Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis Solutions
	Subarea:	Miscellaneous Analysis Solutions
	Original Author:	Biswatosh Purkayastha, Bentley Technical Support Group

During the analysis of a structure suspended from a pivot, STAAD reports an instability warning message. Why?
 

When a structure is suspended from a single point defined as a pinned support, it can swing about that point like a pendulum since a pinned support cannot resist the moments MX , MY or MZ. This is a rigid body motion. Rigid body motions are not handled during static analysis and are reported as an instability condition.

One may assign a weak spring to stabilize the structure rotationally at that support as shown below.

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SUPPORTS

93 FIXED BUT KMX 0.05 KMY 0.05 KMZ 0.05

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This should avert the instability condition. Although a moment would be developed at support, its magnitude will be negligible.

Alternatively, additional supports defined at the lower levels that prevent translation along FX and FZ may also be specified as a remedy to this problem.

Note that only the initial position for geometry and loading are considered during this type of linear static analysis. If the structure does rotate as a rigid body at the site, that condition is not being analyzed with the above arrangement.

In the practical scenario, the structure will try to attain a position where the vertical load acts along a vertical line passing through the support so that there will be no eccentricity. That is, the support and C.G coordinates lie on the same vertical line .

Consider the example shown below.

 

The complete arrangement is being suspended from a single top support. Due to eccentricity of the center
of gravity with respect to the support, the initial loading condition induces an initial rigid body rotation.

If one wishes to avoid using stabilizing rotational springs, the structure has to be analyzed in the equilibrium position.

For this, the eccentricity needs to be determined (the horizontal distance of CG from the X and Z coordinates of the point of suspension). The center of gravity of loading can be found in the output file with the help of the PRINT CG command .

From this data, Θ be computed . Next, rotate the structure by that angle “Θ” , so that the load is aligned with a line passing through the support coordinate. Thus, you are now analyzing the arrangement at the equilibrium position.