I am analyzing a mat using PLATE MAT support and getting a warning " **WARNING, 1 or more joints are in multiple SUPPORT commands ...". What does the warning mean ?
For models with large number of plates, the software automatically splits the PLATE MAT commands into multiple commands such that for each command, the list of plates is less than a certain number. Even if you have assigned the PLATE MAT through a single command that software may split the command as it can only support upto a certain number of entities in one list. This is not a problem but can lead to the warning in question, as explained next.
The software calculates the spring stiffness at each node in the plate mat by multilying the subgrade modulus by the influence area for the node. Each plate connected to the node contributes to this influence area calculation. If these plates are part of separate PLATE MAT commands due to the reason mentioned above, a single node may get multiple spring support assignments from multiple PLATE MAT support commands. This is not a problem as all the spring stiffness at the node would eventually be added to produce the correct spring stiffness at the node. However the software would issue a warning trying to make the user aware that some nodes are getting supports assigned thorugh multiple support commands. In situations like these, the warning may be safely ingnored.
For some other types of support assignment, there may actually be duplicate support assignments at a particular node due to oversight, which can also lead to the same warning. For example sometimes the user may inadvertertently assign two different types of support ( say PINNED and FIXED ) at the same node. In such cases it may be worth checking and fixing such modeling mistakes.