# # Beam axes

## # Symmetry about the axis

In most cases beams have symmetry about one axis or both axes (e.g., a rectangular, circular, I-beam or tee section). In these cases the principal axes of the beam *correspond* with the local axes. This means that the bending behaviour is governed by:

## # No symmetry about the axis

When a section lacks symmetry about both axes, the cross term *y* direction will produce a *displacement* in *y* and *z* directions.

The angle of the principal second moments of area can be calculated from:

The second moments of area in the *u* and *v* axes are:

These relationships can be visualised using a Mohr's circle approach similar to stress.

Note:GSA gives the option to use eitherprincipalsecond moments of area, orlocalsecond moments of area (ignoring theterm). Principal moments most often apply to asymmetric or angle sections; by using this option it is possible to capture sideways deflection. Therefore, the latter option is not recommended.

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