Aquariums have become big attractions at some of the largest venues on the planet. Think of Walt Disney World; the largest mall in the world in Dubai; the AquaDom in Berlin, Germany, which has a 25-meter-tall cylindrical aquarium with a built-in elevator; or the Georgia Aquarium that holds 6.3 million gallons of water. If you are not familiar with these aquariums, I encourage you to step aside from this article and Google them so that you can appreciate their enormity.
|The aquarium at the T-Rex restaurant at Walt Disney World in Orlando, FL, failed in 2014, when a vertical bonded seam opened instantaneously. Image courtesy Paul J. Gramann.|
These aquariums hold thousands of exotic sea animals in extremely demanding environments, all the while putting the spectator inches away from this marine life. What is the single-most important element of the aquarium that makes it possible for these behemoths to exist? Plastic. Specifically, poly(methyl methacrylate) or PMMA, also known as acrylic and sometimes as plastic glass. Acrylic is ideal for this application. It possesses the mechanical properties, such as strength and stiffness, required to support the tremendous water pressure created in these ever-growing marine exhibits. Acrylic has other attributes that make it possible to meet the requirements of the ever increasing size and complexity, as engineers “push the envelope”:
- Acrylic is one of the clearest materials in the world;
- the weight of acrylic is less than half that of glass with up to 17x the impact strength;
- acrylic can be heated and easily formed into curved shapes;
- curved shapes, as well as other shapes, can be bonded together to make massive, continuous structures, such as cylinders and long wall expansions;
- it has relatively, low moisture absorption;
- raw material and fabrication costs are low;
- acrylic is more forgiving than glass if it is out of alignment or not perfectly supported.
Why acrylic aquariums may fail
Acrylic does not come without drawbacks. The engineer needs to have full knowledge and understanding of these drawbacks to successfully design, manufacture and assemble an aquarium that will stand and support aquatic life for years. To ensure longevity, the typical large aquarium is designed with a factor of safety of 11 to 12. This may seem high, but when one considers the implications if one of these large aquariums were to fail, and the sudden, catastrophic event that occurs when they do fail, it becomes more understandable and acceptable. Unfortunately, aquariums have failed for various reasons, leading to tremendous damage, huge monetary losses and, at times, complete loss of the aquatic life. There have been high-profile public aquarium failures, which typically involve huge aquariums, as well as private aquarium failures that range from several hundred to thousands of gallons of water loss. Some common reasons why acrylic aquariums can fail include:
- Poor bonding of acrylic panels creating a weak seam;
- improper installation;
- poor manufacturing of the acrylic panels, resulting in inferior strength and stiffness;
- residual stress molded or formed into the panel during manufacturing; and
- introduction of large gouges or notches that can significantly increase stress in the panel.