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FEM and Cables-Reply

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STAAD has pretensioned cable members which are invoked with the MEMBER CABLE 
command. You specify the cable pretension, but these forces must be 
statically admissible, i.e. the forces specified must correspond to a real, 
achievable equilibrium state in the system of cables. If they are not, the 
final forces in the cable results will be significantly different than those 
input in order for the cable system to be in static equilibrium.  For 
symmetrical cable layouts, *equal* pretensioning forces in all the cables is 
statically admissible and hence final cable forces will all be the same, but 
somewhat less than specified if the cables are, for example, connected to a 
stack where elastic shortening takes place under the cable forces. This is 
one little glitch in the program since in the field the pretensioning force 
is measured (?) as the stack shortens and hence elastic shortening of stack 
is accounted for as pretensioning takes place. In STAAD, you would have to 
specify a somewhat larger cable pretension and adjust by trial and error to 
end up with a specific final pretensioning force.

In the situation where the cables are not symmetrical, it is (usually) 
practically impossible to predict beforehand what a set of statically 
admissible cable forces will be.  To determine this, I first run the problem 
of an equal, unit temperature drop on all the cables. However, MEMBER TRUSS 
members have to be used to represent the cable response rather than MEMBER 
CABLES. This then gives the linear force dependence among the truss members 
(cables) for the particular geometry.  Then, I run the lateral load (e.g. 
wind pressures) together with the MEMBER CABLES. This requires several runs, 
incrementing the cable pretensions, but maintaining their  relative, 
statically admissible set of forces as determined from the temperature drop 
analysis, until a desired design criterion is achieved. One such criterion 
might be that no cables go slack under the design lateral load.

As is usually the case with the STAAD folks, none of this necessary procedure 
is even touched on in the STAAD user manual. My approach I discovered on my 
first unsymmetrical cable situation. Maybe there are other approaches. If so 
I would like to hear about them. Also, in the field, I would like to know how 
the erector goes about measuring a specified cable pretension, particularly a 
relatively small 1500 lb. to 2000 lb. required pretensioning force for small 
(3/8' - 1/2" diam) cables, which I encounter with exhaust stacks.