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.

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. See the attached modified.std model

modified.std


This will get rid of the warning message.