Warning message in output file: This structure is disjointed, accompanied by several instability warnings.

The error message, "this structure is disjointed..." is caused by multiple structures in the model. Multiple structures exist when one portion of the model does not have any way of transferring forces to another portion. It is possible to create a model that visually appears to be a single structure, but upon closer inspection it is revealed that the model contains two or more unconnected structures. Instability warnings may also appear in the output file, due to the fact that the structure appears to be properly supported when, in fact, it is not. Multiple disjointed structures can be detected by selecting the top menu option Tools > Check Multiple Structures.

What do we mean by member connectivity? What determines whether two members are connected so that one can transfer loading to another? Consider two lines that intersect in 3D space, with the starting point of one line lying on the longitudinal axis of the other line. The fact that the end of one member lies on the axis of another is not sufficient to ensure connectivity. Connectivity is ensured only when intersecting members are divided into segments that meet at a single, common node. ("Common node" means a single node number. Two separate node numbers, both having the same X,Y,Z coordinates is called a duplicate node situation. You can test for duplicate nodes in the model by pulling down the Tools menu and selecting the Check Duplicate command, then select the Nodes command from the Check Duplicate sub-menu).

The following wiki contains more information on connectivity related issues

https://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/20702.connectivity-issues