Performance Evaluation of Steel Buckling Restrained Braced Frames Subjected to Far-Field, Near-Field, and Long-Duration Earthquakes

Fokruddin Ahmad

doi:10.7273/000005249

The use of Buckling Restrained Brace Frames (BRBFs) has been increasing in recent decades due to their ability to provide superior seismic performance and enhance the resilience of buildings against earthquakes. However, not many studies have extensively and thoroughly investigated the response and resiliency of prescriptively designed BRBF buildings to varying types of earthquake hazards. This study fills that research gap by investigating the seismic performance of two code-designed BRBFs prototype buildings subjected to far-field, near field with pulse and without pulse, and long-duration ground motion sets. The first phase of the study investigated the seismic resiliency of the prescriptively designed BRBF buildings and compared them to identical prototypes designed with mass timber PT-CLT rocking walls using the FEMA P-58 methodology to compare seismic losses. The seismic loss investigation was part of a larger study evaluating the two types of structural systems using multiple criteria decision analysis across four performance categories of seismic resiliency, global warming potential, superstructure cost, and durability. The global warming potential and superstructure cost estimate was completed by others, but this study completed the seismic resiliency assessment and multiple criteria decision analysis.The second phase of this dissertation work analyzed the structural response of the two BRBF prototype buildings across four sets of ground motions representing different hazard levels in Seattle, WA. The two prototype buildings were modeled in 3D using OpenSeesPy to understand the effect of different ground motion types on the structural responses. The analysis results showed that near-field motions increase the deformation demands, such as inter-story drift and maximum ductility in the pulse direction. Though BRBFs are not a self-centering systems, only the upper two floors of the mid-rise building experienced residual drift higher than 0.2%, which is the threshold for expecting minor repair and structural realignment. None of the stories had residual inter-story drift exceeding 0.5% drift for any motion sets. Overall, the code minimum based BRBF buildings showed excellent performance across all the different hazard types. However, the one caveat of this analysis was that long-duration motions had significantly higher cumulative ductility demand than other motion sets. Therefore, the final phase of this dissertation works further investigated the cumulative deformation demand on BRBF braces under long-duration motions. It is important to verify the ductility of the braces through analysis or testing because they act as the primary structural fuse to dissipate the earthquake energy. The final study compared different loading protocols from different countries to the nonlinear modeling results of long-duration motions. It was determined that the long duration motions had over 80% probability of exceeding the current AISC 341 required testing protocol. To rectify these issues, a new loading protocol appropriate for long-duration earthquakes was proposed that accounts for the increased plastic deformation demand and matches the cyclic content of the nonlinear dynamic analyses. In conclusion, these studies have demonstrated that prescriptively designed BRBFs that meet code minimum requirements are a high performing lateral force resisting system to a range of earthquake hazards. They have excellent seismic resiliency, even when not optimized during design through nonlinear time history analysis, as is common in performance-based earthquake engineering. Additionally, the code-designed BRBF buildings were not predicted to have high residual inter-story drifts, which means they are highly likely to be repairable with minor adjustments and re-alignment. However, it was identified that long-duration earthquakes will increase the ductility demand on the braces significantly compared to far-field and near-field earthquakes and that current minimum testing requirements do not account for this increase. A new protocol was proposed to rectify this one challenge with BRBFs.

Performance Evaluation of Steel Buckling Restrained Braced Frames Subjected to Far-Field, Near-Field, and Long-Duration Earthquakes

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