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This page is devoted to *frequently asked questions* (FAQ) related to [constraints|kb:Constraint].
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h2. 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|etabs:Home], while _edge_ constraint is used in [SAP2000|sap2000:home].


h2. What are the advantages and disadvantages between using rigid links and body constraints?

*Extended Question:* I am using the [bridgeBridge modelerModeler|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|kb:Link] 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|kb:Joint] 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.

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*Related Incident:*

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h2. Why are numerical problems reported for diaphragm constraints?

*Answer:* Please see the [diaphragmDiaphragm constraint|kb:Diaphragm constraint] article for response.


h2. Why do edge constraints have displacement discontinuity?

*Answer:* Please see the [lineLine (edge) constraint|kb:Line (edge) constraint] article for response.


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*When would a body constraint be applied with only some DOF constrained?*
*Answer:* {open-question}
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*What behaviors occur when only some DOF are constrained?*
*Answer:* {open-question}
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*Is there an example of how constraint coupling may cause numerical instability?*
*Answer:* {open-question}
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