What are second-order effects and when must they be considered in structural analysis?

Second-order effects (P-Delta and P-delta) account for additional moments caused by axial loads acting through displaced positions. P-Delta (global) considers story drift effects, while P-delta (member) considers member deformation. They must be considered when stability coefficient theta = (P*delta)/(V*h) exceeds 0.1, typically in tall buildings, slender structures, or high axial loads. Analysis can use geometric stiffness matrices, direct P-Delta analysis, or amplification factors. Ignoring these effects can underestimate moments by 20-30%.