Note that the axis parallel to the back of the channel is the weaker axis (i.e., lesser moment of inertia). The moment of inertia value needed from the standard steel tables for this calculation is dictated by the configuration of the built-up shape. In this instance, it is the channel's weak axis that is parallel to the z axis of the built-up shape.

Using the parallel-axis theorem, the moment of inertia I_z for the built-up shape can be calculated.