Healthcare and Medicine Reference
In-Depth Information
Fig. 1.50 There are two ways to support
objects in our universe: tension or
compression, hanging or bracing. Walls
brace up one brick on top of another to
create a continuous compression structure.
A crane suspends objects via the tension in
the cable. Notice that tension and
compression are always at 90° to each
other: the wall goes into tension horizontally
as the pressure falls vertically, while the
cable goes into compression horizontally as
the tension pulls vertically.
Fig. 1.51 (A) In the class of structures
known as 'tensegrity', the compression
members (dowels) 'float' without touching
each other in a continuous 'sea' of
balanced tension members (elastics).
When deformed by attachments to an
outside medium or via outside forces, the
strain is distributed over the whole
structure, not localized in the area being
deformed. (B) That strain can be
transferred to structures on a higher or
lower level of a tensegrity hierarchy.
(C) Here we see a model within a model,
roughly representing the nucleus within a
cell structure, and we can see how both
can be de- or re-formed by applying or
releasing forces from outside the 'cell'.
(Photo courtesy of Donald Ingber).
c
to reliably change any malalignment of the bones (Fig.
1.51).
tensile-resistant steel rods. These forces are minimal,
though, compared to the compressive forces offered by
gravity operating on the heavy building. Buildings,
however, are seldom measured in terms of design effi-
ciencies such as performance-per-pound. Who among
us knows how much our home weighs?
Biological structures, on the other hand, have been
subjected to the rigorous design parameters of natural
selection. That mandate for material and energetic effi-
ciency has led to the widespread employment of tenseg-
rity principles:
All matter is subject to the same spatial constraints,
regardless of scale or position. ... If is possible that fully
triangulated tensegrity structures may have been selected
through evolution because of their structural efficiency -
their high mechanical strength using a minimum of
materials. 91
Tensegrity structures are
maximally efficient
The brick wall in Figure 1.50 (or almost any city build-
ing) provides a good example of the contrasting common
class of structures based on continuous compression.
The top brick rests on the second brick, the first and
second brick rest on the third, the top three rest on the
fourth, etc., all the way down to the bottom brick, which
must support the weight of all the bricks above it and
transmit that weight to the earth. A tall building, like the
wall above, can also be subject to tensile forces as well
- as when the wind tries to blow it sideways - so that
most compressive-resistant 'bricks' are reinforced with
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