It is very fortunate for the thousands of visitors to aquariums each year that acrylic exists.
With good structural properties and an industrial capability to design, fabricate, and install large windows of diverse shapes, acrylic has completely changed the way we view underwater sea life.
Thanks to acrylic sheets and their versatility and strength, when installed into concrete tanks holding millions of gallons of water, visitors are now able to view aquatic life unhampered by the limitations which would have been imposed by small glass windows.
Let’s take a closer look at how acrylic has been used to transform aquariums using “The Handbook of Acrylics” as a resource.
Acrylic Casting Technology’s Dramatic Progress
Since 1965, the size of acrylic flat panels available for installation in aquaria progressed from 48 x 72 x 4 in (124 x 183 x 10 cm) to 120 x 324 x 15 in (305 x 465 x 38 cm) monolithic panels. In addition, besides flat and curved panels, there are hemispherical bays in the concrete wall, and tubular tunnels through concrete aquaria.
Many people have visited an institutional aquarium where the exhibits are made predominately with small, rectangular-windowed tanks. When glass sheet was the only viewing material available, the small rectangular window was the standard option. Small-cast acrylic did not expand the view much more than the glass allowed, nor did the shaping of small sheets add much to the exhibit viewing experience.
It wasn’t until the development of largecast acrylic panels that designers and architects had the opportunity to create and expand their imaginations. The public was no longer limited to looking at small tanks.
The large-cast acrylic sheets were being used to create virtual walls of water. When standing close to these huge acrylic panels, the public started to experience the feeling of immersion.
There was water when you looked to the left, right, up, and down. People could now be inches away from sharks, rays, and eels and the daunting presence of hundreds of thousands or even millions of gallons of water held back by a virtually invisible acrylic panel. The public started to feel they were actually in the aquarium.
Large, flat acrylic panels have been used where the ends are faceted and butted up to one another to create a three-sided room – even four-sided support panels have now been used. The uniqueness of these rooms is that the walls are seemingly water.
So how are these amazing feats of architecture created? Let’s look at three main considerations.
1) Deflection
The term deflection refers to the give or sag in an acrylic sheet. Dsign engineers must take into consideration tension and the maximal deflection of an acrylic panel in order to maintain safety regulations.
In the case of aquariums, the choice of material thickness or the definition of the permissible water pressure is based on theoretical calculations and confirmed by practical tests.
Deflection must also be taken into account when an acrylic sheet is being fabricated. Routers, band saws, table saws, shapers, lathes, milling machines, and drill motors, as needed for woodworking or metalworking, can be used to machine acrylic.
Experienced woodworkers or machinists should have no trouble in working acrylic shapes; however, they must understand the properties of the material being shaped to achieve good quality finishes and to avoid damage.
The coefficient of thermal conductivity, coefficient of thermal expansion, and tensile modulus of elasticity are the most important properties affecting the machining of acrylic. The low thermal conductivity prevents heat generated by the cutting tool from effectively flowing away from the cutting zone; the high thermal expansion causes reductions in the clearances of the cutting tools in the machining zone, which can result in tool binding; and the low modulus of elasticity results in material deflection at the cutting point, if proper precautions are not taken.
The following criteria are considered industry standard or above standards for deflection.
3-side Support Panel Deflection Standards:
Most panel fabricators will design their 3-side support transparencies to an industry maximum deflection standard of L/400. However, there is an issue to this standard, thus below we shall explore an alternative solution and evaluate the issues with this standard.
Example:
Panel A) 3-side support 100″L x 40″H
Panel B) 3-side support 200″L x 40″H
Issue:
As we can see both panels are of equal height. However, based upon the industry standard design criteria of L/400, panel B would be able to deflect twice that of panel A. Thus the thickness of transparency B could be thinner then transparency A. Also based upon this criteria a panel could theoretically deflect an infinite amount.
Solution:
A more appropriate standard would be <= H/200 OR <=L/400 whichever is less. Thus panel A and panel B would be able to deflect the same amount.
4-side Support Panel Deflection Standards:
Structural loading panels with 4 side supported edges:
Initial predicted deflection <= 1/300 the length of the shortest edge.
Reducing deflection beyond industry standards:
All transparencies should be designed to strict standards for deflection and stress criteria, thus reducing deflection beyond these criteria is solely a cosmetic undertaking. There are several options to achieve this:
- Increase the thickness of the transparency
- Reduce the panel size
- Reduce the water depth
- Increase rebate bearing face support
2) Maximum design stress
of magnitude shorter than in a potential hydrospace application, for example an underwater habitat or aquarium.
In applications where point-impact or contact with abrasive objects is to be expected, the proper response in the design process is to specify laminated window construction, i.e., a layer of transparent elastomeric material is sandwiched between two sheets of acrylic. In such a design the thickness of the interior acrylic layer is calculated to carry all of the pressure loading. In such an arrangement, scratches, gouges, or cracks in the outer layer introduced by point contact do not degrade the structural performance of the window to such an extent that the immediate replacement of the damaged window is required.
Design Stress for 3 side and 4 side support panels:
The minimum tension and flexure design safety factor for panels shall be 11.2. Per ASME-PVHO-1. The minimum compression design safety factor for panels shall be 8 per ASME-PVHO-1.
3) Safety factors
In addition to the physical reaction of acrylic plastic to sustained loading, there is also its chemical reaction to solar or nuclear radiation, moisture, and chemical reagents. Their cumulative effect is to degrade the surface of the material and as a result cause deformation and rupture sooner than predicted by extrapolation of data generated in the laboratory environment.
For this reason, it is preferable to base the design of acrylic structures on long-term experimental data generated in operational environments and to limit the extrapolation of values to a maximum time factor of 10. If this is not feasible, appropriate safety factors should be applied to the extrapolated laboratory data.
The major parameters that influence such test results are temperature, environment, duration of loading phases, duration of relaxation phases, rate of loading, rate of unloading, and the presence or absence of load-direction reversals.
Of these, the effect of temperature is best understood, as it affects the ultimate strength and effective modulus of elasticity in the same
manner that it does under constant long-term loading, i.e., longer fatigue life and effective modulus of elasticity values are associated with lower ambient temperatures. The absence of sunlight and of industrial pollutants in the ambient atmosphere are also known to increase the fatigue life significantly. However, the effects of either continuous or intermittent immersion in seawater are not known.
By taking into consideration deflection, maximum design stress and safety factors, engineers are now only limited by their imagination and not physical constraints when it comes to the size and shape of the transparent walls they create for aquariums.