by the
table on the end of each of the two legs also equals the downward force of
the load 
(orange
arrow). However, initially, the tensile forces being exerted by the rubber
are less 
than the
tensile stresses being induced in it by the downward force that is trying
to spread 
the legs
apart and so the rubber band stretches. That is, the forces acting
on and within the 
triangle
structure are not completely balanced and therefore the structure moves,
or 
deforms,
in the direction the greater force is pointed. As long as you
continue to increase 
the
downward pressure the legs will continue to spread apart, the rubber band
will stretch, 
and the
triangle will not achieve a state of stable equilibrium. 

Now, if
you hold a steady downward pressure on the apex, so that the force does
not 
increase
or decrease, you still feel an upward reaction force but the legs of the
triangle no 
longer
move (this is represented in the forgoing figure by the extreme downward
position 
of the
triangle with its legs spread apart). In that position the internal
tensile forces exerted 
by the
rubber band now equal the internal stresses induced in it by the load and
so it does 
not
continue to stretch. The triangle does not deform any more because the sum
of all the 
downward, or negative forces, is equal to, or in equilibrium with, the sum
of all the upward, 
or
positive, reaction forces. That is, the sum of the forces equals
zero. This also shows that 
the
internal strength of the weakest member of the structure (the rubber band)
must be 
equal to
or greater than the internal tensile or compressive stresses it is
subjected to for the 
structure to be stable and resist deformation. 

Now
replace the rubber band with a Polymorf rectangle (REC) panel for the base
and try to 
push
down on the apex. You can feel that the upward reaction force is now
significantly 
greater.
And the legs of the triangle do not spread apart. This is because
the tensile forces 
. 



Fig. 129  Demonstrating the forces and 
reactions of a stable triangle structure 
(demonstration model) 


. 
being
exerted by the material comprising the base panel matches the tensile
stresses being 
induced
in it by the downward force of your push. The harder you push the
harder the 
structure pushes back. Here again the sum of the external and
internal forces equal zero 
and the
triangle is in a state of stable equilibrium. Actually the tensile
strength of the 
plastic
in the base panel is so great that the joints will rupture before it
stretches noticeably 
causing
the triangle structure to collapse into a heap in order to reach a new
state of 
equilibrium. Try collapsing the structure yourself to see that this
is so. In doing this you are 
destructive testing the structure to empirically determine its point and
mode of failure. 
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Page 83 
Building Stability  Forces and reactions 

