PEER has just published (December 2019) Report No. 2019/09: "Seismic Behavior of Special Concentric Braced Frames under Short- and Long-Duration Ground Motions." It was authored by Ali Hammad and Mohamed A. Moustafa, University of Nevada, Reno.
Over the past decade, several long-duration subduction earthquakes took place in different locations around the world, e.g., Chile in 2010, Japan in 2011, China in 2008, and Indonesia in 2004. Recent research has revealed that long-duration, large-magnitude earthquakes may occur along the Cascadia subduction zone of the Pacific Northwest Coast of the U.S. The duration of an earthquake often affects the response of structures. 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 duration of the ground motion on structural performance and its design implications is an important issue.
The goal of this study was to investigate how the duration of an earthquake affects 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, with the goal of improving seismic design provisions by incorporating these effects. 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 performed a sensitivity analysis that used OpenSees models preliminarily calibrated against previous experimental results for different configuration of SCBFs. 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. Based on the results obtained from the experimental program, these models were revisited for further calibration and validation in the post-test analysis.
The experimental program included three large-scale shake-table tests of identical single-story single-bay SCBF with a 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; testing continued with an ever-increasing ground-motion scale until failure occurred, e.g., until both braces ruptured. The shake table tests showed that the duration of the earthquake may lead to premature seismic failure or lower capacities, supporting the initiative 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.
Post-test analysis focused first on calibrating an OpenSees model to capture the experimental behavior of the test specimens. The calibration started by matching the initial stiffness and overall global response. Next, 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 currently available in the literature does not reflect the observed experimental results. New values for the fatigue parameters are suggested herein based on the results of the three shake-table tests.
The calibrated model was then used to conduct incremental dynamic analysis (IDA) using 44 pairs of spectrally-matched short- and long-duration ground motions. To compare the effect of the duration of ground motion, this analysis aimed at incorporating ground-motion variability for more generalized observations and developing collapse fragility curves using different intensity measures (IMs). The difference in the median fragility was found to be 45% in the drift capacity at failure and about 10% in the spectral acceleration (Sa). Using regression analysis, the obtained drift capacity from analysis was found to be reduced by about 8% on average for every additional 10 sec in the duration of the ground motion.
The last stage of this study extended the calibrated model to SCBF archetype buildings to study the effect of the duration of ground motion on full-sized structures. Two buildings were studied: a three-story and nine-story build that resembled the original SAC buildings but were modified with SCBFs as lateral support system instead of moment resisting frames. 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 effect of duration, 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, a larger percentage of fractured braces under long-duration cases. The archetype SCBFs analysis provided similar conclusions on duration effects as the experimental and numerical results on the single-story single-bay frame.