STAAD.Pro - continuous cable - intermediate supports

Please try adding small dummy vertical members at the intermediate support locations. Release the horizontal shear DOF at the upper end of these vertical members. Assign a dummy material property to these members ( High E, zero density ). An example is attached for your reference.

 

Structure2.std

 

How is the situation different from say, a bridge beam supported by a roller support which prevents movement in the vertical direction, but allows movement in the horizontal direction? One normally applies a "FIXED BUT FX MX MY MZ" support in such cases. If the beam is continuous across the support, the segments on both sides of the support have the same displacement at that point and the same set of forces and moments.

You write that

"It seems to me the support is displacing and that STAAD does not understand the cable continuity across the support."

Can you elaborate on what result you are looking at that tells you that the cable is not behaving as you intend?

Regarding your question :

Is that the actual support displacing, or does the program understand that the cable is free to move but the support stays in place?

Our response:

Think of the support as a collection of 6 springs, one for each degree of freedom (d.o.f). A d.o.f that is fully restrained has a spring constant (stiffness) that is infinity, while the springs associated with d.o.f's that are free to displace have zero stiffness. So, when you ask if the support is displacing, the answer is that the springs corresponding to the unrestrained d.o.f are moving while those corresponding to full restraint are not. The joint representing that end of the cable is attached to all 6 springs, so, the joint and hence the cable moves in tandem with all those 6 springs. If you apply the "FIXED BUT FX MX MY MZ" support as mentioned above, you are telling the program that the cable is free to move in X but prevented from moving in Y at that point.

Thank you. Your response obviously makes sense. It should behave no differently than any other member with a roller support. I think we model so few beams on rollers which also have an applied tension, that the results appeared odd when in fact they are exactly what they should be.

Thank you for the response. This may be closer to my actual condition anyway. It's not actually the simple cable running thru the hole in a column as I described (for purposes of framing the question simply).