How nature builds: hierarchy and interfaces

 

Scanning electron micrograph of cancellous (spongy) bone tissue

There are seven key distinctions to guide biomimetic
thinking, summarized below:

Structure and materials are indistinguishable in nature, which is a radically different way of thinking to grasp in an architectural context, where these concerns are more easily separated, and where traditional manufacturing techniques and drawing packages both reinforce this separation.

Nature organises structure and materials together through hierarchy.

Perhaps a good way to begin exploring hierarchy is to visualise a range of bridge designs. One means of spanning a modest distance would be to use two solid steel beams that sit on piers at either end.

This would represent a monolithic approach with no hierarchy. A more efficient way to span the distance would be to use a pair of steel trusses instead. That represents one level of hierarchy. Supposing we went one step further so that each compression member in the truss became a small box truss and each tension member became a cable made from stranded steel.

That would represent two levels of hierarchy. With increasing levels of hierarchy, the structure becomes more efficient in terms of the amount of material used to achieve a given objective.

Diagram showing how, with levels of hierarchy, an element of structure can be further refined to use as little as 2 per cent of the material of a solid section (after work by Adriaan Beukers and Ed van Hinte in Lightness: The Inevitable Renaissance of Minimum Energy Structures, with input from Fluid Structures)

Trusses within trusses within trusses on the Eiffel Tower – showing three levels of hierarchy

The Eiffel Tower demonstrates three levels of hierarchy, but the majority of human engineering uses only one level.

 In biology, it is not uncommon to find six levels of hierarchy and proportionately higher performance because the structure benefits from bonds at every level from atoms to molecules to cells to organisms and upwards.

 

The bridge example shows the material decreasing in quantity and changing form at the same time (solid steel sections become steel cable). At this point some readers may be feeling confused about the difference between structures and materials and that is quite justifiable because in biology there really is no distinction between the two.

The way that nature makes things from the bottom up, molecule by molecule, means that what we might think of as a biological material is also a structure. Wood, for instance, is a microstructure of lignified cell walls, and bone is a hierarchical structure of calcium phosphate and collagen molecules in fibrous, laminar, particulate and porous form.

Scanning electron micrograph showing the microstructure of oak (Quercus robar)

Scanning electron micrograph of cancellous (spongy) bone tissue

Hierarchical structures also deliver benefits in stiffness and fracture control and this is achieved through interfaces between, and within, each level of hierarchy.

The abalone shell is made from platelets of aragonite (a form of calcium carbonate) bonded together with a flexible polymer mortar.
The polymer forms the interface and, as is typical in biology, the material used at these points is weaker than the surrounding material.

Materials scientist J. E. Gordon explains: ‘this is not because Nature is too incompetent to glue them together properly but because, properly contrived, the weak interfaces strengthen the material and make it tough’. Toughness, in engineering terms, means resistance to fracture.

Although 95 per cent of an abalone shell is made from the same raw material as chalk, it achieves 3,000 times the toughness through its hierarchical structure and interfaces. Artificial nacre is already being constructed, with research aiming to create ceramic materials and composites with far greater strength than has conventionally been possible.

Currently, these experiments are at relatively small scale (by architects’ standards, that is – an abalone would take the opposite view) but could lead to substantial increases in spanning capabilities of Guastavino vaulting and related forms of construction.

 

 

Book reference:

Biomimicry in Architecture by Pawlyn, Michael
https://amzn.to/3GgvxGy