United States Department of Transportation
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Impact of cascadia subduction zone earthquake on the seismic evaluation criteria of bridges : technical report : SPR 770.
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2016-12-01
[PDF-13.57 MB]
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Abstract:A large magnitude long duration subduction earthquake is impending in the Pacific Northwest, which lies near the
Cascadia Subduction Zone (CSZ). Great subduction zone earthquakes are the largest earthquakes in the world and are the sole source
zones that can produce earthquakes greater than M8.5. For this reason, structures such as reinforced concrete bridges are facing high
seismic hazards and risk. The seismic risk used for the bridge design and retrofit is defined by hazard maps of ground acceleration
values. The maps combine multiple regional sources of ground shaking using a Probabilistic Seismic Hazard Analysis (PSHA). Each
source has a different intensity, probability of occurrence, and distance to a specific location. One key source of ground shaking in
PSHA in Oregon is from the Cascadia Subduction Zone; however, a CSZ has several potential scenarios (M8.3 and M9.0) that can have
significantly different ground motion estimates as a standalone event than what is captured in the values derived from PSHA. In this
study, a computer model called CSZ14 was developed to obtain the acceleration values expected from a full rupture CSZ event. These
values were also compared to previous CSZ models as well as to uniform hazard of various return periods from USGS hazard adopted in
2002, which continues to be used for the current design of bridges. Also, the increased duration of a CSZ earthquake may result in more
structural damage than expected. Recent long duration subduction earthquakes occurred in Maule, Chile (Mw 8.8, 2010) and Tohoku,
Japan (Mw 9.0, 2011) are a reminder of the importance of the effect ground motion duration on structural performance. As part of the
research on the potential impacts, the dynamic performance of circular reinforced concrete bridge columns was experimentally evaluated
using shake table tests by comparing the column response from crustal and subduction ground motions. Three continuous reinforced
columns and three laps-spliced columns were tested using records from 1989 Loma Prieta, 2010 Maule and 2011 Tohoku. The results
demonstrated that duration of the motion can affect the imposed damage and the displacement capacity of the bridge column.
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