Towers and
space trusses 
. 
Nonspace enclosing structures such as towers, antennas, and scaffolds can
also be 
triangulated for strength and stability. Another one of the first
structural uses for the octet 
truss
geometry was a twentyfive meter tall observation tower built in Nova
Scotia by Dr. 
Bell in
1907. Notice that the top of the tower is an octet truss that is
sliced parallel to the 
111
plane to form the viewing platform. The legs are sliced from the
octet truss parallel to 
the
squared 100 plane. 
. 



◄ Fig. 297  Bell's
tower (1907) 

scale visualization model ► 
(built with all LT) 

click image to enlarge 


. 
Other
freestanding tower designs can incorporate elements of the octet truss.
Fig. 298 
shows
two tower designs based on the octahedron. Fig. 298 a) is a lattice tower, called 
Type A,
comprised of octahedral cells joined face to face. Readers of
the previous lesson 
. 




Fig. 298  Type A and B 
Fig. 299  Type B 
towers 
unit cell 

M = 20 J = 9 
(demonstration models) 
20 < 3 ( 9 )  6 
click image to enlarge 
unstable, needs +1 M 

ST (blue), RT (green) 



. 
on
crystallography will recognize it as being structurally identical to a
section of the HCP 
crystal
lattice structure (a variation of the octet truss). Since the tower
is completely 
triangulated, it is inherently stable. Fig. 298 b) shows a tower, Called type B, built from 
octahedra that are joined vertex to vertex. Vertical braces, or
stays (green), are used to 
position
the octahedra upright. The unit cell of this structure, pictured in
Fig. 299, is not 
stable.
Therefore it must be braced against torsional or twisting movement.
This can be 
done,
redundantly, by cross bracing the face of the rectangular shaped sections
with cable. 

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 Building stability  Towers and space trusses 

