RETROFIT OPTIMIZATION FOR RESILIENCE ENHANCEMENT OF BRIDGES MULTIHAZARD SCENARIO 

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RETROFIT OPTIMIZATION FOR RESILIENCE ENHANCEMENT OF BRIDGES MULTIHAZARD SCENARIO

ABSTRACT

 

Bridge performance is often controlled by the strength of its critical sub-structural components (Teng et. al 2000). The seismic response of highway bridges is governed

significantly by the axial strength and ductility of its columns.  Prior to 1971, bridge failures were often characterized by column failures at the plastic hinge zones due to poor detailing and insufficient confinement (Ramanathan 2012). The Caltrans Seismic Retrofit Program adopted the technique of column jacketing to provide the additional confinement required to restrict the lateral disintegration of column concrete. Steel jackets have been established as a means to increase the deformation capacity of concrete beyond its unconfined compressive strength and consequently improve the column rotational ductility under lateral loading. However, fiber reinforced polymer (FRP) is growing as a preferred alternative owing to its high strength to weight ratio, resistance to corrosion and superior confinement through hoop action coming from the orientation of its constituent fibers (Hajsadeghi et. al 2010).  FRPs also provide the advantage of easy and economical installation, although their manufacturing costs are much higher in comparison to steel. The inherent disparity in the mechanical properties and the associated costs of these different materials gives rise to a trade-off between cost and performance when it comes to retrofit operations. This study explores the aforesaid trade-off and aims to optimize bridge retrofit design configurations with respect to cost and resilience.

The study is a two-objective optimization problem that aims to minimize column jacket retrofit cost and simultaneously maximize the retrofitted performance measured in terms of bridge resilience. Multi-objective evolutionary algorithm, namely Non-dominated Sorting Genetic Algorithm II is used to carry out the optimization owing to its implicit elitism and simplicity in use.  The variables in the parameter space include the choice of material for the retrofit, the choice of column in the bridge to be retrofitted and the thickness of the retrofit material for each bridge column. Three different materials, steel, carbon fiber and glass fiber composites are investigated, each associated with different values of strength and unit cost. Required thickness of jacket and unit cost of jacketing differ for each material for the same target resilience.  The algorithm hence, searches the domain to arrive at parameter values which are most favorable in terms of cost as well the resulting resilience of the retrofitted structure. Results from the optimization, called Pareto-optimal set, include solutions that are distinct from each other in terms of the associated cost, contribution to resilience enhancement, and values of design parameters.  The user is offered a wide range of superior solutions to choose from, based on more specific preferences.

It is of interest to investigate seismic resilience enhancement due to column retrofit under the multi-hazard effect of earthquake and flood-induced scour in continuation to previous study by Prasad and Banerjee (2013). Hence the example bridge is evaluated for its seismic resilience for various retrofit configurations taking into account the bridge columns being exposed to preexisting scour resulting in reduced structural stiffness.

 

 

 

TABLE OF CONTENTS

List of Figures………………………………………………………………………………………………………. vi

List of Tables……………………………………………………………………………………………………….. vii

Acknowledgements………………………………………………………………………………………………. viii

 

Chapter 1 Introduction ………………………………………………………………………………………………. 1!

1.1 Seismic Retrofit of bridge columns …………………………………………………………………. 1!

1.2 Multi-hazard scenario ……………………………………………………………………………………. 2!

1.3 Optimization problem …………………………………………………………………………………… 2!

Chapter 2 Disaster Mitigation and Enhancement in Resilience through Retrofit Measures …. 6!

2.1 Loss model ………………………………………………………………………………………………….. 8!

2.1.1 Direct losses ……………………………………………………………………………………….. 9!

2.1.2 Indirect losses …………………………………………………………………………………….. 11!

2.2 System recovery …………………………………………………………………………………………… 12!

2.3 Seismic hazard model ……………………………………………………………………………………. 12!

Chapter 3 Example Bridge Model ……………………………………………………………………………….. 15!

3.1 Bridge schematic ………………………………………………………………………………………….. 15!

3.2 Modeling of bridge components …………………………………………………………………….. 17!

3.3 Material model …………………………………………………………………………………………….. 18!

3.3.1 Material model validation …………………………………………………………………….. 18!

3.4 Mechanical properties of chosen retrofit materials ……………………………………………. 20!

3.5 Bridge fragility curves …………………………………………………………………………………… 21!

Chapter 4 Multi-Objective Optimization Analysis and Results ……………………………………….. 27!

4.1 Pareto optimality ………………………………………………………………………………………….. 27!

4.2 Optimization problem statement …………………………………………………………………….. 28!

4.3 Analysis using all three retrofit materials ………………………………………………………… 29!

4.4 Optimization run II with only GFRP options ……………………………………………………. 32! 4.5 Optimization run III with only steel jacket options ……………………………………………. 35!

4.6 Partial jacketing options ………………………………………………………………………………… 36!

Chapter 5 Summary and conclusions …………………………………………………………………………… 38!

Further study …………………………………………………………………………………………………….. 40! References ………………………………………………………………………………………………………… 41!

Chapter 1  

 

Introduction

1.1 Seismic Retrofit of bridge columns

Bridge performance is often controlled by the strength of its critical sub-structural components. The seismic response of highway bridges is governed significantly by the axial strength and ductility of its columns.  Prior to 1971, bridge failures were often characterized by column failures at the plastic hinge zones due to poor detailing and insufficient confinement (Ramanathan 2012). The Caltrans Seismic Retrofit Program adopted the technique of column jacketing to provide the additional confinement required to restrict the lateral disintegration of column concrete. Steel jackets have been established as a means to increase the deformation capacity of the concrete beyond its unconfined compressive strength and consequently control the column rotational ductility under lateral loading. However, fiber reinforced polymer (FRP) is growing as a preferred alternative owing to its high strength to weight ratio, resistance to corrosion and superior confinement through hoop action coming from the orientation of the constituent fibers (Hajsadeghi et. al 2010).

Mander model originally developed to describe the behavior of concrete confined by transverse steel reinforcement has been widely used to characterize the stress-strain behavior of concrete confined by steel jackets (Mander et. al 1988). The Mander model was adopted to analyze FRP confined concrete until subsequent studies proved this use to be inaccurate owing to the elastic brittle nature of composite jackets as opposed to the ductile behavior of steel jackets

(Saadatmanesh et. al 1994, Seible et. al 1995, Teng and Lam 2004). However in this study, the

Finite Element model of the FRP retrofitted bridge column section is based on the Mander model.

In future study this may be modified to incorporate a more recent model namely the Teng and Lam model (Teng and Lam, 2004).

1.2 Multi-hazard scenario

Present civil engineering practice relies on individual hazard models to analyze natural and manmade hazards.  Consequently, hazard mitigation techniques are developed based on structural failure probabilities under discrete hazard conditions.  However, structures may be exposed to a variety of natural hazard conditions, including multihazard, during their service lives. Previous study demonstrated that among several possible combinations of extreme hazards, earthquake in the presence of flood-induced scour is a critical scenario for highway bridges located in seismically active, flood-prone regions (Prasad and Banerjee 2013). Seismic vulnerability analysis of typical California bridges pre-exposed to foundation scour resulting from various intensity flood events revealed significant deterioration of their seismic performance even under the effect of moderate scour (Banerjee and Prasad 2013). Flood induced scour on bridge piers has the effect of reducing structural lateral stiffness and consequently increasing the rotational response of the bridge columns under lateral loading. In this study it is of interest to investigate the effect of column jacket retrofitting in the scenario of increased bridge flexibility under the effect of pre-existing scour on pier bases.

1.3 Optimization problem

From more than a decade long research, confinement of bridge columns using wraparound jackets has been established to have a positive impact on the seismic response of bridge structures owing to enhanced shear and flexural capacity of the columns (Priestley et. al 1996, Haroun and Elsanadedy 2005).  With the advent of displacement based seismic design in the early 1990s column jacketing has been widely adopted by several DOTs as a bridge rehabilitation and retrofit technique to ensure a ductile mode of failure in columns. Although steel has been the traditional choice of material for external confinement, the associated cost and time consumption in the installation process have led to exploring alternative lightweight materials with superior strength properties.

Fiber Reinforced Polymer composites were extensively being used in aerospace and shipbuilding before being applied for the first time for column wrapping by FYFE in the US in the1980s using carbon fiber (Priestley et. al, 1992). Bridge retrofit and rehabilitation is an integral part of highway bridge network maintenance. Today, with the variety of composite materials finding their place in the area of structural enhancement of bridges, it becomes relevant to the bridge owners and stakeholders to base their decisions regarding bridge retrofit on the relative cost-benefits of the various options laid out to them.

The inherent disparity in the mechanical properties and the associated costs of these different materials give rise to a trade-off between cost and performance when it comes to retrofit operations. This study explores the aforesaid trade-off and aims to optimize bridge retrofit design configurations with respect to cost and resilience.

The study is a two-objective optimization problem that aims to minimize column jacket retrofit cost and simultaneously maximize the retrofitted performance which is measured in terms of bridge resilience. Multi-objective evolutionary algorithm, namely Non-dominated Sorting Genetic Algorithm II is used to carry out the optimization owing to its implicit elitism and simplicity (Deb 2001, Deb et. al 2002).  The variables in the parameter space include the choice of material for the retrofit, the choice of column in the bridge to be retrofitted and the thickness of the retrofit material for each bridge column. Three different retrofit materials, steel, carbon fiber and glass fiber composites are investigated, each with different values of strength and unit cost.

Required thickness of jacket and unit cost of jacketing differ for each material for the same target resilience.  The algorithm hence, searches the domain to arrive at parameter values which are most favorable in terms of cost as well the resulting resilience of the retrofitted structure. Results from the optimization, are Pareto near-optimal solutions, that are distinct from each other in terms of associated cost, contribution to resilience enhancement, and values of design parameters namely, material properties and thickness of jacket.  The user is offered a wide range of superior solutions to choose from based on more specific preferences.

A flow chart presented in Figure 1-1 shows the overall optimization process.  The genetic algorithm treats every retrofit design parameter set (consisting of specifications for choice of material, thickness for each column) like an individual in the biological evolutionary process (Ferrolho and Crisostomo 2005). Every such ‘individual’ needs to be evaluated for its fitness based on which it either gets eliminated or is retained in the pool. Retained individuals generate ‘off-springs’ through recombination and mutation to keep the population pool replenished. The new population pool that includes both parents and off-springs is in turn evaluated and reduced; with every such cycle, the population fitness keeps improving due to elimination of unfit individuals and generation of fitter individuals.

Analogically speaking, the retrofit solution sets are the individuals generated by the algorithm which are incorporated into the bridge model. Nonlinear time history analyses are performed to generate bridge fragility information for resilience estimation of the bridge. The fitness of every parameter set is evaluated in terms of the two objectives i.e. seismic resilience and retrofit cost. Higher the retrofitted resilience and lower the cost, better is the fitness of the design parameter set.

Figure 1-1. Flowchart of the optimization process

RETROFIT OPTIMIZATION FOR RESILIENCE ENHANCEMENT OF BRIDGES MULTIHAZARD SCENARIO

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