What does a zero stiffness warning message in the STAAD output file mean?
The procedure used by STAAD in calculating displacements and forces in a structure is the stiffness method. One of the steps involved in this method is the assembly of the global stiffness matrix. During this process, STAAD verifies that no active degree of freedom (d.o.f) has a zero value, because a zero value could be a potential cause of instability in the model along that d.o.f. It means that the structure has no ability to resist a load acting along that d.o.f.
A warning message is printed in the STAAD output file highlighting the node number and the d.o.f at which the zero stiffness condition exists.
Examples of cases which give rise to these conditions :
Consider a frame structure where some of the members are defined to be trusses. On this model, if a joint exists where the only structural components connected at that node are truss members, there is no rotational stiffness at that node along any of the global d.o.f. If the structure is defined as STAAD PLANE, it will result in a warning along the MZ d.o.f at that node. If it were declared as STAAD SPACE, there will be at least 3 warnings, one for each of MX, MY and MZ, and perhaps additional warnings for the translational d.o.f.
These warnings can also appear when other structural conditions such as member releases and element releases deprive the structure of stiffness at the associated nodes along the global translational or rotational directions. A tower held down by cables, defined as a PLANE or SPACE frame, where cable members are pinned supported at their base will also generate these warnings for the rotational d.o.f. at the supported nodes of the cables.
Solid elements have no rotational stiffness at their nodes. So, at all nodes where you have only solids, these zero stiffness warning messages will appear.
These are warnings and not errors because :
The reason why these conditions are reported as warnings and not errors is due to the fact that they may not necessarily be detrimental to the proper transfer of loads from the structure to the supports. If no load acts at and along the d.o.f where the stiffness is zero, that point may not be a trouble-spot.
What is the usefulness of these messages :
A zero stiffness message can be a tool for investigating the cause of instabilities in the model. An instability is a condition where a load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through, and may result in a lack of equilibrium between the applied load and the support reaction. A zero stiffness message can tell us whether any of those d.o.f are obstacles to the flow of the load.