SA 78: How Sidesway Creates Second-Order Moments in Frames

This lecture provides an introduction to second-order analysis of structures. We begin by explaining why first-order analysis can miss an important effect in sway frames: the additional moment created when gravity loads act through the displaced shape of the structure. This effect is known as the P-Delta effect. Using a one-story frame example, we show how sidesway creates secondary moments, how those moments affect member design forces, and why first-order analysis can underestimate critical demands. The lecture also introduces the Moment Amplification Method as an approximate way to account for second-order effects without performing a full nonlinear analysis.