Swagata Banerjee

Socio-economic effect of seismic retrofit of bridges for highway transportation networks: a pilot study

A simulation-based study is performed to evaluate the socio-economic effect of seismic retrofit of bridges using the California Department of Transportation (Caltrans) Los Angeles area highway network as the testbed. 47 scenario earthquakes that represent the regional seismic hazard, consistent with the US Geological Survey (USGS) hazard map, are considered. Two sets of bridge fragility curves, before and after seismic retrofit, are used to simulate the seismic performance of the network in both cases. Analysis estimates the total social cost arising from driver delay and loss of opportunity in the degraded network. The benefit of seismic retrofitting is computed in present values as equal to the total future economic losses avoided from social cost and repair/restoration cost over the remaining bridge service lives. Estimated benefit is compared with the retrofit cost to investigate the benefit-cost ratio. Study shows that from the Caltrans point of interest, bridge seismic retrofit is cost-effective when loss avoided due to social cost is considered.

Nonlinear Static Procedure for Seismic Vulnerability Assessment of Bridges

The impact of an earthquake event on the performance of a highway transportation network depends on the extent of damage sustained by its individual components, particularly bridges. Seismic damageability of bridges expressed in the form of fragility curves can easily be incorporated into the scheme of risk analysis of a highway network under the seismic hazard. In this context, this article focuses on a nonlinear static method of developing fragility curves for a typical type of concrete bridge in California. The method makes use of the capacity spectrum method (CSM) for identification of spectral displacement, which is converted to rotations at bridge column ends. To check the reliability of this current analytical procedure, developed fragility curves are compared with those obtained by nonlinear time history analysis. Results indicate that analytically developed fragility curves obtained from nonlinear static and time history analyses are consistent.

Kalman Filter-Based Identification of Bridge Fragility Parameters

The study describes a procedure to identify bridge fragility parameters utilizing its vibration response recorded during experimental study. For this purpose, bridge damage data observed in a near full-scale shake table experiment is utilized. The bridge was tested under a sequence of earthquake ground motions with increasing intensities. Low and high amplitude tests were performed in series to observe the seismic performance of the bridge starting from yielding to complete failure. In the present study, recorded bridge acceleration during high amplitude tests is utilized and further analyzed to evaluate the degraded performance of the bridge after each high amplitude test. This is done by using extended Kalman filtering (EKF) technique as a tool. The degraded performance of the bridge after each run is measured in terms of degraded stiffness of the bridge at pier ends. In parallel, finite element (FE) model of the same bridge is developed in order to perform time history analysis under a set of earthquake ground motions with various hazard levels. Before applying the ground motions, the FE model is updated with the degraded stiffness of the bridge obtained from EKF after each high amplitude test. This is important to numerically simulate the gradual progression of bridge damage when subjected to earthquake ground motions in sequence. After each time history analysis, bridge response is obtained in terms of the rotation at bridge pier ends. Thus obtained response from time history analyses is used for fragility curve development. The change in fragility parameters represents the progressive damage of the bridge when subjected to ground motions with incremental intensity.