Export NASTRAN File: Entities

Description

Nodes

Strand7 exports nodes as GRID entries using large-field format. For each entry, Strand7 supplies the node number and position.

Local coordinate systems

NASTRAN GRID entries have an optional field, CD, which references a local coordinate system. This system is used in conjunction with displacements, degrees of freedom, constraints and solution vectors.

Strand7 exports a coordinate system ID to this field if the node has a restraint defined in a local system. However, Strand7 nodes that have restraints in different freedom cases may reference different coordinate systems.

Strand7 supplies the first coordinate system referenced by a restraint on that node, checking freedom cases sequentially.

This is a reasonable compromise. Although Strand7 does allow a node restraint to reference different coordinate systems in different freedom cases, this will be an unusual modelling requirement. It will be far more common for restraints on a given node to all reference the global XYZ system, or else the same local system.

Beam Elements

Strand7 exports standard beams and user-defined beams as CBEAM entries. Other beam types (bar, cable, cutoff bar, point contact, spring-damper) are exported as CROD entries. Both types of entry are in small-field format. For each entry, Strand7 supplies the beam number, property number and node connections.

Orientation

CBEAM entries are always exported with an orientation vector (X1, X2, X3) which points in the local 2 direction of the beam. If the beam has no reference node, Strand7 chooses the direction in a consistent manner; if the beam does have a reference node, the local 2 direction will be in the plane defined by the two ends of the beam and the reference node. Also, if the beam has a non-zero local axis angle, the local 2 direction will be rotated about the beam axis by this amount.
End-releases

If the beam has any end-releases, these are written out in the fields PA and PB. Only full-releases are written out, not partial releases. For example, if the directions DX, DY and RZ are fully released, Strand7 writes out "126". NASTRAN and Strand7 both define beam end-releases in the beam principal axis system.
Offsets

If the beam has any offsets, these are written out in the fields W1A to W3B. The same vector will be written out at each end, since Strand7 only supports a single offset vector, whereas NASTRAN supports a vector at each end node.

Strand7 offsets are stored as components in the beam's principal axis system. Before exporting, they are converted to components in the global coordinate system. If either end node was exported with a local coordinate system, the corresponding offset is then converted to components in that system.

Plate Elements

Tri, Quad

Strand7 exports Tri3 elements as CTRIA3 , Tri6 elements as CTRIA6, Quad4 elements as either CQUAD4 or CQUAD4R (depending on the option selected), and Quad8 and Quad9 elements as CQUAD8. All entries are in small-field format. In the case of Quad9 entries the centre node is ignored when exporting. For each entry, Strand7 supplies the plate number, property number and node connections.

Offsets

If the plate has an offset, it is written to the ZOFFS field. Strand7 and NASTRAN describe plate offsets in the same manner: a positive value indicates an offset in the direction of the local +z axis of the plate.
Material orientation

If the plate has a non-zero local angle, it is written out to the NASTRAN file. However, the local axis systems in Strand7 and NASTRAN are not defined the same way. Usually, there is a non-zero angle between the local x-directions as defined by Strand7 and NASTRAN. This angle difference is added to the local axis angle before it is written to NASTRAN, so that the local axes are in alignment.

Brick Elements

Tetra

Strand7 exports Tetra4 and Tetra10 elements as CTETRA entries. All entries are in small-field format. For each entry, Strand7 supplies the brick number, property number and node connections.

Pyra

Strand7 exports Pyra5 and Pyra13 elements as either collapsed hexahedra or as CPYRAM, depending on the option selected.

Wedge

Strand7 exports Wedge6 and Wedge15 elements as CPENTA entries. All entries are in small-field format. For each entry, Strand7 supplies the brick number, property number and node connections.

Hexa

Strand7 exports Hexa8, Hexa16 and Hexa20 elements as CHEXA entries. All entries are in small-field format. For each entry, Strand7 supplies the brick number, property number and node connections.

Entity Numbering

Node numbers are preserved when exporting to a NASTRAN file.
Element numbers are not preserved. Elements are numbered globally, starting from 1. For example: if there are 120 beams, 55 plates and 175 bricks in a given Strand7 model, the NASTRAN file will contain beams numbered 1-120, plates numbered 121-175 and bricks numbered 176-350. Node masses, exported as CONM2 entries, are also given a global element number, because NASTRAN treats node masses as elements.

Links

Rigid Links

Strand7 exports rigid links as RBAR rigid body elements. Dependent degrees of freedom are set to "123456", "126", "234" or "135" for XYZ, Plane XY, Plane YZ and Plane ZX rigid links respectively.

Pinned Links

Strand7 exports pinned links as RROD rigid pin-ended rod elements. The dependent translational degree of freedom is set to the first global direction found for which (a) the corresponding end node is not fixed in that direction in the global coordinate system, and (b) the direction is not perpendicular to the axis of the pinned link.

Master-Slave Links

Strand7 exports Master-Slave links as one or more MPC (multi-point constraint) entities. Strand7 Master-Slave links may have up to six active degrees of freedom, whereas NASTRAN multi-point constraints may only reference a single degree of freedom, so one MPC is exported for each active degree of freedom for each Master-Slave link.

Master-Slave links are independent of freedom case, whereas multi-point constraints are freedom case-specific, so each set of MPC's is exported once for every freedom case.

The coefficients assigned to the two MPC grid points are +1.0 and -1.0 if the Master-Slave link is a positive link, and +1.0 and +1.0 is the link is negated.