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


unit cell


M = 11    J = 5

(demonstration models)

11 < 3 ( 6 ) - 6

click image to enlarge



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.


Page 156 - Building stability - Towers and space trusses

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