**2. Seismic hazard and inventory characterization for the study region**

In [10] the 1811, 1812 sequence of three distinct earthquakes corresponding to rupture along separate segments of the irregular shaped New Madrid fault is described. More recently, in the FEMA study [3], a scenario established for emergency planning purposes comprising a single M7.7 event consisting of sequential rupture along all three segments. Seismicity of smaller events recorded using a strong motion instrument array during an almost 30 year span is plotted in Figure 1 to which the approximate location of the study region has been added.

According to the 2012 data compiled for the National Bridge Inventory (NBI) [6] in the US by the Federal Highway Administration (FHWA), a total of 18,459 highway bridges are found in the 82 counties in the state of Mississippi (MS). The study region contains only a small subset of this inventory and may be approximately characterized as the counties located in north MS most likely to experience moderate ground shaking from a major event in the NMSZ. Based on default inventory data contained in the GIS-based software, Hazards US-Multihazard (HAZUS-MH) [5] created under sponsorship by FEMA for use in emergency management planning, 1133 bridges are exposed to the moderate seismic hazard.

The seismic vulnerability of all bridges in MS has been examined from a risk or loss estimation point of view in both [3] and [13]. In each study, the HAZUS-MH methodology has been implemented which depends on use of fragility curves assigned to bridge classes included in the NBI system. No study has yet been performed to assess seismic vulnerability using FE as the basis of the loss estimation. The present study provides a first step toward such a more comprehensive study and focuses on five bridges at a variety of sites in the study region FE Based Vulnerability Assessment of Highway Bridges Exposed to Moderate Seismic Hazard http://dx.doi.org/10.5772/55334 187

consideration, a simulation based approach is adopted to highlight the salient features of both the input and response at the site. The vulnerability assessment requires that reasonable behavioral response and multiple failure limit states be examined under a range of possible ground motion intensities. While a probabilistic approach is desirable overall, a deterministic approach enables the examination of the key response characteristics and the detailed information needed to establish relative importance of different limit states including soil

The bridge seismic vulnerability studies in this chapter highlight the challenges posed by the need to balance the level of sophistication of the finite element (FE) simulation with the:

**1.** state of knowledge of the bridge facilities, their seismic exposure, and local site conditions

**2.** project objectives in order to provide safe and economic decision making for hazard

Lessons learned and discussed herein are the result of over a decade of research at UM sponsored at the multidisciplinary Center for Community Earthquake Preparedness (CCEP) and graduate level studies by a number of students supported by the Department of Civil

**2. Seismic hazard and inventory characterization for the study region**

In [10] the 1811, 1812 sequence of three distinct earthquakes corresponding to rupture along separate segments of the irregular shaped New Madrid fault is described. More recently, in the FEMA study [3], a scenario established for emergency planning purposes comprising a single M7.7 event consisting of sequential rupture along all three segments. Seismicity of smaller events recorded using a strong motion instrument array during an almost 30 year span is plotted in Figure 1 to which the approximate location of the study region has been added.

According to the 2012 data compiled for the National Bridge Inventory (NBI) [6] in the US by the Federal Highway Administration (FHWA), a total of 18,459 highway bridges are found in the 82 counties in the state of Mississippi (MS). The study region contains only a small subset of this inventory and may be approximately characterized as the counties located in north MS most likely to experience moderate ground shaking from a major event in the NMSZ. Based on default inventory data contained in the GIS-based software, Hazards US-Multihazard (HAZUS-MH) [5] created under sponsorship by FEMA for use in emergency management

The seismic vulnerability of all bridges in MS has been examined from a risk or loss estimation point of view in both [3] and [13]. In each study, the HAZUS-MH methodology has been implemented which depends on use of fragility curves assigned to bridge classes included in the NBI system. No study has yet been performed to assess seismic vulnerability using FE as the basis of the loss estimation. The present study provides a first step toward such a more comprehensive study and focuses on five bridges at a variety of sites in the study region

planning, 1133 bridges are exposed to the moderate seismic hazard.

capacity, pile/column axial and flexural strength, and member/system instability.

mitigation and emergency response and mobilization planning

186 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

Engineering.

**Figure 1.** Recent seismicity in NMSZ and surrounding multi-state region exposed to risk of a repeat of historic cata‐ strophic events (M7-M8); red circles give epicenters for events > M2.5 during the period, 1974-2002 [from 19]; the study region is represented by the blue shaded area

**Figure 2.** NBI bridge inventory in study region shown by open circles; green circles show bridges located in north Mis‐ sissippi on the major access routes for the Memphis metropolitan area, those investigated using FE based seismic vul‐ nerability analysis lie within shaded areas; red lines indicate highways on federal and state system; blue lines represent major rivers to show critical water crossings

investigated during three separate projects. Figure 2 shows the locations of the sites in relation to the NBI inventory supplied in HAZUS-MH and federal and state highway system.

Specifications appeared in 1994, the ground motion demand at the site was only about

FE Based Vulnerability Assessment of Highway Bridges Exposed to Moderate Seismic Hazard

http://dx.doi.org/10.5772/55334

189

The third and most recent study was performed for MEMA to investigate findings of the FEMA sponsored NMSZ catastrophic earthquake study [3] on the impact of an *M* 7.7 event on bridges inMS.UsingaHAZUS-MHfragilitycurvebasedanalysiswhichestimatedconditionalprobabil‐ ities of damage at four basic limit states (slight, moderate, extensive, and complete), the study foundthatonlysixbridgesintheentirestatewouldhaveasignificantprobabilityexceedingslight damage. The purpose of the FEMA study was to provide states affected by the NMSZ a basis for establishing earthquake components of their federally mandated mitigation plans. The MEMA study used an FE based approach to establish vulnerability considering more site and facility specificinformation.InconsultationwithMDOTpersonnel,threebridgesshowninFigure5were identified for study. All are located on major evacuation/mobilization routes which crossed the Coldwater River. The bridges were deemed near the edge of significant ground shaking based on the FEMA study. The rationale was that if these showed evidence of significant vulnerabili‐

**Figure 4.** Bridge carrying *MS* 302 over Interstate highway *I* 55; (left) looking toward Southaven, MS, a fast growing city forming part of the metropolitan area of the City of Memphis; (right) view of the intermediate bents and girders

**Figure 5.** Three bridges crossing the Coldwater River on lifelines serving the study area; (left) view of southbound *I* 55 bridge; (middle) nearby *US* 51 bridge showing piled bents carrying simple spans; (right) view of northbound *US* 78 bridge

ty then bridges closer to the NMSZ would then be at similar or higher risk.

PGA=0.15g.

of the two closely spaced bridges

The select bridges studied have been modeled to varying degrees of complexity with both twodimensional (2D) and three-dimensional (3D) computational simulations including eigenval‐ ue, linear dynamic, nonlinear static, and nonlinear dynamic. Earthquake time histories have been generated to capture a range of intensities from M6 to M8 and peak ground accelerations (PGA) in the approximate range, PGA=5-50% g, depending on the site, study objectives, and methodology. It is noteworthy that, over the period of the studies, significant changes have occurred in the understanding of the earthquake risk and level of ground shaking to be expected. Each study used the best available knowledge at the time.

The earliest study was performed for MEMA in the context of a broader study of the seismic vulnerability of facilities located on the UM campus [17]. The motivation for the study was the belief that the University was and remains a key to economic development in the state as well as aplaceofbothhistoricvalueandapopulationcenterofrelativelyhighdensity.Thestudyincluded anapproximately70yearoldbridgethatservesasamajorentranceasseeninFigure3.Thebridge was designed by MDOT prior to any recognition of a significant seismic threat in the applicable designcode.Thisbridgeservedasthefirstattemptatadetailed3DFE-basedevaluationofseismic response and vulnerability assessment [12]. The evaluation was performed with both fixed base and soil-foundation-substructure interaction boundary conditions to capture the influence of high embankments on the response of the structural components with emphasis on the pier columns.

**Figure 3.** East Gate Bridge carrying traffic from University Avenue in the City of Oxford at the entrance to the UM main campus [17]; present day bridge, old private railroad tracks, and right-of-way have been replaced by a modern city roadway

A second study [14] was performed for MDOT for what the Bridge Division deemed a critical facility that provides access from a major interstate highway to a vital economic development region in the state. The region is located within the fastest growing county in the state and one of the fastest growing in the nation due to its proximity to the metropolitan area of the city of Memphis, Tennessee. Approximately 30 years old, this bridge shown in Figure 4 was built to low seismic standards. The code recognized by MDOT was and remains the one published by the American Association of State Highway and Transportation Officials (AASHTO). Even when the first edition of the AASHTO Bridge Load and Resistance Factor Design (LRFD) Specifications appeared in 1994, the ground motion demand at the site was only about PGA=0.15g.

investigated during three separate projects. Figure 2 shows the locations of the sites in relation

The select bridges studied have been modeled to varying degrees of complexity with both twodimensional (2D) and three-dimensional (3D) computational simulations including eigenval‐ ue, linear dynamic, nonlinear static, and nonlinear dynamic. Earthquake time histories have been generated to capture a range of intensities from M6 to M8 and peak ground accelerations (PGA) in the approximate range, PGA=5-50% g, depending on the site, study objectives, and methodology. It is noteworthy that, over the period of the studies, significant changes have occurred in the understanding of the earthquake risk and level of ground shaking to be

The earliest study was performed for MEMA in the context of a broader study of the seismic vulnerability of facilities located on the UM campus [17]. The motivation for the study was the belief that the University was and remains a key to economic development in the state as well as aplaceofbothhistoricvalueandapopulationcenterofrelativelyhighdensity.Thestudyincluded anapproximately70yearoldbridgethatservesasamajorentranceasseeninFigure3.Thebridge was designed by MDOT prior to any recognition of a significant seismic threat in the applicable designcode.Thisbridgeservedasthefirstattemptatadetailed3DFE-basedevaluationofseismic response and vulnerability assessment [12]. The evaluation was performed with both fixed base and soil-foundation-substructure interaction boundary conditions to capture the influence of high embankments on the response of the structural components with emphasis on the pier

**Figure 3.** East Gate Bridge carrying traffic from University Avenue in the City of Oxford at the entrance to the UM main campus [17]; present day bridge, old private railroad tracks, and right-of-way have been replaced by a modern city

A second study [14] was performed for MDOT for what the Bridge Division deemed a critical facility that provides access from a major interstate highway to a vital economic development region in the state. The region is located within the fastest growing county in the state and one of the fastest growing in the nation due to its proximity to the metropolitan area of the city of Memphis, Tennessee. Approximately 30 years old, this bridge shown in Figure 4 was built to low seismic standards. The code recognized by MDOT was and remains the one published by the American Association of State Highway and Transportation Officials (AASHTO). Even when the first edition of the AASHTO Bridge Load and Resistance Factor Design (LRFD)

to the NBI inventory supplied in HAZUS-MH and federal and state highway system.

expected. Each study used the best available knowledge at the time.

188 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

columns.

roadway

The third and most recent study was performed for MEMA to investigate findings of the FEMA sponsored NMSZ catastrophic earthquake study [3] on the impact of an *M* 7.7 event on bridges inMS.UsingaHAZUS-MHfragilitycurvebasedanalysiswhichestimatedconditionalprobabil‐ ities of damage at four basic limit states (slight, moderate, extensive, and complete), the study foundthatonlysixbridgesintheentirestatewouldhaveasignificantprobabilityexceedingslight damage. The purpose of the FEMA study was to provide states affected by the NMSZ a basis for establishing earthquake components of their federally mandated mitigation plans. The MEMA study used an FE based approach to establish vulnerability considering more site and facility specificinformation.InconsultationwithMDOTpersonnel,threebridgesshowninFigure5were identified for study. All are located on major evacuation/mobilization routes which crossed the Coldwater River. The bridges were deemed near the edge of significant ground shaking based on the FEMA study. The rationale was that if these showed evidence of significant vulnerabili‐ ty then bridges closer to the NMSZ would then be at similar or higher risk.

**Figure 4.** Bridge carrying *MS* 302 over Interstate highway *I* 55; (left) looking toward Southaven, MS, a fast growing city forming part of the metropolitan area of the City of Memphis; (right) view of the intermediate bents and girders of the two closely spaced bridges

**Figure 5.** Three bridges crossing the Coldwater River on lifelines serving the study area; (left) view of southbound *I* 55 bridge; (middle) nearby *US* 51 bridge showing piled bents carrying simple spans; (right) view of northbound *US* 78 bridge
