Alignment of beam elements

Posted28 January 2022

The local axis of a beam element in Frame is defined as shown in Figure 1 below.

analysis img_06 — Figure 1: Alignment of local axes – I & H sections

The alignment of the local axes for beam elements in Frame can be considered for two
cases:

Beams: By default, Frame sees the local y-z plane of a beam as vertical. This means
that the web of the beam element will always be vertical. Entering a value for the βangle will then rotate the beam element around the z-axis.
Columns: When beam elements are used as columns, the orientation of the column is
determined by aligning the web (y-z plane) parallel to the global X-axis.

Consider the model in Figure 2:

analysis img_07 — Figure 2: Frame model – Column appears to be rotated.

All four columns consist of the same beam section and the β-angle for all columns is equal
to zero. Yet one of the columns appears to be rotated. Upon closer inspection, the nodes for
the “rotated” columns are not vertically aligned.

analysis img_08 — Figure 3: Nodes “3” and “4” are not aligned vertically.

Since the nodes that define the “rotated” column are not vertically aligned, column 3-4 is no
longer seen as a column (even though the nodes differ by only 1mm in both the X and Z
directions). The two nodes are now connected by the beam element and the web (y-z
plane) is aligned vertically. This seems to be a small difference but can potentially lead to
large differences in certain analyses.