Constraint FAQ

What is the difference between a line constraint and an edge constraint?

Answer: Line and edge constraint both refer to the same modeling feature except that line constraint is terminology specific to ETABS, while edge constraint is used in SAP2000.

What are the advantages and disadvantages between using rigid links and body constraints?

Extended Question: I am using the Bridge Modeler to model an offset rocker bearing. A combination of link objects seems to simulate the connection between girder and pier systems. Is there an explanation for the difference between links and body constraints?

Answer: Application of either rigid links or body constraints is suitable for modeling a rigid connection. A comparison of these modeling techniques is described as follows:

Advantages of using links:

• Links are clearly shown in the model, which may assist review.

• Internal forces are obtained for links, which is not the case for body constraints.

• Link stiffness may be modified, if necessary, to match that of the component being modeled. Joints constrained using body constraints are always fully fixed.

Advantages of using body constraints:

• Using body constraints reduces the number of degrees of freedom (DOF) in the model, which speeds numerical solution.

Why are numerical problems reported for diaphragm constraints?

Answer: Please see the Diaphragm constraint article for response.

Why do edge constraints have displacement discontinuity?

Answer: Please see the Line (edge) constraint article for response.