The
following models show towers that use octahedra in combination with
tetrahedra for |
stability. Fig. 300 c) is a tower, called
Type C, built by joining octahedra edge to edge with |
tetrahedra wedged between them for good measure. The tower shown in
Fig. 300 d), |
called
Type D, is identical to the Type C tower except that the octahedra are
split in two. |
. |
 |
|
 |
|
Fig. 300 - Type C and D |
Fig. 301 - Type D |
towers |
unit cell |
|
M = 11 J = 5 |
(demonstration models) |
11 < 3 ( 6 ) - 6 |
click image to enlarge |
unstable |
|
needs + 1 (red brace) |
|
|
|
. |
This
exposes the unstable square face midsection of the octahedra, which must
be braced |
as shown
in Fig. 301 to the right. |
. |
 |
A
unique lattice tower can also be built with only tetrahedra cells
that are |
joined face to face. It is called a tetra helix. Its
twisted structure bears a |
striking resemblance to the double helix structure of DNA. |
|
◄
Fig. 302 - Tetra helix tower |
(demonstration model) |
click image to enlarge |
|
|
. |
These tower-like constructions can also be used as structural |
 |
members, called space trusses, in extended frameworks. The |
image to the right shows an octahedral shaped lattice structure |
whose members are segments of space trusses, identical to the |
Type C tower design, that are joined together end to end like |
struts. This octahedra structure could in turn be stacked face
to |
face with others like it to build a multi-frequency Type A tower. |
|
Fig. 303 - Octahedral structure built
from Type C space trusses |
|
|
. |
In the
next lesson on mechanical engineering you will see how machines are
designed |
by
balancing the stable (i.e. unmoving) and unstable (i.e. moving) components
to achieve |
the
desired function. |
|
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Page 156
- Building stability - Towers and space trusses |
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