A structural engineer is designing the exterior columns of a single-story commercial building. The roof structure applies a significant gravity load.

Scenario: During a hurricane, high wind pressures act directly on the exterior walls, which transfer the load horizontally to the columns.

Solution: The columns can no longer be designed purely as compression members. They must be designed as beam-columns because they are subjected to a high axial load ($P_u$) from the roof and a high lateral flexural load ($M_{ux}$) from the wind pressure acting along their length.

The interaction equation ensures that the combination of these stresses does not exceed the capacity of the steel. In many cases, an exterior column that passes a gravity-only check will fail the interaction check under wind loading, requiring a section with a much larger moment of inertia ($I_x$) or a thicker web to resist the combined stresses.

A multi-story steel frame building with a tall, open first floor (a "soft story") undergoes lateral displacement ($\Delta$) during a seismic event.

Scenario: The building sways laterally. The massive weight of the upper floors is now displaced horizontally from the foundation.

Solution: This displacement creates a Second-Order Effect ($P$-$\Delta$). The gravity load ($P$) acting on the laterally displaced column ($\Delta$) creates an additional secondary bending moment ($M_{secondary} = P \times \Delta$).

This secondary moment adds to the primary moments caused by the earthquake, further increasing the total moment on the beam-column. To account for this, the AISC Direct Analysis Method requires engineers to apply an amplification factor ($B_1$ and $B_2$) to the primary moments when evaluating the interaction equations, ensuring the columns are designed for the true, amplified forces they will experience.