Effect of eccentric moments on seismic ratcheting of single-degree-of-freedom structures
This study describes the background and provides validation of new provisions included in the 2016 amendment to NZS1170.5 to address ratcheting due to the presence of static eccentric moments. The validation was done by performing nonlinear time history analysis using a fibre model of a bridge column with a cracked section period of 1.0 s subjected to the FEMA P695 far-field ground motion suite. Peak and residual displacements were used as indicators of the degree of ratcheting. The effects of member axial loads and design force reduction factors on ratcheting were also investigated. It was found that the peak displacement demands increase with an increasing eccentric moment. The increase in displacement when the ratio of effective lateral strengths in the back-andforth directions considering the presence of eccentric moment is 1.15 or less is lower than 10% compared to when equal lateral strengths were provided. Based on these findings, the ratcheting effect can be neglected for cases with low eccentric moments. When the effective strength ratio is increased to 1.86 and the eccentric moment applied is equal to 30% of the yield moment capacity, the peak displacements increased by more than 65% and residual displacements may also become close to the peak displacements. Design curves for estimating the displacement demands for different eccentric moments are provided.