Seismic Behavior of Special Concentric Braced Frames under Long Duration Ground Motions
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Over the past decade, several long duration subduction earthquakes took place in different locations around the world such as Chile in 2010, Japan in 2011, China in 2008, and Indonesia in 2004. Long-duration and large-magnitude earthquakes are also possible to occur in the Cascadia subduction zone along the Pacific Northwest Coast of the United States. The duration of an earthquake is expected to affect the response of structures. However, current seismic design specifications mostly use response spectra to identify the hazard and do not consider duration effects. Thus, a comprehensive understanding of the effect of the ground motion duration on structural performance and its design implications is an important issue.The goal of this study was to investigate the influence of earthquake duration on the structural response of Special Concentric Braced Frames (SCBFs). A comprehensive experimental program and detailed analytical investigations were conducted to understand and quantify the effect of duration on collapse capacity of SCBFs to possibly incorporate these effects in improved seismic design provisions. The experimental program included large-scale shake table tests and the analytical program consisted of pre-test and post-test phases. The pre-test analysis phase used OpenSEES models that were preliminarily calibrated against previous experimental results for different configuration of SCBFs to conduct a sensitivity analysis. A tornado diagram framework was used to rank the influence of the different modeling parameters, e.g. low-cycle fatigue, on the seismic response of SCBFs under short and long duration ground motions. These models were revisited for further calibration and validation in the post-test analysis using the experimental program.The experimental program included three large-scale shake table tests of identical single-story single-bay SCBF with chevron brace configuration tested under different ground motions. Two specimens were tested under a set of spectrally-matched short and long duration ground motions. The third specimen was tested under another long duration ground motion. All tests started with a 100% scale of the selected ground motions then testing continued with increasing ground motion scale until failure, i.e. until both braces ruptured. The conducted shake table tests showed that earthquake duration effects can lead to premature seismic failures or lower capacities, which provides more confidence for future initiatives to consider duration effects as part of the seismic design provisions. Identical frames failed at different displacements demands because of the damage accumulation associated with the earthquake duration with about 40% reduction in the displacement capacity of the two specimens tested under long duration earthquakes versus the short duration one.Using tests results, the post-test analysis phase focused first on calibrating an OpenSEES model for the test specimens to capture the experimental behavior. The calibration started by matching the initial stiffness and overall global response, then the low-cycle fatigue parameters were fine-tuned to properly capture the experimental local behavior, i.e. brace buckling and rupture. The post-test analysis showed that the input for the low-cycle fatigue models available in the literature cannot capture the observed experimental results. Hence, new values for the fatigue parameters were suggested in this study based on the three shake-table tests. The calibrated model for the test specimens was used to conduct incremental dynamic analysis using 44 pairs of spectrally-matched short and long duration ground motions. This analysis aimed at incorporating ground motions variability for better generalized observations and developing collapse fragility curves using different intensity measures to compare the effect of ground motion duration. The difference in the median fragility was found to be 45% in the drift capacity at failure and about 10% in the spectral acceleration. Using regression analysis, the obtained drift capacity from analysis was found to be reduced by about 8% on average for every additional 10 seconds in ground motions significant duration. The last stage of this study extended the calibrated model to full SCBF archetype buildings to study the effect of ground motion duration on full structures. Two buildings, three-story and nine-story, that resembled original SAC buildings but modified to have SCBFs as lateral support system instead of moment resisting frames, were used. Two planer frames were adopted from the two buildings and used for the analysis. The same 44 spectrally-matched pairs previously used in post-test analysis were used to conduct nonlinear time history analysis and study the effect of duration. All the ground motions were scaled to two hazard levels for the deterministic time history analysis: 10% exceedance in 50 years and 2% exceedance in 50 years. All analysis results were interpreted in a comparative way to isolate the ground motions duration effects, which was the main variable in the ground motion pairs. In general, the results showed that the analyzed SCBFs experienced higher drift values under the long duration suite of ground motions, and in turn, larger percentage of fractured braces was observed under long duration cases. The archetype SCBFs analysis provided similar conclusion on duration effects as the experimental and numerical results on the single-story single-bay frame.