Consideration of time-evolving capacity distributions and improved degradation models for seismic fragility assessment of aging highway bridges Jayadipta Ghosh a,n , Piyush Sood b a Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India b School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta 30318, USA article info Article history: Received 10 December 2015 Received in revised form 29 April 2016 Accepted 2 June 2016 Available online 7 June 2016 Keywords: Seismic fragility Chloride-induced deterioration Highway Bridges Corrosion Pitting Probability Reliability Time-evolving capacity abstract This paper presents a methodology to develop seismic fragility curves for deteriorating highway bridges by uniquely accounting for realistic pitting corrosion deterioration and time-dependent capacity dis- tributions for reinforced concrete columns under chloride attacks. The proposed framework offers dis- tinct improvements over state-of-the-art procedures for fragility assessment of degrading bridges which typically assume simplified uniform corrosion deterioration model and pristine limit state capacities. Depending on the time in service life and deterioration mechanism, this study finds that capacity limit states for deteriorating bridge columns follow either lognormal distribution or generalized extreme value distributions (particularly for pitting corrosion). Impact of column degradation mechanism on seismic response and fragility of bridge components and system is assessed using nonlinear time history analysis of three-dimensional finite element bridge models reflecting the uncertainties across structural mod- eling parameters, deterioration parameters and ground motion. Comparisons are drawn between the proposed methodology and traditional approaches to develop aging bridge fragility curves. Results in- dicate considerable underestimations of system level fragility across different damage states using the traditional approach compared to the proposed realistic pitting model for chloride induced corrosion. Time-dependent predictive functions are provided to interpolate logistic regression coefficients for continuous seismic reliability evaluation along the service life with reasonable accuracy. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction The last decade and especially the past few years have wit- nessed significant efforts towards quantifying the seismic vulner- ability of deteriorating highway bridges. Given that across the globe a significant percentage of these critical infrastructure ele- ments are nearing the end of their useful service life, the time is opportune to investigate joint seismic and aging threats to bridge structures, especially for those located in moderate to high seismic zones. For instance, the average age of highway bridges within the US inventory at present is 42 years [6] and approximately 234,238 bridges out of over 600,000 within the inventory are located in moderate to high seismic zones [24]. While such comprehensive statistics maybe difficult to track for all regions across the globe, deterioration of infrastructure systems alone (primarily due to corrosion) contributes to over 3% of the GDP of industrialized countries [68]. The dollar impact of environmental deterioration coupled with seismic threats necessitates the vulnerability as- sessment of aging highway bridges to ensure the socio-economic welfare of a nation. Multiple degradation mechanisms may potentially affect the structural performance of highway bridges during its service life, such as erosion, fatigue, sulfate and acid attacks on concrete, car- bonation and chloride induced corrosion of steel components, freeze-thaw cycles in bridges in cold regions, oxidation of rubber bearing pads, amongst others [21,29]. Amongst these, corrosion deterioration (particularly from chloride ions) has received sig- nificant attention with reference to vulnerability quantification of bridges located in seismic zones. This deterioration mechanism potentially affects multiple critical bridge components comprising of embedded or exposed steel elements, such as reinforced con- crete (RC) columns, steel bearings, bearing anchor bolts or dowel bars, reinforced deck slabs and exposed steel girders [12,2,27,39,63]. The vulnerability of aging as well as pristine (as- built or non-deteriorating) highway bridge structures under seis- mic shaking is typically expressed in the form of seismic fragility curves which quantifies the likelihood of meeting or exceeding a particular damage state given the intensity of ground motion. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ress Reliability Engineering and System Safety http://dx.doi.org/10.1016/j.ress.2016.06.001 0951-8320/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: jghosh@iitb.ac.in (J. Ghosh). Reliability Engineering and System Safety 154 (2016) 197–218