• To: seaint(--nospam--at)seaint.org
• From: James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)
• Date: Tue, 6 Apr 1999 09:21:00 -0400
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.