Structure Distributes Stress

Biological Strategy

Scots pine

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Conifers and Others (Pines, Spruce, Gingko)

Clade Gymnosperms (“naked seeds“): Conifers, cycads, Gingko, gnetophytes

There are approximately 1,000 known species of Gymnosperms. These include well known needle-bearing plants like pines, firs, and spruce, as well as some with broad leaves like the Gingko and the gnetophytes. Instead of flowers, gymnosperms produce unenclosed (“naked”)seeds exposed on the surface of cone scales. Economically, conifers make up 45 percent of the world’s lumber production. The first gymnosperms evolved over 390 million years ago in the Paleozoic Era and dominated landscapes for a quarter of a billion years before the evolution and diversification of flowering plants.

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Trunks and branches of trees withstand external stresses through load-adaptive growth.

Trees and bones achieve an even distribution of mechanical tension through the efficient use of material and adaptive structural design, optimizing strength, resilience, and material for a wide variety of load conditions. For example, to distribute stress uniformly, trees add wood to points of greatest mechanical load, while bones go a step further, removing material where it is not needed, lightweighting their structure for their dynamic workloads. At the scale of the cell, trees arrange fibers in the direction of the flow of force, or principal stress trajectories, to minimize shear stress. Engineers have incorporated these and other lessons learned from trees and bones into software design programs that optimize the weight and performance of fiber-composite materials. For example, car parts and entire cars designed with these principles have resulted in new vehicle designs that are as crash-safe as conventional cars, but up to 30% lighter.

Image: Tuohirulla / Wikimedia Commons /

Image: Ronja Addams-Moring / Wikimedia Commons /

Image: The Biomimicry Institute /

Collage of images relating to lightweighting

Image: Sherry Ritter /

This tree has larger rings on the downwind side of the tree as a response to high winds in this area (Waterton Lake, Alberta, CA). The grasses in the photo are bent over from the high wind.

Helical Wound Fibres Increase Toughness

Plants

Cellulose fibers in plant stems increase toughness by winding around tubes at an angle

Last Updated August 21, 2019

References

“The analogy that [Claus] Mattheck wants to pursue, though, is not that between trees and other organisms, but between trees and engineered artefacts. If trees achieve longevity and structural stability, aren’t these the qualities of reliability and integrity that engineers want to design into products?

“The key to this is Mattheck’s contention that the structural optimisation in trees and apparent in other natural structures such as animal bones is all about making the external and internal stresses as uniform across the whole structure as possible. Mattheck calls this the ‘axiom of uniform stress’ and adds that, though he can cite plenty of examples of it, he cannot prove it exists…Mattheck’s contention is that trees are constantly readjusting this balance by adding more material at points of high stress and adding no material at points of low or no stress. (Bones, he contends, go one stage further by actually shrinking at points of low stress.)

“In trees, junctions between main trunks and branches, for instance, are places of concentrated stresses. Trees compensate for this extra stress by adding more material to the shoulder.” (Pullin 1998:17-18)

Journal article

The author's reply to the discussion on “A survey of X-rays in engineering and industry”

Journal of the Institution of Electrical Engineers - Part I: General | 11/04/2014 | V.E. Pullin

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Book

Design in Nature: Learning from Trees

27/02/2004 | Claus Mattheck

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Journal article

Teacher tree: The evolution of notch shape optimization from complex to simple

Engineering Fracture Mechanics | 05/04/2006 | C. Mattheck

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Journal article

Talking to the trees

Pullin, John

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