Bulk Data Entry Defines a connection between opposite edges/faces of the structure. This entry is used to apply Periodic Boundary Conditions to the model.


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PERBC 2 45 32 0.01


Field Contents SI Unit Example
ID Set identification number.

(Integer > 0)

GSID Identification number of a SET of grid points on one side of the structure.

(Integer > 0)

RLID A RELOC Bulk Data Entry identification number that maps the nodes on one side of the structure to the other. An exact one-to-one match between the two sides is required.

(Integer > 0)

TOL Specifies the numeric value defining the maximum distance between two grid points to allow equivalence. All grid points in the set defined by GSID and the set of grid points on the other side of the structure (mapping defined by the RELOC entry) are considered for equivalence based on the tolerance.

(Real > 0.0)


  1. The PERBC entry can be used to apply periodic boundary conditions, to match results from one side of the structure to another.
  2. The referenced RELOC Bulk Data Entry should be defined with TYPE=MOVE or TYPE=ROTATE. For TYPE=ROTATE, a simple rotation about a single axis should only be defined. All grids on one side of the structure (defined via GSID) and matching grids on the other side cannot have a coordinate system (field CD on GRID entry) defined. If RELOC(ROTATE) is specified, then all matching grids on both sides must have a cylindrical coordinate system assigned with the Z-axis matching the axis used for RELOC rotation.
  3. Periodic Boundary Conditions are supported for all implicit solution sequences (except OptiStruct-Multibody Dynamics (OS-MBD)) and all optimization types. In shape optimization, when GRID's identified on the PERBC entry are in or near the optimized zone, their location changes should be properly linked through DVGRID, otherwise the results may be incorrect.