Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Modeling Solutions
	Subarea:	Specifications
	Original Author:	Bentley Technical Support Group

While using the "Relationships" command to define a rigid diaphragm in a framed multi-storied structure, what criteria should one adopt to determine the "Control node"?

For an ordinary static analysis, any of the joints in a control/dependent system can be the control joint.

A second point to understand about the STAAD.Pro  input is that the command

DEPENDENT ZX CONTROL j JOINT joint-spec

should be used for ordinary floor rigid diaphragms (assuming they are parallel to the ZX axes). This command sets the ZX plane to behave rigidly for inplane actions; while out-of-plane shear and bending will behave flexibly.

The command

DEPENDENT RIGID CONTROL j JOINT joint-spec

should be used if the control/dependent connections behave rigidly in all directions and rotations.

For a natural frequency calculation, seismic analysis and dynamic analysis, the control joint should be the joint nearest to the c.g. of the masses associated with the control/dependent joints.

If the c.g. is well off the structure as in an L shaped structure with narrow wings, you may want to add a joint at the c.g. just for the purpose of being the control; [remember to connect that joint to another joint with a dummy (low E value) member if it is not already connected].