distance
it can span for a given load, all other factors being equal. For example,
the model 
of
unit
cell c) should be able to span twice the distance or carry twice the load
of unit cell 
a),
because c) has twice the depth as a) but the same module size. Consequently the depth/ 
module
ratio of c) is twice that of a). As a result the diagonal struts of
cell c) are aligned 
more parallel to a perpendicularly applied load. This reduces the internal
stresses its web 
struts are
subjected to. However, as with truss bridges, there is a limit to
how deep a double 
layer
spaceframe lattice can be before other factors such as the slenderness
ratio of the 
struts,
the dead weight of the structure, and its cost must be taken into account.
Note also 
that
when the depth/module ratio of a Tri1 unit cell is equal to 0.816, all of
the struts of the 
spaceframe are the same length. That is, it is a standard octet truss. 
. 
Another
factor affecting the load bearing capacity of a spaceframe is how its
perimeter is 
supported. In general, the more support that is given to the
spaceframe's perimeter, the 
longer
it can span, and the greater load it can carry, all other factors being
equal. For 
example,
for a given load, the required depth to span ratio for a spaceframe
entirely 
supported around its perimeter is about onehalf of that for one that is
just supported at its 
corners. 
. 

Fig. 235  Tri1 spaceframes 

with equal spans but 
different depths 
◄ depth = 5.9 cm = .183 
span 32 cm (all LT) 

depth = 2.9 cm = .092 ► 
span
32 cm (all RT) 

. 
This is
due to the fact that
the maximum bending moment exerted by a load located mid 
span of the structure is a
function of its distance from the nearest support, as you saw before 
with
beams and truss bridges. Supporting the entire perimeter of the
spaceframe reduces 
this distance overall compared to just corner supports. 
. 
The
Tri1 spaceframe is a very efficient design due to its omnitriangulated
geometry that 
minimizes material usage while maximizing its surface area. It is
particularly appropriate 
for
maximum load bearing situations. 
. 

Fig. 236  Tri1 spaceframe 

structures 

◄ offshore oil rig 
band shell ► 
(scale visualization models) 
click image to enlarge 

. 
Variations of the Tri1 spaceframe design are generated by selectively
removing struts from 
the
structure in a regular pattern. Two common examples are Tri3 and
Tri5. 
. 
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 Building stability  Tri1 spaceframe 

