Shells and Nature
Shells. Sometimes it’s a game. You are supposed to guess where the pea is hidden after someone moves your options around until they feel you haven’t kept up and then the question comes for you to commit. Is it a trick, a matter of probabilities, or a metaphor for you to pay attention to life’s ever-changing nature? Success is always about paying attention.
Shell structures are found in nature utilizing a variety of forms, materials and strategies. There are the kind that most people are familiar with that are encountered near beaches, old inland seas or gift shops. Then there are the kind that are familiar to architects and engineers. We know these structures as performance systems that give us a variety of benefits that utilize minimal materials and internal structure. Shells are structurally configured using an surface and a thin superstructure. Thin being relative to the span, curvature, and loading. Thin can comprise itself of multiple layers of structure. The behaviour is in the relative thickness. This relative nature lets you decide if you are dealing with a shell or ‘spaceframe’ or truss structure.
Shells have the capacity to take local loading effects and transfer them to the global system without failure. They use deflection, planar force transfer and geometry to accomplish it. Shells often have a rigid perimeter to provide them the necessary stiffness or rigidity to perform as we expect.
Shells in nature use a minimal amount of material to enclose a volume and produce rigid surfaces. The egg is an example of a shell structure. A hermit crab shell uses the shell methodology in conjunction with a curvature stategy to provide a lightweight, strong defense to the inhabitant.
Shells also have a redundant capacity to them. A portion of the shell can be buckled or removed and the remaining shell remains intact. You can drill a hole, crack a portion away, or even damage its perimeter support and the shell still is capable. A global failure of shells is usually a buckling effect. The whole shell reverses curvature past its neutral plane and fails. There are rubber toys that simulate this. You push them inside-out and in a couple seconds they find their original form and ‘pop’ off the table. These are made of rubber. Whereas for most rigid materials, a global buckling means ‘failure’ in the form of a collapse.
Nature’s shells provide architects the ability to use the same strategies to enclose volumes and cover spaces using single-layer structures and a membrane. We have always liked shell structures for their beautiful shapes, engineering strength and their elegant simplicity. Their design and analysis is another issue altogether. The shell may appear simple, but the behaviour and mechanisms that provide it benefits to us are sophisticated and require experienced engineers. Shells are meant to be light rigid structures, and understanding their behaviour and strategies provides the path for getting there effectively. What’s your application?
