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Computer model with tension only bracing

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Pat Ebner wrote:

>>I want to make a computer model of a tower, as truss members, with tension
only diagonal bracing.  If the structure is symmetrical, how do I make the
model stable with symmetrical gravity loading only?<<

Let's see if I am envisioning this correctly.  You have a completely 
symmetrical tower (possibly with tapered legs) with completely symmetrical 
gravity loading with the diagonals as tension only members and you want to 
make the model stable.

Recalling from statics, the "ball at the top of a hill" being described as 
being "unstable equilibrium," I would say that the tower without diagonals is 
in the same situation --- it is in equilibrium, but it is precariously 
stable as only a slight lateral force makes it unstable.  But, your 
conditions preclude a lateral force.

I would suggest modeling the tower without diagonals since it is stable 
without lateral forces.  The horizontal members connecting the legs where the 
diagonals attach would have a fictitious force in them that can be resolved 
later by considering FBD's of the joints.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona