Statistical and Mechanistic Fragility Analysis of Concrete Bridges

M. Shinozuka, S. Banerjee and S-H. Kim

MCEER-07-0015 | 09/10/2007 | 214 pages

Cover of Statistical and Mechanistic Fragility Analysis of Concrete Bridges.

Keywords: Seismic performance.  Reinforced concrete (RC).  Bridges.  Ground motion.  Fragility analyses.  Probabilistic analyses.  Mechanistic aspects.  Finite element (FE) analyses.  Damage.  Plastic hinges.  Columns.  Failure modes.  Retrofitting.  Seismic vulnerability analyses.

Abstract: This report elaborates on the seismic performance of reinforced concrete (RC) bridges subject to earthquake ground motion by integrating probabilistic, statistical and mechanistic aspects of bridge damageability in the form of two-parameter lognormal fragility curves.  To simulate general patterns of the progressive nature of bridge damage and failure mechanisms, the study performs nonlinear time history analyses of typical California RC bridges by finite element method (FEM).  The analyses demonstrate that under normal conditions, the most prominent bridge damage that is first observed after a significant earthquake is the formation of plastic hinges at the ends of bridge columns.  Therefore, damage due to pounding of girders at expansion joints, unseating of bridge decks and shear failure of bridge columns are considered but not as governing failure modes in this study.  Another purpose of these finite element method (FEM) analyses is to simulate the enhancement of bridge fragility characteristics due to seismic retrofit, primarily because neither empirical nor experimental results are available to evaluate such enhancements.  In addition, these FEM analyses develop fragility curves with and without retrofit.  The main purpose of these FEM analyses is to develop analytical fragility curves without retrofit that can be calibrated with empirical fragility curves.  In the process of calibration, intervals of rotational ductility values that represent states of bridge damage are adjusted to form contiguous intervals over the one-dimensional space of rotational ductility.  If the rotational ductility is between upper and lower bounds in one of the intervals, the bridge is assumed to have suffered from the state of damage corresponding to the rotational ductility specified by that interval.  This calibration is made for each bridge separately, although it is envisioned to perform combined calibration to derive common damage states for all types of bridges.  Nonlinear static procedure is also performed to assess seismic vulnerability of the bridge and results from these two methods are in agreement.  The effect of ground motion directionality on bridge fragility characteristics is demonstrated, indicating that directionality may significantly influence bridge seismic damageability.