A structural engineer is analyzing a 5-story building and deciding between using a moment-resisting frame (unbraced) or a concentrically braced frame for lateral stability.

Scenario: The choice directly affects the design of the gravity columns.

Solution: In a braced frame, lateral translation at the floor levels is prevented by the diagonal braces. The columns can be designed with an effective length factor ($K$) near $1.0$.

In an unbraced moment frame, the frame must sway to resist lateral loads (sidesway uninhibited). The columns in an unbraced frame typically have $K$ values greater than $1.0$ (sometimes well over $2.0$), significantly increasing their effective slenderness ($KL/r$) and reducing their axial compressive capacity. Consequently, the columns in the unbraced frame must be much larger and heavier.

Tall steel storage racks in a warehouse are failing under heavy pallet loads. The columns are built-up channel sections.

Scenario: The racks are buckling, but not in the direction one might expect. The engineers must diagnose the failure mode.

Solution: An inspection reveals that the columns are buckling globally, but specifically about their weak axis (the y-axis). While the beams connecting the columns provide strong-axis bracing, there is insufficient bracing in the longitudinal direction of the aisle.

Because a column will always buckle about the axis with the largest $KL/r$ ratio, the lack of weak-axis bracing caused $KL_y / r_y$ to exceed the critical limit, leading to premature failure. The retrofit involved adding diagonal cross-bracing along the aisles to reduce the unbraced length $L_y$.