Effects of long duration earthquakes on bridge structures

Blandine C. Valle

The main objective of this research was to assess the response of multi-column bent bridges, with columns expected to behave primarily in shear, subject to long-duration earthquake. Recent geological evidence indicates that the potential exists for large earthquakes resulting in long-duration ground motions in the Pacific Northwest due to rupturing of the locked interface between the Juan de Fuca and the North American Plate. Three Washington State Department of Transportation bridges were selected for this study, bridges 5/227, 5/649 and 512/29. All three bridges are located in close proximity to Olympia and Seattle. Ten earthquake records with return periods ranging from 475 to 2475 years were used to study the effect of duration on bridge response; six long-duration and four short-duration. Since the column aspect ratios were similar for the three bridges (approximately 3), other bridge characteristics were more influential on the variation of the bridge seismic responses. The bridge deck design, monolithic or non-monolithic, and the bridge geometry greatly influenced the behavior. Each bridge was unique enough that in order to accurately assess the seismic vulnerability of each bridge, nonlinear time history analyses were needed rather than basing predictions merely on bridge member detailing, as is often the case due to limited resources. In general, the 475-year return period earthquakes induced light to moderate cracking in the column plastic hinge regions for all bridges. The 975-year return period earthquakes created more severe cracking with bearing pad failures in one of the bridges. The 2475-year return period earthquakes induced failures in the center bent columns as well as numerous bearing pad failures for all three bridges. The damage estimations for each earthquake were based on damage recorded in experimental column testing. Overall, long-duration earthquakes created more damage in the three bridges than short-duration earthquakes. For the smaller earthquakes, the duration had little effect on the bridge response since multiple cycles at low ductility demands did not lead to damage of the columns. As the intensity of the earthquake and the duration increased, damage in the columns increased. Therefore, both earthquake intensity and ground motion duration affect the bridge response; however, large intensity alone can lead to significant demand on the bridges, while duration is not influential on the bridge demand unless the intensity is high as well.

Effects of long duration earthquakes on bridge structures

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