NASTRAN File: Conventions

Description

Some differences between conventions used in NASTRAN and Straus7 are described below.

Beam Local Axis System

In NASTRAN, the 1 direction is along the beam; the 2 direction at right-angles to 1 in the plane of the reference node or reference vector; the 3 direction is at right-angles to 1 and 2.

In Straus7, the 3 direction is along the beam; the 2 direction is at right angles to 3 in the plane of the reference node; the 1 direction is at right-angles to 2 and 3. The corresponding beam axis mappings are:

Straus7	NASTRAN
+1	-3
+2	+2
+3	+1

See Beam Elements: Local and Principal Axes.

Plate Local Axis system

The local x-axis of the plate element in NASTRAN is defined below:

NASTRAN Plate Type	Local x-axis
CQUAD4	Bisects the angle formed by the diagonals N1-N3 and N2-N4.
CQUADR	Bisects the angle formed by the diagonals N1-N3 and N2-N4.
CQUAD8	Follows lines of constant eta (equals CQUAD4 definition when plate edges are straight). Since the local x-axis direction varies across the surface of the plate, Straus7 approximates this as being parallel to the line N8-N6.
CTRIA3	Parallel to the line N1-N2.
CTRIAR	Parallel to the line N1-N2.
CTRIA6	Follows lines of constant eta (approaches CTRIA3 definition when plate edges are straight). Since the local x-axis direction varies across the surface of the plate, Straus7 approximates this as being parallel to the line N1-N3.
CTRIAX6	Parallel to the radial direction.

In Straus7, the local x-axis of plate elements are defined as:

Straus7 Plate Type	Local x-axis
Tri3	Passes through N1, and bisects the line N2-N3.
Quad4	Bisects the lines N2-N3 and N4-N1.
Tri6	Parallel to the line N1-N5.
Quad8	Parallel to the line N8-N6.
Quad9	Parallel to the line N8-N6.

See Plate Elements: Local Axes.

PBEAM, PBEAML and shear centre offsets

Straus7 and Nastran use different conventions for the location of the cross section at the nodes of beam elements: whereas Straus7 positions the centroid of the cross section to the node, Nastran positions the shear centre to the node. For doubly-symmetric sections, the two are the same. For cross sections where the shear centre and the centroid are not coincident, an offset is required for both importing and exporting to/from Straus7 and Nastran, in order to produce the same structure. Straus7 performs this offset automatically.

When beam section data is imported/exported using the PBEAM option, the automatic offset produces near identical results. The PBEAM card explicitly specifies the shear centre location, so the offset performed by Straus7 will produce matching element stiffness and element-based loads for both import and export.

When beam section data is imported/exported using the PBEAML option, which does not specify the shear centre location, the automatic offset might not produce identical results. This is because the procedures used by Straus7 and Nastran for the calculation of the shear centre are not the same and therefore they do not produce identical shear centre locations.

Location of Beam Element Loads

NASTRAN gives beam locations as length values (if the load scale type is LE or LEPR), or fractional values (if the load scale type is FR or FRPR). In the case of distributed loads, NASTRAN provides positions P1 and P2 relative to the first end of the beam.

Straus7 converts all locations to fractional values. In the case of distributed loads, Straus7 converts locations so that P1 is relative to the first end, and P2 is relative to the second end. For example, suppose a NASTRAN beam is 6.0 metres long, with P1 at 2.0 metres, and P2 at 4.0 metres. Straus7 will represent these locations as (0.33, 0.33).

Direction of Plate Normal Pressure

In NASTRAN, a positive pressure is in the same direction as the surface normal. The surface normal is defined according to the right hand rule with respect to the sequence of nodes defining the plate.

In Straus7, plate normal pressure can be applied to the +z and/or -z surfaces, with positive value indicating inward pointing pressure.