Models are typically generated with columns attached to ground. In order to use load panels it is necessary to close loops of beam (or 1D) elements. Create Base Dummy Elements creates a set of dummy beam elements to form enclosed areas. The base level is considered to be z elevation value specified in the specification.
Normally a set of grid lines will be defined prior to building a GSA model. This option tries to generate a series of grid lines base4d on the geometry defined in the model.
This is available from the Model > Create rigid membranes from storeys menu command. A rigid constraint is created that corresponds to each storey. The constraint type is set to xy plane so that the floor is considered rigid in-plane but can deform in the out-of-plane direction.