Why is displacement a more reliable performance indicator compared to force when assessing structural performance using nonlinear methods?

Displacement is a more reliable performance indicator compared to force when assessing structural performance using nonlinear methods because it directly relates to the damage and deformation experienced by the structure, particularly after yielding has occurred. In nonlinear analysis, the relationship between force and displacement is no longer linear; after a structural element yields, the force it can resist may increase very little even with significant increases in displacement. Therefore, relying solely on force as a performance indicator can be misleading because it doesn't accurately reflect the extent of damage or the structure's proximity to collapse. Displacement, on the other hand, provides a more direct measure of the deformation demands on structural elements and their ability to accommodate these deformations without failure. Performance criteria in nonlinear seismic design are often defined in terms of displacement limits or drift ratios (displacement divided by height), which are directly related to the strain in structural members and the potential for damage to both structural and non-structural components. Furthermore, displacement-based assessment allows for a better understanding of the structure's overall behavior and its ability to dissipate energy through inelastic deformation. For example, a structure might be able to withstand a certain force level, but if the corresponding displacement exceeds the allowable drift limits, it indicates that the structure is undergoing excessive deformation and is at risk of damage or collapse. Therefore, displacement provides a more reliable and direct measure of structural performance in nonlinear analysis.