Avoiding the unBEARable: Design Considerations for Animal Exhibit Glazing
By Stewart Jeske
Several years ago, I was getting ready to design our first polar bear exhibit glazing for a zoo. This was a challenging design with eight adjacent glazed openings of 13 feet by 6 inches tall by 7 feet wide and water levels at 8 feet on the glazing. My mind was swirling
with questions and considerations around this significant design.
As I was thinking, I also searched Google to compare pictures of polar bear exhibits.
That’s when I found a concerning video. It was a similar application, with laminated glass retaining deep water in which a polar bear was swimming. The polar bear went down to the bottom of the pool, picked up a large rock and let it loose as it went up for air. The rock went down, scraping against the glass and shattering the outer layer. I was floored. Why would the zoo allow rocks to be in the pool?
I’m sure that the structural engineer who designed this glass did not design it for rocks being dropped into it by a polar bear. This unforeseen load on the glazing was allowed mistakenly.
Animal Exhibit Designs
Clarity and transparency are essential considerations for animal exhibits. The whole
point is the view. The glazing should offer the best possible clarity while minimizing
optical distortion. If laminated glass is used, it’s important for specifications to minimize
roller-wave distortion in the glass. In addition, low-iron glass should be used because
of its superior optical clarity. Using acrylic has some drawbacks in terms of clarity associated with “fishbowl” effects, UV-degradation with a yellowing appearance and stress-hazing over time.
Visitor safety is even more important than clarity and transparency. Laminated glass
typically is designed with significant safety factors because it is very brittle. Impact from
animals, whether they are running or flying, must be considered and translated into a
structural design load. If an animal can manipulate objects, the design should consider
objects thrown into the glass. Communication with the zoo is important to minimize
the placement of items such as rocks that can damage the glazing severely.
Typical safety factors used with animal impact are approximately ten on the mean fracture strength of glass. It is a standard practice when designing animal exhibits to consider
the case of a broken lite of laminated glass. In other words, if one outer ply is compromised by sudden breakage, the remaining glass plies are designed to withstand environmental loads so that the visitors can evacuate safely.
Acrylic is not as brittle as glass, but also not as strong. It requires approximately 2-3
times the thickness of laminated glass to resist loads. Seismic sloshing of the water is another safety design load for underwater glazing. During seismic events, the ground moves rapidly and may cause additional water loads on the glazing system.
Maintenance is also important in the design of animal exhibit glazing. Acrylic is much softer than glass and may be marred by regular interaction with visitors and animals. This marring and degradation of the surface over time may affect view clarity. Acrylic is more sensitive to chemicals used to clean glazing systems than glass. If the exhibit glazing is underwater, coatings may be used to minimize water marks and staining and reduce maintenance.
The joints surrounding the glazed opening are an additional maintenance item. Our experience has shown that some animals like to scrape, dig out or even eat sealant. Protective cover plates could be used at those joints to cover sealant that would otherwise be exposed.
This is not a comprehensive list of considerations but a broad overview of factors influencing the design of animal exhibit glazing. Understanding the unique design factors may help guide glazing contractors, engineers and architects to successful exhibit projects.
Stewart Jeske is president and owner of JEI Structural Engineering in Kansas, Mo.
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