A Diaphragm Constraint causes all of its constrained joints to move together as a planar diaphragm that is rigid against membrane (in-plane) deformation. This constraint can be used to model concrete floors (or concrete-filled decks) in building structures, which typically have very high in-plane stiffness.
The constraint must apply to at least two joints to have any effect on the model. It is intended to be used on a set of joints that lie in a flat plane. If the joints are not co-planar, the constraint will effectively restrain the joints against out-of-plane (plate) bending, which unrealistically stiffens the structure.
An option is available to automatically create multiple, independent constraints at different elevations from the single set of joints assigned this constraint.
Use the following areas on the Diaphragm Constraints form to make the appropriate entries:
Constraint Name. Use the default or enter a unique name for the constraint being defined. If the multiple-constraint option is used (see below), the independent, automatic diaphragm constraints still share the same name.
Coordinate System. Choose the coordinate system in which the constraint axis (below) is being defined. If the constraint axis is automatic, the choice of coordinate system has no effect on the constraint.
Constraint Axis. Use this area to select the constraint axis. The constraint equations will be written for a diaphragm in a plane perpendicular to the selected axis. If the joints lie in a unique plane, it is usually best to choose the Auto option and let the program automatically determine the axis. This axis does not necessarily need to be parallel to a coordinate-system direction. If the joints are not coplanar, or to force a particular direction, choose the axis in the chosen coordinate system that is perpendicular to the diaphragm. For example, if the Z axis is chosen, this constraint is equivalent to a body constraint in the same coordinate system with translations X and Y and rotation Z chosen. The Z direction must be chosen to use the automatic, multiple-constraint feature (see the next bullet.)
Assign a different diaphragm constraint to each different selected Z level check box. The Assign a different diaphragm constraint to each different selected Z level check box is available if Z is the chosen constraint axis. When this check box is checked, the program will automatically create a separate diaphragm constraint for each subset of joints that are assigned this constraint and have the same Z coordinate. For example, in a simple structure one diaphragm could be assigned to the entire model, and separate diaphragm constraints would be created at each floor level. This assumes that there are no intermediate joints on the columns or walls, and all joints at each floor are at the same elevation. Use with caution!
If the Advanced > Define > Joint Constraints command was used to access the Diaphragm Constraints form, the defined constraint must be assigned to the joints.
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Access the Diaphragm Constraint form as follows:
Click the Advanced > Define > Joint Constraints command to display the Define Constraints form, or select the joints to be constrained and click the Advanced > Assign > Joints > Constraints command to display the Define/Assign Constraints form. Then do one of the following:
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