Spurious Degrees of Freedom Warning

Modestas — Tue, 25 Apr 2023 12:35:56 GMT

Current Revision posted to RAM | STAAD | ADINA Wiki by Modestas on 4/25/2023 12:35:56 PM


	Applies To

	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Analysis Solutions
	Subarea:	Instability and Zero Stiffness
	Original Author:	Bentley Technical Support Group

Question:

Even after creating proper beam plate connectivity, why does Staad still report an instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

The STAAD in-plane plate has one "spurious displacement mode" in addition to the 3 in-plane rigid body modes (and 9 elastic displacement modes). Normally, conditions such as edge beam members, supports which suppress rotation about the element local Z axis, other out-of-plane elements which are connected to the element in question, and irregular grids suppress this spurious mode. However, if none of these conditions exist, STAAD automatically creates a fictitious weak spring at one of the nodes to correct this instability, so there is no actual problem, just warning messages.

The structure in your case is one where none of the compensating conditions exist. In other words, the X-Z planar nature of your model, and support conditions or the column tip on which plates rests allows the rotation about the global Y axis meaning that there is nothing to suppress the spurious displacement mode, which is why STAAD is forced to create the weak spring.

You can also introduce an additional dummy beam inside the plate connecting the common node. This will get rid of the warning message.

Tags: analysis, STAAD.Pro, spurious, Instability, degrees of freedom, drilling

Spurious Degrees of Freedom Warning

Seth Guthrie — Fri, 11 May 2018 16:56:21 GMT

Revision 7 posted to RAM | STAAD | ADINA Wiki by Seth Guthrie on 5/11/2018 4:56:21 PM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Area:	Analysis
	Subarea:	Instability and Zero Stiffness

Question:

Even after creating proper beam plate connectivity, why does Staad still report an instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

Tags: analysis, STAAD.Pro, spurious, Instability, degrees of freedom, drilling

Spurious Degrees of Freedom

Seth Guthrie — Wed, 08 Feb 2017 22:47:54 GMT

Revision 6 posted to RAM | STAAD | ADINA Wiki by Seth Guthrie on 2/8/2017 10:47:54 PM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Environment:	N/A
	Area:	Instability and Zero Stiffness
	Subarea:	Spurious Degrees of Freedom
	Original Author:	Biswatosh Purkayastha

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

Tags: spurious, Instability, degrees of freedom, drilling

Spurious Degrees of Freedom

Biswatosh — Wed, 17 Aug 2016 04:11:35 GMT

Revision 5 posted to RAM | STAAD | ADINA Wiki by Biswatosh on 8/17/2016 4:11:35 AM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Environment:	N/A
	Area:	Instability and Zero Stiffness
	Subarea:	Spurious Degrees of Freedom
	Original Author:	Biswatosh Purkayastha

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

modified.std

This will get rid of the warning message.

Tags: drilling degree of freedom, Instability, MY instability, spurious degrees of freedom

Spurious Degrees of Freedom

Sye — Tue, 28 Jun 2016 23:35:45 GMT

Revision 4 posted to RAM | STAAD | ADINA Wiki by Sye on 6/28/2016 11:35:45 PM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Environment:	N/A
	Area:	Instability and Zero Stiffness
	Subarea:	Spurious Degrees of Freedom
	Original Author:	Bentley Technical Support Group

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

modified.std

This will get rid of the warning message.

Tags: drilling degree of freedom, Instability, MY instability, spurious degrees of freedom

Spurious Degrees of Freedom

Sye — Tue, 28 Jun 2016 23:35:10 GMT

Revision 3 posted to RAM | STAAD | ADINA Wiki by Sye on 6/28/2016 11:35:10 PM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Environment:	N/A
	Area:	Instability and Zero Stiffness
	Subarea:	Spurious Degrees of Freedom
	Original Author:	Bentley Technical Support Group

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

modified.std

This will get rid of the warning message.

Tags: drilling degree of freedom, Instability, spurious degrees of freedom

Spurious Degrees of Freedom

Steve Crabtree — Thu, 23 Jun 2016 17:36:13 GMT

Revision 2 posted to RAM | STAAD | ADINA Wiki by Steve Crabtree on 6/23/2016 5:36:13 PM


	Applies To

	Product(s):	STAAD.Foundation Advanced
	Version(s):	All
	Environment:	N/A
	Area:	Instability and Zero Stiffness
	Subarea:	Spurious Degrees of Freedom
	Original Author:	Bentley Technical Support Group

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

modified.std

This will get rid of the warning message.

Tags: spurious degrees of freedom

Spurious Degrees of Freedom

Steve Crabtree — Wed, 15 Jun 2016 17:36:52 GMT

Revision 1 posted to RAM | STAAD | ADINA Wiki by Steve Crabtree on 6/15/2016 5:36:52 PM

Spurious Degrees of Freedom

Question:

Even after creating proper beam plate connectivity, why Staad still reports instability warning in my model?

model.std

Explanation:

The plate element tends to deflect inplane about the connected beam plate node by the action of lateral loading and hence due to spurious degree of freedom instability arises.

modified.std

This will get rid of the warning message.

Tags: spurious degrees of freedom