Compound Section Design

Compound sections where a secondary section is added to primary section are designed as follows. 

The the major axis forces are taken by the primary section and minor axis forces are taken by the secondary section.

In the lateral torsional buckling check of the major axis section, the full compound section properties are used in the calculation of Mcr, therefore the two sections acting together are used to benefit the LTB stability check. Like wise for the LTB of the minor axis secondary section.

The compound section properties are also used at analysis time, therefore the member deflection is based on the full compound section.