Seismic ratcheting of rc columns with eccentric gravity loadings and non-symmetric lateral stiffness and strength.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Structures with different lateral force resistance or different stiffness in both directions due to eccentric gravity loading and/or strength differences may have a tendency to displace more in once direction than the other during strong earthquake shaking. This effect is known as “ratcheting”.
This thesis examines current provisions to consider ratcheting, provides methods to estimate the displacement demands for single-degree-of-freedom reinforced concrete (RC) and steel structures that have eccentric gravity loading and subjected to strong ground motion shaking by performing inelastic time history analyses using a fibre section model with different force reduction factors, periods, eccentric moments, and axial load ratios. Methods to mitigate ratcheting were developed and a number of approaches to best consider ratcheting effects are also evaluated.
It was found that the wording used to define some parameters in NZS1170.5 (2016) is confusing and can be interpreted in different ways, and new definitions to clarify these complexities were proposed. It was also found that the new provisions are conservative in estimating the maximum displacement for RC structures with eccentric gravity loading for different eccentric moments and periods. However, it needs to be modified to account for higher values of force reduction factor (R>5) and lower axial load ratios (P/Po<0.1). The results have shown that when there is no mitigation of ratcheting, the steel structures had significantly higher ratcheting displacements than those modelled as reinforced concrete. Graphs and empirical equations were developed to estimate these displacements for a set of earthquake records. Increasing the strength in the weak side of the column by 3.4 times the eccentric moment was required to mitigate maximum displacement ratcheting for RC columns. Furthermore, it was shown that of the methods evaluated to account for ratcheting, the best one was the period ratio, which is based on calculating the period in each direction, when the eccentric moment was less than 0.4.