to the extreme pressure underground, let alone
be useful for building mountains, trees, or |
houses for little pigs above ground.
Structural materials get their strength and stability from |
the way that their atoms (i.e. bricks) are
packed together (i.e. stacked) by the force of their |
atomic bonds (i.e. mortar) so as to resist
being pushed out of shape by external forces (i.e. |
wolf puffs). The reason an
element like carbon can be soft and flaky like the graphite
in a |
pencil, and yet hard and brittle like
diamond, is due solely to the way the carbon atoms |
are arranged structurally. That is,
certain structures are inherently more stable than others |
regardless of what they are made of.
This will be demonstrated in the following section. |
|
Two-dimensional stability |
|
For example, build a chain made from six small
square (SS) Polymorf panels that are |
joined together by flexible pin hinges, or
pinges. The chain is obviously not very stable |
|
 |
Fig. 119 - Six member flexible chain |
with hingeable joints |
(demonstration model) |
|
|
since both ends of it and the middle can be
moved in any direction with the slightest push. |
The only restriction is that the individual
members are linked together. Now join the ends |
of the chain together forming a six-member
hexagonal ring. Notice that the ends of the |
 |
chain are no longer free to move independently of each other |
and
the movements of the individual members are confined to |
the
ring's perimeter. But the hexagon is still very flexible. |
Very
little force is required to move the members and distort |
the
hexagon's shape. |
|
◄
Fig. 120 - Flexibility of a hexagon.
(demonstration model) |
|
|
|
Next remove one of the members from the ring
and then pin the free ends of this five- |
member chain back together forming a pentagon.
The pentagon is still pretty flexible but |
 |
the
movement of the individual members is |
more restricted than before. Remove
another |
member forming a four-member parallelogram. |
This shape still flexes but opposite members can |
only move parallel to each other. Finally remove |
another
member forming a triangle. This shape |
is now stable! Since the individual
members of |
Fig. 121 - Flexibility of polygons with |
the
triangle are held together by pinges, its |
decreasing number of members. |
stability can only be due to two things - the |
(demonstration models) |
rigidity of the individual members and the |
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Back to
Knowhere |
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Page 78 - Building Stability -
Two-dimensional stability |
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