correspond to the bottom chords of the bridge, experience mainly tensile
stresses. The 
diagonal
struts making up the web can experience either type of stress depending on
their 
position
in the framework relative to the load. Such spaceframes are statically
determinate. 
That is,
their internal forces can be predicted using static analysis techniques.
It is common 
practice, however, to construct real life spaceframes using rigid joints
for the hubs to reduce 
the
forces experienced by the members and the amount of deflection the
spaceframes 
exhibit.
Like the rigid gusset plate joints of truss bridges, and the fixed ends of
columns and 
beams,
rigid hubs impart additional bending stresses in the struts which have to
be figured 
into the
load calculations. Accounting for these perpendicular acting forces
in addition to 
the
axial forces increases the required cross sectional area of the struts, and
the amount of 
material
used for them, compared to spaceframes with flexible joints. In
general the small 
benefit
achieved by rigid hubs is offset by the increased cost of materials.
Although both 
types of
structures are referred to here as spaceframes, technically, lattices with
flexible 
hubs
should be referred to as space trusses, and those with rigid hubs as
spaceframes. 
. 
Just as
you saw that there is a relationship between the H/S ratio of a truss
bridge and the 
maximum
load it can carry, so too the ratio of the depth of a spaceframe to its
span affects 
its load
bearing capacity significantly. In the following example, the depth
of the unit cell 
of the
Tri1 spaceframe is varied. This involves varying the length of the
struts in the inner 
and
outer layers, as well as the web diagonals. You can model this with
Polymorf by simply 
building
the unit cells out of either small triangles (ST), or right triangles (RT),
or large 
triangles (LT), or isosceles triangles (IT) as shown by the following. 
. 
side
view 




depth/module = .41 
depth/module = .82 
depth/module = .82 
depth/module = 1.29 
top view 




a) w/right tri. (RT) 
b) w/small tri. (ST) 
c) w/large tri. (LT) 
d) w/isosceles tri. (IT) 
click image to enlarge 
Fig. 234  Tri1 unit cells with varying
depth/module size ratios (cm)
(demonstration models) 

. 
As shown
by the forgoing models, The length of each strut comprising the grid
layers is 
referred
to as the spaceframe's module size [e.g. cell a) module = 7.18 cm].
And the 
distance
between the layers is its depth [e.g. cell a) depth = 2.95 cm]. As
with Aframe and 
truss
bridge structures, increasing the depth, or height, of a spaceframe
increases the 
. 
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 Building stability  Tri1 spaceframe 

