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| MX3D Bridge By Joris Larman |
Nature makes extremely economical use of materials, often achieved through evolved ingenuity of form. Using folding, vaulting, ribs, inflation and other means, natural organisms have created effective forms that demonstrate astonishing efficiency. The many manifestations of this in natural organisms provide a rich source book of ideas for structures that could be radically more efficient than those found in conventional architecture.
Hollow tubes
Nature builds simply and economically, often meeting
both goals simultaneously by making hollow tubes.
Nature is abundant in examples that demonstrate this
structural principle, such as human bones, plant stems
and feather quills.
If one takes a square cross-section of solid material with a side dimension 24 mm, it will have the same bending resistance as a circular solid section of diameter 25 mm with only 81.7 per cent of the material.
Similarly, a hollow tube with only 20 per cent of the material of the solid square can achieve the same stiffness. In engineering terms, material has been removed from areas close to the neutral axis and placed where it can deliver much greater resistance to bending – achieving the same result but with a fraction of the material.
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| Sketch showing how four equally stiff structural elements can be made with varying degrees of efficiency. By using shape and putting the material where it needs to be, it is possible to use only 14 per cent of the material of a solid square section (after work by Adriaan Beukers and Ed van Hinte in Lightness: The Inevitable Renaissance of Minimum Energy Structures) |
One plant in particular shows how hollow tubes can be
applied at larger scales in nature. Bamboo species can
reach 40 m in height.
How do they maintain strength
over this length?
One of the ways in which a tubular
element can fail under loading is through one side of
the tube collapsing in towards the central axis, leading
to overall buckling. Bamboo solves this by interrupting
smooth tubular growth with regular nodes, which
act like bulkheads .
The nodes provide great
resistance to structural failure, and are part of what has
facilitated bamboo’s lofty accomplishments.
Bamboo is,
by strict taxonomy, actually a species of grass which has
achieved such wild success that it resembles the scale
of a tree. This plant’s solution seems to apply so widely
that it begs the question: why aren’t more trees hollow
tubes? The answer derives from the different forms that
they strive to grow into: trees generally create a canopy
of cantilevering branches, rather than the multiplicity
of stems characteristic of grasses.
Bamboo offers
solutions to tubular structural elements, while trees
offer a biomimic further solutions to holistic structural
issues, since they face different pressures than grasses.
Trees: solid forms
In nature, biological forms follow a simple rule, which he describes as the axiom of uniform stress. In locations of stress concentration, material is built up until there is enough to evenly distribute the forces; in unloaded areas, there is no material.
Trees also demonstrate the idea of optimized junction shapes that avoid stress concentrations and can adapt over time. The result approaches optimal efficiency, in which there is no waste material and all the material that exists is carrying its fair share of the load.
By contrast, many steel and concrete structures are designed so that the most onerous load conditions (which only occur in specific locations) determine the size of the whole beam or column.
The program allows designers to subject a rough structural computer model to the kind of forces that would be experienced in reality.
These include snow, wind and seismic loading, as well as loads imposed by the building’s use.
The first stage uses ‘Soft Kill Option’ (SKO) software to eliminate material in zones where there is little, or no, stress. Then a ‘Computer Aided Optimization’ (CAO) program refines the shapes and, where necessary, builds up material at the junctions to minimise stress concentrations that could lead to failure.
The designer is free to decide whether they like the output and find alternative ways to achieve structural integrity. Mattheck likens this process to starting with a roughly axed piece of timber, which is then carved to the near-final shape (the SKO stage) before being sanded and polished (CAO). The results can be surprisingly organic in form, and far more efficient than conventional structures. The designer Joris Larman used this to develop a number of elegant pieces of furniture and a bridge that is to be 3D printed and will span over a canal.
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| Diagram showing Claus Mattheck’s design refinement process using ‘Soft Kill Option’ (SKO) and ‘Computer Aided Optimization’ (CAO) software |
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| 3D-printed bridge by Joris Larman Lab demonstrating the expressive and material efficient results of designing with SKO software |
Trees consequently grow as solid forms. This might seem surprising, given the hollowness of many bones. The explanation probably lies in the fact that there is not the same selective pressure for lightness in stationary trees as there is in animals that must move at speed to either catch, or avoid becoming, prey.
Most of the bulk of a tree is dead material (only the outer layers remain alive), whereas bones are continually being reformed and recycled.
One other possible explanation is that the solid core of trees functions to some extent as a compression core to resist the tension created by the outer sapwood, which grows in helical patterns up and around the trunk. This structural form has some similarities with Future Systems’ Coexistence Tower.
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| Coexistence Tower by Future Systems. The compression core and the helical arrangement of tension members around the perimeter have functional similarities with the structure of tree trunks |
Trees growing in the shallow soils of rain forests have
evolved buttress roots that resist overturning
The formation of a wide, stiff base effectively moves the pivot point some distance from the trunk and, on the opposite side, a branching network of roots mobilizes a vast amount of soil as ballast to resist overturning.
In rain forests, where soils can be relatively shallow and therefore cannot provide the same resistance as those in temperate climates, trees have evolved pronounced buttresses which actually work in tension to prevent overturning.
Book reference:
Biomimicry in Architecture by Pawlyn, Michael.
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