chance to build on prior experience and keep improving outcomes for clients, visitors and inhabitants. The following examples offer insights into recent aquariums benefiting from ongoing research and development.

SeaWorld Abu Dhabi exemplifies the integration of concrete aquarium pools into the framework of a complex building configuration. Containing 25 million liters of water, it is the world's largest and most expansive marine-life aquarium, housing more than 68,000 animals, including sharks, schools of fish, manta rays and sea turtles.

The focal point for visitors is the “Endless Vista,” a 20-meter-tall vertical window across multiple levels that reveals stunning aquatic scenes. As part of a global effort, the strategic structural design involved careful analysis and coordination with the LSS team. The solution accommodates the facility's vast scale by employing the multi-level aquarium's reinforced concrete walls both as the primary liquid containing structural system and the lateral force resisting system. Additionally, as part of the lateral system, the pool walls are designed to remain water-tight under all but the most severe seismic motion estimated at the site.

The New York Aquarium's Ocean Wonders: Sharks! exhibit is a three-story cast-in-place concrete structure designed to reflect the Eastern Seaboard's natural environment. Eleven salt-water tanks reveal the mysteries of sea life right next to New York City with a variety of experiences, including a coral reef tunnel and expansive shark tank with overhead viewing through a shipwreck. Framing consists of mild steel reinforced concrete slabs and beams, concrete shear walls and a mat foundation. The lowest building level is partially submerged.

Unique design challenges included buoyancy design for high groundwater as well as 100-year flooding; hurricane level wind forces; heavy saltwater tank and life support system loads; and potential for liquefaction of the native, sandy soils. Using the exhibit pool walls to support the back-of-house mezzanine level keeps the guest area free of columns and allows for expansive, dramatic and uninterrupted views into the main shark exhibit.

Michael Leonard is the principal and owner of MLA Engineering, with more than 40 years of experience providing engineering for regional, national and international projects.