SEISMIC RESILIENCE OF HIGHWAY BRIDGES
ABSTRACT
Highway transportation network is one of the important civil infrastructure systems. Various components of this system, especially bridges, are highly vulnerable to extreme natural hazards such as earthquakes. Extreme events can cause severe damage to highway bridges and thus can result in disrupted functionality of highway transportation systems. Bridge damage not only causes direct economic losses due to post-event bridge repair and restoration, but also produces indirect losses arising from network downtime and traffic delay. Therefore, it is always desirable to minimize these negative consequences from extreme events and to maximize disaster resilience of highway transportation systems.
Seismic retrofitting of highway bridges is one of the most common approaches undertaken by state Departments of Transportation (state DOTs) and by bridge owners to enhance system performance during seismic events. In this relation, this study evaluates the effectiveness of different retrofit techniques to enhance seismic resilience of highway bridges. The study considers a reinforced concrete bridge in La Cienega-Venice Boulevard sector of the I-10 freeway in Los Angeles, California. The bridge was severely damaged during the 1994 Northridge earthquake due to shear failure of one of the bridge piers. Post-event reconnaissance indicated that the failure was initiated from inadequate lateral confinement of bridge piers due to which vertical load carrying capacity of the bridge reduced significantly resulting in crushing of core concrete and buckling of longitudinal re-bars of bridge piers during the seismic event. Seismic vulnerability of the
as-built bridge is estimated through finite element (FE) analysis of the bridge under a suite of time histories that represent seismic hazard at the bridge site. Appropriate loss and recovery models are used to calculate seismic resilience of the as-built bridge. Three retrofit strategies are applied to the bridge to observe the effectiveness of these strategies in enhancing bridge seismic resilience in case the bridge was retrofitted prior to the earthquake event. Seismic vulnerability of retrofitted bridge models are estimated through time history analyses of these models under the same set of ground motions used for the as-built bridge. Seismic resilience of a highway bridge can be represented as a combined measure of bridge seismic performance and its recovery after the occurrence of seismic events. For each method of seismic retrofit, seismic resilience of the bridge is calculated. Difference in resilience obtained before and after bridge retrofits and the respective costs incurred are evaluated. Comparison of these values indicates the relative effectiveness of different retrofit techniques in enhancing the seismic resilience of the bridge. Results show that one of the retrofit strategies is most effective in enhancing the seismic resilience of the bridge, whereas other two strategies are not effective at all. Cost benefit analysis is performed to calculate the benefit obtained from the most effective retrofit strategy over a period of 30-80 years after bridge retrofit. Hence, results obtained from this study help in educated decision-making for selecting efficient and cost-effective seismic design and/or retrofit strategies for highway bridges.
Keywords: Concrete bridges, fragility analysis, resilience, sensitivity study, cost analysis
TABLE OF CONTENTS
List of figures………………………………………………………………………..vii
List of tables………………………………………………………………………..viii
Acknowledgement……………………………………………………………………ix
Chapter 1 Introduction ………………………………………………………………………………………………. 1
Chapter 2 Seismic Resilience ……………………………………………………………………………………… 5
2.1. Vulnerability of a system or system component ………………………………………………. 7 2.2. Loss function ………………………………………………………………………………………………. 8
2.3. Recovery function ……………………………………………………………………………………….. 11
Chapter 3 Test-bed Bridge …………………………………………………………………………………………. 13
3.1. Bridge modeling and validation …………………………………………………………………….. 15
3.1.1 Modeling of bridge components……………………………………………………………. 15
3.1.2. Northridge earthquake ground motions records ……………………………………… 24
3.1.3. Bridge response under the Northridge earthquake and model validation …… 26
Chapter 4 Seismic Resilience of the bridge ………………………………………………………………….. 29
4.1. Bridge fragility curves………………………………………………………………………………….. 30
4.2. Direct and indirect losses due to the Northridge earthquake………………………………. 39
4.3. Post-event seismic recovery ………………………………………………………………………….. 40
Chapter 5 Proposed Retrofit Techniques and Enhancement in Seismic Resilience ……………. 42
5.1. Steel Jacketing …………………………………………………………………………………………….. 42
5.1.1. Moment Rotation plots ……………………………………………………………………….. 44
5.1.2. Shear capacity of jacketed piers …………………………………………………………… 46
5.1.3. Fragility analysis of the retrofitted bridge ……………………………………………… 48
5.2. Abutment model ………………………………………………………………………………………….. 51
5.2.1. Longitudinal Response due to backwall and backfill ………………………………. 51
5.2.2. Longitudinal Response due to bearing ………………………………………………….. 53
5.2.3. Transverse Response in wingwall ………………………………………………………… 55
5.3. Shear Key model …………………………………………………………………………………………. 57 5.4 System Resilience ………………………………………………………………………………………… 58
Chapter 6 Sensitivity Study ……………………………………………………………………………………….. 60
6.1. Recovery time …………………………………………………………………………………………….. 60
6.2. Control time ……………………………………………………………………………………………….. 60 6.3. Indirect to Direct Loss Cost Ratio ………………………………………………………………….. 61
6.4. Fragility Parameters …………………………………………………………………………………….. 61
- 5. Results from Sensitivity Analysis …………………………………………………………………. 62 Chapter 7 Cost-Benefit Analysis ………………………………………………………………………………… 64
7.1. Cost of Retrofit ……………………………………………………………………………………………. 64
7.2. Benefit ……………………………………………………………………………………………………….. 65
7.2.1 Annual Benefit …………………………………………………………………………………… 65
7.2.2. Total Benefit ……………………………………………………………………………………… 68
7.2.3. Cost-Effectiveness ……………………………………………………………………………… 69
Chapter 8 Conclusions and Recommendations ……………………………………………………………… 71
8.1. Conclusions ………………………………………………………………………………………………… 71
8.2. Applications of current study ………………………………………………………………………… 73
8.3. Future Study ……………………………………………………………………………………………….. 76
References ……………………………………………………………………………………………………………….. 77
Appendix: Cost-Benefit Analysis tables ………………………………………………………………………. 88
Chapter 1
Introduction
A natural disaster is the consequence of an extreme natural hazard like earthquake, flood, hurricane, tornado, and landslide. It leads to economic, human and/or environmental losses to a society. The resulting loss depends on the resistance of the affected population to survive against the disastrous event, called its resilience. Disaster resilience of a civil infrastructure system is defined in literature as a function that indicates the capability of the system to sustain a level of functionality over a period of time decided by owners or the society.
Bridges constitute a significant part of highway transportation systems that serve as a key mode for ground transportation, and sometimes act as an important feeder system to other modes of transportation such as railroad systems, port facilities and air travel. Damage of highway bridges due to extreme events may cause severe disruption to the normal functionality of highway transportation systems, which may have further impact on the performance of other modes of transport. Bridge damage not only causes direct economic losses due to post-event bridge repair and restoration, but also produces indirect losses arising from network downtime, traffic delay and business interruptions.
Shirole and Holt (1991) documented that about 823 bridges failed over a period of 40 years since 1950 due to extreme events such as earthquakes, fire and vehicle overloading. Failure of a large number of highway bridges was observed in California during 1971 San Fernando, 1989 Loma Prieta and 1994 Northridge earthquakes. These extreme natural events severely disrupted the normal functionality of regional highway transportation systems (Penzien et al. 2003) and caused sudden and undesired changes in technical, organizational, societal and economic conditions of communities served by these systems. Prevention is better than cure – this simple yet powerful adage becomes of profound importance when such a disaster condition is thought of. Along this line, ‘recovery’ and ‘resilience’ have become key points in dealing with extreme events, if not to prevent completely, but to minimize their negative consequences and to maximize disaster resilience of highway transportation systems. Seismic retrofitting of highway bridges is one of the most common approaches undertaken by state Departments of Transportation (state DOTs) and by bridge owners to enhance system performance during seismic events. Types of seismic retrofit strategies applied to a bridge depend on various factors including bridge attributes and configurations, accessibility and seismic demand. Common seismic retrofit techniques for bridges include lateral confinement of bridge piers using steel or composite jackets, installation of restrainers at abutments and expansion joints, seismic isolation through bearings and installation of bigger foundations (Caltrans 2011, WSDOT 2011, Wright et al. 2011). While all these bridge retrofit techniques may be effective in mitigating seismic risk of bridges, adequacy of their application and relative effectiveness greatly depend on demand from seismic hazard specific to a bridge, enhancement in seismic functionality of highway transportation systems, and benefit to cost ratio for bridge retrofitting. In this relation, calculation of resilience is identified as a meaningful way to express loss and recovery of system functionality immediately after a natural disaster (Chang and Nojima 2001, Bruneau et al. 2003, Chang and Chamberlin. 2004, Shinozuka et al. 2004, Rose and Liao. 2005, Amdal and Swigart. 2010, Cimellaro et al. 2010). Seismic resilience of a highway bridge can be represented as a combined measure of bridge seismic performance and its recovery after the occurrence of seismic events. Therefore, calculation of bridge resilience before and after the application of a retrofit strategy will not only indicate the effectiveness of this strategy in improving bridge seismic performance, but also exhibit its impact on system functionality during seismic hazard.
This study evaluates effectiveness of various retrofit techniques to enhance seismic resilience of highway bridges. A reinforced concrete bridge in La
Cienega-Venice Boulevard sector of Santa Monica (I-10) freeway in Los Angeles in California is selected as a test-bed bridge. This freeway runs across 8 states from Florida to the Pacific. In 1993, this freeway was reported to be the world’s busiest freeway carrying an average daily traffic (ADT) of 261,000, approximately (U.S Department of Transportation, 2002). During the 1994 Northridge earthquake, the bridge under consideration was severely damaged primarily due to shear failure of one of the bridge piers. Post-event reconnaissance indicated that the failure was initiated from inadequate lateral confinement of bridge piers due to which vertical load carrying capacity of the bridge reduced significantly resulting in crushing of core concrete and buckling of longitudinal re-bars of bridge piers during the seismic event. Seismic vulnerability of the as-built bridge prior to the seismic event is estimated through finite element (FE) analysis of the bridge under a suite of time histories that represent seismic hazard at the bridge site. Appropriate loss and recovery models are applied to calculate seismic resilience of the as-built bridge. To observe the effectiveness of various seismic retrofit techniques in enhancing bridge seismic resilience, three retrofit strategies are considered assuming that the bridge was retrofitted prior to the earthquake event. Seismic vulnerability of three retrofitted bridges is estimated, and for each method of seismic retrofit, seismic resilience of retrofitted bridges is calculated. First order second moment (FOSM) reliability analysis is performed to evaluate sensitivity of bridge resilience on various parameters associated with bridge vulnerability analysis and resilience estimation modules. A tornado diagram is also developed to understand sensitivity of various parameters and to further support the observation from FOSM analysis. Cost-benefit study is performed for a range of 30-60 years of service life of the bridge after retrofitting.