Products: ABAQUS/Standard ABAQUS/Explicit ABAQUS/CAE
The orientation of a beam cross-section:
is defined in terms of a local, right-handed axis system; and
can be user-defined or calculated by ABAQUS.
The orientation of a beam cross-section is defined in ABAQUS in terms of a local, right-handed (
, 
, 
) axis system, where 
is the tangent to the axis of the element, positive in the direction from the first to the second node of the element, and and are basis vectors that define the local 1- and 2-directions of the cross-section. is referred to as the first beam section axis, and is referred to as the normal to the beam. This beam cross-sectional axis system is illustrated in Figure 15.3.4–1.
For beams in a plane the -direction is always (0.0, 0.0, –1.0); that is, normal to the plane in which the motion occurs. Therefore, planar beams can bend only about the first beam-section axis.
For beams in space the approximate direction of must be defined directly as part of the beam section definition or by specifying an additional node off the beam axis as part of the element definition (see “Element definition,” Section 2.2.1). This additional node is included in the element's connectivity list.
If an additional node is specified, the approximate direction of is defined by the vector extending from the first node of the element to the additional node.
If is defined directly for the section and an additional node is specified, the direction calculated by using the additional node will take precedence.
If the approximate direction is not defined by either of the above methods, the default value is (0.0, 0.0, –1.0).
-direction may be used to determine the -direction (discussed below). Once the -direction has been defined or calculated, the actual -direction will be calculated as 
, possibly resulting in a direction that is different from the specified direction.| Input File Usage: | Use the following option to specify the -direction directly for a beam section integrated during the analysis: |
*BEAM SECTION Use the following option to specify the *BEAM GENERAL SECTION Use the following option to specify an additional node off the beam axis to define the *ELEMENT |
| ABAQUS/CAE Usage: | Property module: Assign |
| Specifying an additional node off the beam axis is not supported in ABAQUS/CAE. |
For beams in space you can define the nodal normal (-direction) by giving its direction cosines as the fourth, fifth, and sixth coordinates of each node definition or by giving them in a user-specified normal definition; see “Normal definitions at nodes,” Section 2.1.4, for details. Otherwise, the nodal normal will be calculated by ABAQUS, as described below.
If the nodal normal is defined as part of the node definition, this normal is used for all of the structural elements attached to the node except those for which a user-specified normal is defined. If a user-specified normal is defined at a node for a particular element, this normal definition takes precedence over the normal defined as part of the node definition. If the specified normal subtends an angle that is greater than 20° with the plane perpendicular to the element axis, a warning message is issued in the data (.dat) file. If the angle between the normal defined as part of the node definition or the user-specified normal and 
is greater than 90°, the reverse of the specified normal is used.
| Input File Usage: | Use the following option to specify the -direction as part of the node definition: |
*NODE node number, nodal coordinates, nodal normal coordinates Use the following option to define a user-specified normal: *NORMAL |
| ABAQUS/CAE Usage: | Defining the nodal normal is not supported in ABAQUS/CAE; the nodal normal calculated by ABAQUS is always used. |
If the nodal normal is not defined as part of the node definition, normal directions at the node are calculated for all beam and shell elements for which a user-specified normal is not defined (the “remaining” elements). For beam elements the normal direction is calculated by forming the cross product . For shell elements the normal direction is orthogonal to the shell midsurface, as described in “Shell elements: overview,” Section 15.6.1. The following algorithm is then used to obtain an average normal (or multiple averaged normals) for the remaining elements:
The normal direction belonging to the lowest numbered remaining element is taken as the reference normal used for averaging.
The normal direction of each remaining element is then compared with the reference normal. If the angle with the reference normal is less than 20° (for shell elements with or without reversal of the normal direction), averaging will be used.
The nodal normal used by the elements satisfying the above criteria is then calculated by averaging the element normal directions.
To ensure proper application of loads that act normal to the beam cross-section, it is important to have beam normals that correctly define the plane of the cross-section. When linear beams are used to model a curved geometry, appropriate beam normals are the normals that are averaged at the nodes. For such cases it is preferable to define the cross-sectional axis system such that beam normals lie in the plane of curvature and are properly averaged at the nodes.
In ABAQUS/Standard normal direction definitions can result in a beam element having an initial curvature or an initial twist, which will affect the behavior of some elements.
When the normal to an element is not perpendicular to the beam axis (obtained by interpolation using the nodes of the element), the beam element is curved. Initial curvature can result when you define the normal directly (as part of the node definition or as a user-specified normal) or can result when beams intersect at a node and the normals to the beams are averaged as described above. The effect of this initial curvature is considered in cubic beam elements. Initial curvature resulting from normal definitions is not considered in quadratic beam elements; however, these elements do properly account for any initial curvature represented by the node positions.
Similarly, nodal-normal directions that are in different orientations about the beam axis at different nodes imply a twist. The effect of an initial twist, which could result from normal averaging or user-defined normal definitions, is considered in quadratic beam elements.
In ABAQUS/Explicit initial curvature of the beam is not taken into account: all beam elements are assumed to be initially straight. The element's cross-section orientation is calculated by averaging the - and -directions associated with its nodes. These two vectors are then projected onto the plane that is perpendicular to the beam element's axis. These projected directions and are made orthogonal to each other by rotating in this plane by an equal and opposite angle.
Beam Section Orientation: select region and enter the