Multiple earthquake effect on steel structures

Abstract

This thesis provides an extensive investigation into the seismic performance of steel moment-resisting frames that undergo mainshock-aftershock (MS-AS) sequences, while also considering the impact of several aftershocks and retrofitting options. Nonlinear time history analysis was invoked to assess 3-, 9-, and 20-story SAC buildings modelled and analyzed with real ground motion records that represent either near-fault or far-fault ground shaking. The intended novelty was the realistic modelling of sequential earthquake events (including extended aftershocks) to be able to quantify their cumulative structural damage effects. The analysis began by assessing the original (non-retrofitted) frames subjected to either mainshock-only and MS-AS sequences. The most important observation was that aftershocks could influence interstory drift ratios and base shear demands. This was especially significant in the short and mid-rise structures. Damage accumulation is considered the evolution and distribution of plastic hinges. Of the three frames, the 3-story frame experienced the most significant increase in terms of drift and hinge damage due to aftershocks, while the 20-story building was the most stable relative to overall behavior in the early stages of the MS-AS sequences. Seismic resilience can be improved through retrofitting methods that rely on concentric braces in exterior bays of moment-resisting frames. The retrofitted frames showed a considerable improvement in performance. While it was anticipated that base shear would increase to be consistent with the additional lateral stiffness found in the retrofitted frames, the maximum interstory drifts were considerably decreased. The data demonstrates that retrofitting to achieve seismic resilience will significantly reduce damage and improve the overall structural performance under repeated seismic loading and aftershocks. In the extended special case, the 3-, 9-, and 20-story buildings were subjected to up to five consecutive aftershocks following a mainshock, with the sequences derived from the Mammoth Lakes earthquake. The analysis revealed that structural responses (i.e. maximum drift and base shear) vastly increased after the first and second aftershocks, however, structural response plateaued after the third aftershock, with only marginal additional drift or damage observed with subsequent events. This plateau indicates an upper bound for the damage state for the structure, or redistribution of internal forces within the system, which likely precluded further damage or drift. Notably, while the number of plastic hinges appeared to increase for the 20-story model, the number of CP-level hinges did not increase proportionally, which suggests that tall buildings are tolerant when subjected to sequential earthquakes. These results signify the importance of including aftershock sequences and cumulative seismic demand in the seismic design and assessment of structures. Even if designers only consider the loads from a mainshock on their designs, it is easy to suspect that they potentially underestimate the actual risk of seismic loading, and this study has shown that seismic load in the presence of aftershocks for previously designed and built structures is far from negligible. Additionally, this study shows that retrofitting involves planning simple designs and that retrofitting enhances deformation and damage control, ensuring safety during multiple earthquake events. To conclude, this study contributes to understanding the realistic structural behavior of steel moment-resisting frames subjected to sequential earthquakes. The inclusion of the aftershock effect in the design and retrofitting of steel buildings is essential to achieve a safer and resilient structure.