Fundamental study on structural damage detedtion in vibration response of long span suspension bridge

DEPARTMENT OF CIVIL ENGINEERING YOKOHAMA NATIONAL UNIVERSITY FUNDAMENTAL STUDY ON STRUCTURAL DAMAGE DETECTION IN VIBRATION RESPONSE OF LONG-SPAN SUSPENSION BRIDGE NGUYEN DANH THANG A

DEPARTMENT OF CIVIL ENGINEERING YOKOHAMA NATIONAL UNIVERSITY

FUNDAMENTAL STUDY ON STRUCTURAL DAMAGE DETECTION IN VIBRATION RESPONSE

OF LONG-SPAN SUSPENSION BRIDGE

NGUYEN DANH THANG

A dissertation submitted in partial fulfillment of the requirements for the

degree of Doctor of Engineering

Academic Advisors:

Prof Hitoshi Yamada Prof Hiroshi Katsuchi Assoc Prof Eiichi Sasaki

September, 2010

ABSTRACT

Nowadays, a great number of long-span bridges were constructed all over the world Most

of long-span bridges which are common over the sea are particularly difficult to maintain because of their specific conditions: a severe natural environment including strong winds, strong tidal currents and salt air, a large degree of continuing deformation of structures, an extremely large variety of structural members and materials, and the need to cope with the fatigue of structural steel especially in the case of any bridges which carry trains as well as road traffic However, their service time have to be more than 100 years because these bridges are very expensive to design, construct and maintain As a result, the health, durability, and safety of these bridges in a long-term service period are now attracting a lot

of scientists and engineers An issue arising will be a methodology how the structural damages can be detected from the monitoring data

Besides, with increasing of span and slenderizing of structure, long-span bridges become more and more sensitive to wind For a long-span bridge, with limited torsional stiffness, wind-induced forces, such as self-excited force and buffeting force, can cause destructive phenomena Self-excited forces causing flutter are in general dependent on the geometric profile of the bridge deck section, angle of wind attack and wind velocity expressed as reduced frequency Meanwhile, buffeting is defined as the unsteady loading of a structure

by velocity fluctuations in the oncoming flow In addition, earthquake is also extreme excitation for long-span bridge and can cause a lot of structural damage Therefore, both wind and earthquake are required special attention for long-span bridge

Although many advances in design, construction as well as maintenance have been developed day to day, many problems of structure still remain unknown or unsolved The ability to detect structural damages in a bridge before it endangers the structure has been of interest to engineers for many years

This study was carried out to investigate how the structural damage affects the induced and earthquake responses of a long-span suspension bridge Besides, this study was focused on how to detect damage of long-span bridge in vibration response To illustrate this purpose, a detailed finite element model of a long-span bridge was developed and verified using field data, making this model as accurate as possible in representing the actual structural behavior Using this finite element model, the reliability analysis of the bridge is performed considering dead load, wind load and earthquake loading After that, based on the realistic deteriorations, various types of structural damages of a long-span bridge are simulated to facilitate the discussion All of the dynamic data for comparing damaged with undamaged cases were generated numerically from the finite element model The obtained results showed that monitoring data can be used for detecting some damages cases, but existing monitoring systems are not sufficient for damage detection Lessons archived from this study are expected not only to maintain this bridge but also to improve our understanding of the real bridge performance as well as to provide useful feedbacks for future design

wind-ACKNOWLEDGEMENTS

After going through almost three years of hard work, it is time to thank all those who have pulled me through this period and made my stay at Yokohama National University a pleasant one

First of all, I would like to express my sincere gratitude and thanks to my advisors: Professor Hitoshi Yamada, Professor Hiroshi Katsuchi and Associate Professor Eiichi Sasaki for their kind advices, valuable suggestions, invaluable guidance, moral support and effective encouragement throughout the course of this study

I would like to extend my gratitude to Professor Tatsuya Tsubaki and Associate Professor Kimitoshi Hayano for their helpful comments, suggestions and serving as members of the examination committee

I take this opportunity to thank Ms Matsuda, secretary of Wind and Structures Laboratory, for her kindness, support and spiritual encouragement

My special gratitude is due to all my dear friends for making my time spent at Yokohama National University an unforgettable memory as well as for what they helped for me to overcome lot of difficulties in foreign environment

I would like to thank to the Ministry of Education, Science and Culture of Japan (Monbukagakusho) for the full financial support and the research facilities they provided during my study

Lastly, but not the least, I want to express all my gratitude to my wife and my son, my father and mother, and other members of my great families for all the trust, support that they gave me Without their love, encouragement, inspiration and sacrifice, this work could hardly be completed

Nguyen Danh Thang

September, 2010

TABLE OF CONTENTS

2 MONITORING SYSTEM AND DAMAGE DETECTION OF

2.1.2 Application of structural health monitoring for long-span bridges 11

3.1 Finite element model of long-span bridge 31

3.2.5 Wind-induced response of long-span bridge 43

4.1 Damage detection by global vibration response 52

4.2 Damage detection by local vibration response 77

5.2.3 Change in local frequency 106

LIST OF FIGURES

Page

Figure 2.3 Recorded wind parameters and responses of Akashi Kaikyo Bridge

Figure 2.9 Location of sensor in cross frame of Tsing Ma Bridge 20

Figure 2.10 Mean speed and direction of onset wind recorded in Tsing Ma Bridge 20

U

α

Figure 2.13 Seto Bridge 22

Figure 2.14 Deteriorating grade of maintenance ways of Seto Bridge 23

Figure 2.16 Broken wires and reduction of cross section of Alvsborg Bridge, Sweden 24

Figure 2.17 Fatigued strand of Severn Bridge, England 25

Figure 3.1 3D finite element model of Akashi Kaikyo Bridge 32

Figure 3.2 Cross section of Akashi Kaikyo Bridge deck 33

Figure 3.3 Static aerodynamic coefficients of Akashi Kaikyo Bridge as a function of

Figure 3.4 First symmetric lateral deflection mode of model 34

Figure 3.5 First symmetric vertical deflection mode of model 34

Figure 3.7 Three-degree-of-freedom model of bridge deck 39

Figure 3.8 Maximum wind speed at 10m of altitude at Kobe City and Akashi Town 42

Figure 3.9 Location of Akashi Kaikyo Bridge and two wind recorded places 42

Figure 3.10 Two components of time history wind speed fluctuation at 20 m/s 44

Figure 3.11 Applied three components of wind forces on the bridge deck 44

Figure 3.12 Time history wind forces at 20 m/s applied at middle span of the bridge 45

Figure 3.13 Time history wind-induced response at middle main span of the bridge at

Figure 3.14 Time history wind-induced velocity at middle main span of the bridge at 47

20 m/s

Figure 3.15 Time history wind-induced acceleration at middle main span of the

Figure 3.16 Time history ground motion of two applied earthquakes on the bridge 49

Figure 3.18 Response at middle span of the bridge during EQ1 50

Figure 3.19 Response at middle span of the bridge during EQ2 51

Figure 4.1 Assumed severe damaged cases of Akashi Kaikyo Bridge 54

Figure 4.2 Changes in mean relative value of structural response for type 1 of

Figure 4.10 Changes in mean relative value of structural acceleration for type 3 and

Figure 4.11 Changes in whole structure basic natural frequencies of structure 65

Figure 4.12 Changes in higher order of whole structure natural frequencies 66

Figure 4.13 Changes in STD of structural response for type 1 damage 68

Figure 4.14 Changes in STD of structural response for type 2 damage 69

Figure 4.15 Changes in STD of structural response for type 3 and type 4 damages 70

Figure 4.16 Changes in STD of structural velocity for type 1 damage 71

Figure 4.17 Changes in STD of structural velocity for type 2 damage 72

Figure 4.18 Changes in STD of structural velocity for type 3 and type 4 damages 73

Figure 4.19 Changes in STD of structural acceleration for type 1 damage 74

Figure 4.20 Changes in STD of structural acceleration for type 2 damage 75

Figure 4.21 Changes in STD of structural acceleration for type 3 and type 4 damages 76

Figure 4.22 Three selected typical hanger for damage detection 80

Figure 4.24 Response at damage point in case of 5% of hanger area was lost 82

Figure 4.25 Local frequencies at damage point in case of 5% of hanger area was lost 83

Figure 4.26 Damage point response in case of 30% of hanger area was lost 84

Figure 4.27 Local frequencies at damage point in case of 30% of hanger area was lost 85

Figure 4.28 Local frequencies at upper neighbor point in case of 30% of hanger area

Figure 4.29 Local frequencies at lower neighbor point in case of 30% of hanger area 87

was lost

Figure 4.31 Damaged point response in case of 50% of hanger area was lost 89

Figure 4.32 Local frequencies at damaged point in case of 5% of hanger area was lost 90

Figure 4.33 Local frequencies at neighbor point in case of 50% of hanger area was

Figure 4.35 Damaged point response in case of 50% of hanger area was lost 93

Figure 4.36 Local frequencies at damaged point in case of 5% of hanger area was lost 94

Figure 4.37 Local frequencies at neighbor point in case of 5% of hanger area was lost 95

Figure 4.38 Change in frequency between healthy and damage condition 96

Figure 4.40 Changes in local frequencies at middle span of bridge for all severe cases 98

Figure 4.41 Changes in local frequencies at middle side span of bridge for all severe

Figure 5.1 Analysed earthquake cases of long-span bridge 105

Figure 5.2 Changes in DON value in case of the medium hanger was damaged under

Figure 5.5 Changes in DON value in case of the shortest hanger was damaged under

Figure 5.6 Changes in DON value along the bridge in case of the medium hanger

Figure 5.5

Changes in local frequency in case of the medium hanger lost 30% of

cross section area for upper, damage and lower neighbor point,

respectively

108

Figure 5.6 PSD error in case of the medium hanger damaged 109

LIST OF TABLES

Page

Table 2.1 Measurement items at Akashi Kaikyo Bridge 13

Table 2.2 Criteria for deterioration evaluation of galvanized structure of Seto

Table 3.1 Comparison of natural frequencies between FEM and measured 33

Table 4.1 Change in natural frequencies according to damages cases 67

in the case of any bridges which carry trains as well as road traffic However, their service time have to be more than 100 years because these bridges are very expensive to design, construct and maintain As a result, the health, durability, and safety of these bridges in a long-term service period are now attracting a lot of scientists and engineers Some long-span suspension bridges are monitored for the purpose of their health monitoring An issue arising will be a methodology how the structural damages can be detected from the monitoring data

Besides, with increasing of span and slenderizing of structure, long-span bridges become more and more sensitive to wind For a long-span bridge, with limited torsional stiffness, wind-induced forces, such as self-excited force and buffeting force, can cause destructive phenomena and need special attention, especially after the collapse of suspension Tacoma Bridge on November 7th, 1940 by normal wind (Figure 1.1) Self-excited forces causing flutter are in general dependent on the geometric profile of the bridge deck section, angle

of wind attack and wind velocity expressed as reduced frequency Meanwhile, buffeting is

defined as the unsteady loading of a structure by velocity fluctuations in the oncoming flow

Figure 1.1 The collapse of Tacoma Bridge

Because of important role and expensive cost, long-span bridges are require inspection from time to time to ascertain that they are still safe and capable of withstanding various environmental effects Such inspections and associated non-destructive testing procedures can reveal progressive damage, and allow appropriate repair measures to be taken before the damage deteriorates to the extent of making the structure unserviceable Even for new infrastructure, particularly large structures with high initial construction costs, it is now recognized that monitoring programs are desirable right from the outset in order to detect any signs of damage as early as possible, and allow appropriate interventions to be taken Programs of this nature, if properly implemented, can extend the useful life of the structure quite considerably, with the utility value gained more than justifying the costs of the monitoring itself This philosophy has gained considerable momentum in areas such as Japan, China, and Korea, where long-span bridges are abundant However, this thinking is more widespread, and much research on the issues of monitoring, damage detection and long-term performance of structures is going on not only in Asia, but also in America (USA and Canada) and Europe (Germany, Belgium, UK, etc.)

On the other hand, structural health monitoring of bridges is a very complicated issue The principal developments concentrate on issues that the ordinary bridge owner is not interested in A common language between technology achievements and bridge owners has not been found and the method statement that appeal to bridge owners are lacking

Besides, the development community has not been able to explain the new methods do not eliminate the problem of aging or damaged bridges but are only better at being able to identify problems However, the performed monitoring campaigns are often so expensive that they are only scientific interest

In addition, the cost of Structural Health Monitoring of bridges is also expensive Cost will depend on the depth of investigation and vary from simple quick investigation until permanent online Structural Health Monitoring In general, an in-depth inspection currently costs approximately 10,000 Euro per 100 m of bridge In order to be able to equip Structural Health Monitoring system, bridge owners are required sufficient capital to invest

in the expensive monitoring equipment necessary The cost for a 32-channel Structural Health Monitoring system is in the region of 100,000 Euro with a life expectation of 3 years [1]

Figure 1.2 The collapse of I-35W Bridge (photo by BBC)

Although many advances in design, construction as well as maintenance have been developed day to day, many problems of structure still remain unknown or unsolved The ability to detect structural damages in a bridge before it endangers the structure has been of interest to engineers for many years Currently, bridge condition assessment is largely carried out by visual inspection at intervals of one to five years, followed by more detailed

examination and analysis if necessary However, it is possible for significant damage to have developed in the intervening period, putting structures at risk There have been some disastrous failures of bridges due to undetected progressive damage in the past, e.g the collapse of I-35W Bridge in Minneapolis, USA without any warning on August 1st, 2007 (Figure 1.2) Therefore there is considerable interest in continuous monitoring of bridges

1.2 Necessary of study

The design of civil structures is characterized by two main features: load-carrying capacity and serviceability However, each structural system undergoes various environmental and loading influences during its service life, which can cause a significant damage accumulation Consequently, the structural carrying capacity and serviceability are enormously affected Therefore, the need for reliable nondestructive evaluation technique and detection of damage at the earliest possible stage has been pervasive throughout the civil engineering community in the last decade The process of implementing damage detection strategies can be referred to as “structural health monitoring” The so-called vibration-based health monitoring techniques rely on the fact that damage causes changes

in the local structural damping (energy dissipation) and stiffness As a consequence, the global dynamic properties of the structure, e.g eigen frequencies, mode shapes, modal damping, etc., should be influenced

Structural Monitoring is basically an activity where actual data related to civil structures is measured and registered This has been performed through all times by responsible designers, contractors and owners with almost identical objectives - to check that the structures behave as intended Historically the activity has required specialists, has been time consuming and hence costly and as a result hereof only a limited number of performance indicators - typically geometry - have been measured a periodically and supplemented by regular visual observations

At the core of any structural health monitoring framework system are the diagnostic and prognostic algorithms used to detect the presence, magnitude and extent of structural faults The emergence of this field has led to a variety of diagnostic methods for detecting, locating and quantifying varying degrees of damage

Several methods of structure lifetime estimation are known today However, their performance is heavily influenced by the quality of the recorded data: length of the time series, presence of measurement and system noise, system excitation, etc In addition, different types of energy dissipation could be present at any given time instant Some of them can be associated with material properties, others with the system boundary conditions Effect of contact friction can be observed in some cases as well Thus, the estimation procedure requires a very careful use of numerical procedures Moreover, an engineering understanding and critical considerations are important for a reliable identification of the presented damping properties.

To have a better design for long-span bridge, the study of the wind load and earthquake load on bridge is of vital importance Many works have been conducted on the study of damage of steel bridges [2] However, there has been very little research on the wind-induced damage especially for long-span suspension bridges With the increase of span length of modern suspension bridges, the investigation for evaluating wind induced damage becomes more and more significant for long-span suspension bridges, which were common located at a typhoon prone region

The dynamic response against strong wind and earthquake are subjected to unknown factors those are uneasy to predict Therefore, it is necessary to establish a monitoring system that can collect data on dynamic response of the bridge in order to verify the assumptions and constant used for the design due to strong wind and earthquake The wind load for long-span bridges has great importance in their structural design It usually consists of time averaged wind force and some contribution of the dynamic response due to the wind fluctuation, but there still remain uncertainties in expression of wind characteristics to define the accurate and reliable wind load To overcome this it will be important to compile information of the wind at many bridge site Here, as the example of monitoring results, the deformation characteristics of the bridge response due to strong wind are elucidated By comparing the analyzed simulation results through wind tunnel test and field measured results, the reliability of the current monitoring system is confirmed

Besides, with the development of the structural health monitoring system [3, 4 and 5] for long-span suspension bridges, it becomes possible to obtain field data of dynamic response induced by a typhoon for the bridge with permanent installed monitoring system However,

the possibility of damage detection by using monitoring data is still an attractive problem for engineers as well as scientist and requires more investigation

1.3 Objectives and outline of study

This study was carried out to investigate how the structural damage affects the induced and earthquake responses of a long-span suspension bridge Besides, this study was focused on how to detect damage from vibration monitoring data of long-span bridge

wind-To illustrate this purpose, a detailed finite element model of a long-span bridge was developed and verified using field data, making this model as accurate as possible in representing the actual structural behavior Using this finite element model, the reliability analysis of the bridge is performed considering dead load, wind load and earthquake load After that, based on the realistic deteriorations, various types of structural damages of a long-span bridge are simulated to facilitate the discussion All of the dynamic data for comparing damaged with undamaged cases were generated numerically from the finite element model Lessons archived from this study are expected not only to maintain this bridge but also to improve our understanding of the real bridge performance as well as to provide useful feedbacks for future design

This study was divided into 6 chapters After introduction in Chapter 1 on background and necessity of the study, a review of structural health monitoring system and the past study

on damage detection of long-span bridge is presented in Chapter 2 Chapter 3 provides all needed data for numerical simulation of long-span bridge, wind and earthquake loading Then, Chapter 4 is devoted to structural damage detection by time history wind-induced response In the next step, Chapter 5 was focus on how to detect damage by earthquake response Finally, some conclusions and recommendations are given in Chapter 6

CHAPTER 2

MONITORING SYSTEM AND DAMAGE DETECTION

OF LONG-SPAN BRIDGE

2.1 Monitoring system of long-span bridge

2.1.1 Structural health monitoring

Structural Monitoring is basically an activity where actual data related to civil structures is measured and registered This has been performed through all times by responsible designers, contractors and owners with almost identical objectives - to check that the structures behave as intended Historically the activity has required specialists, has been time consuming and hence costly and as a result hereof only a limited number of performance indicators - typically geometry - have been measured a periodically and supplemented by regular visual observations

This situation has been dramatically changed by the enormous development within information technology in the last two decades High performance sensors, precision signal conditioning units, broad band analogue-to-digital converters, optical or wireless networks, global positioning systems etc have all paved the way for a far more accurate, fast and cost efficient acquisition of data Very sophisticated and powerful software for structural analysis has become available and increases the beneficial use of the large amounts of data that can be acquired Finally, significant developments have been made regarding deterioration mechanisms and environmental loads on civil structures These developments open the way for a wide range of applications related to efficient operation and maintenance of structures

Structural monitoring has thus emerged as a distinct technical discipline as the new technologies have been introduced in the field of civil engineering Numerous and rather sophisticated systems have been established The development of many of these systems seems to have been driven more by the technological possibilities than by well defined objectives for application areas of design verification, trouble shooting, user safety and maintenance planning formulated by the “traditional key players”: the designers, contractors, operators and owners Most likely this is due to the complexity of the new methodologies and systems and the vendors dedicated efforts to market new products, but scientific curiosity and enthusiasm may also have played a role As a consequence of weakly defined objectives it seems as if the owners have not achieved the optimal benefit from the – often rather significant – investment in the structural monitoring systems and their occasionally extensive operation.

Current long span bridge monitoring system was developed to be a reliable device to observe the bridge in earthquake and/or typhoon accurately, besides have a self-check function to sense the disorder of the system itself

Generally, the objectives of long bridge monitoring are [3]:

Design verification:

+ To provide data on structural dynamic response to verify design assumptions used for the strong wind and earthquake

+ To provide data for developing a better further design in a more rational way

+ To develop a reliable health monitoring system that has a self-check function to monitor disorder of the system itself

+ To provide data to adjust level of safety traffic control due to earthquake or strong wind

+ To provide data for assessing post-earthquake or post-typhoon structural reliability to manage traffic flow

The scope of monitoring includes two major types of parameters: load effects and responses The load effects refer to those due to wind, earthquake, temperature and live loads (movements, highways or railways) The responses refer to displacements, velocity, accelerations, stresses, strains and forces of the members of bridge structure, and displacements and stresses of main cables

More, recently, long-span bridge owners and operators are beginning to consider a more comprehensive strategy for monitoring the health of long-span bridges Long-span bridges are unique because they fall outside the present Design Standard Specifications, and there

is little generic experience related to maintaining their performance, especially after they age and/or following any damage arising from acts of nature or artificial hazards In the US more than 800 of the long-span bridges in the national bridge inventory (NBI) are classified as fracture-critical Thus, any strategy that would aid managing this critical infrastructure is of interest to bridge owners/operators, including new types of sensors, wireless data transfer and information data processing, display, management and storage However, there are still many unanswered questions that arise regarding the use and reliability of current structural health monitoring technology

Health Monitoring offers a unified perspective of the entire realm of performance and safety evaluation and management of existing constructed systems, and also serves as a critical prerequisite for intelligent infrastructure systems

Bridges are the flagships of civil engineering They attract the greatest attention within the engineering community This is due to their small safety margins and their great exposure

to the public The global higher transportation network operates about 2.5 million bridges Current bridge management systems rate them using various methodologies and approaches This result is very inhomogeneous statistics In 2005, the US Federal Highway Agency (FHWA) stated that 28% of their 595,000 bridges are rated as being deficient, with only 15% of these being deficient for structural reason The result in Europe is about 10%

of bridges As structural health monitoring should be used in a preventive capacity before

bridges becomes deficient, this considerable increase the number of its application above the global estimate of 10% that are structurally deficient

Structural health monitoring is the implementation of a damage identification strategy to the civil engineering infrastructure Structural damage is defined as changes in material and/or geometric properties, including changes in boundary condition and system connectivity Damage affects the current as well as future performance of structure

Monitoring of bridges can provide many benefits to designers, constructors and bridge owners helping to build and maintain more durable and safer structures The main objective of structural health monitoring is to identify and deficiency that a structure might develop in the future However, before future behavior, a monitoring system must be able

to identify the current behavior of structure The difficulties in interpretation of data to determine the current behavior of monitored structures is actually a major knowledge gap

to be bridged for the general acceptance of structural health monitoring by bridge owners

On other hand, for long-span bridge design, wind parameters for stability analysis and wind tunnel testing are based on wind data collected by weather observation stations far from the bridge site Therefore we must observe wind conditions at the site itself to validate design assumptions and parameters Recently, new health monitoring system – Wind And Structural Health Monitoring System (WASHMS) – was developed by Highway Department of the Hong Kong SAR Government [6] to monitor the integrity, durability and reliability of Tsing Ma Bridge (Hong Kong) This system comprises a total

of approximately 350 sensors divided into seven groups, namely accelerometers, strain gauges, displacement transducers, level sensors, anemometers, temperature sensors and weigh-in-motion sensors They are installed permanently on the bridge with the accompanying data acquisition and processing system Five main steps of this system are

as follows:

 Record wind speeds in three orthogonal directions and responses of accelerations, displacements, and strains;

 Extract the parameters of hourly mean winds, three-second gust winds, roughness

of terrain, wind directions, wind incidences, wind turbulence intensities, and wind power spectra from the wind data under both typhoon and non-typhoon conditions

These parameters will be compared with the design extreme values for monitoring

of wind loading on the bridges;

 Plot the instant wind speeds against instant bridge motions at tower tops and mid main span deck sections under typhoon conditions These plots will be compared with similar plots from aeroelastic wind tunnel testing for design validation and wind response monitoring;

 Plot bridge response spectra under typhoons to determine the mechanical magnification factor or admittance function of the bridge structures;

 Plot the stress demand ratio diagrams in key wind resisting components for wind loading monitoring

In addition, in order to remain competitive in the current global economic environment, it

is necessary to minimize service disruptions to civil engineering structures because of routine maintenance or repairs following extreme events, such as earthquakes or typhoons

By providing instant information about issues such as serviceability, safety and durability,

a structural health monitoring system can help civil engineers cope with these types of disruptions Monitoring and evaluating the integrity of large civil structures, while they are

in service, optimizes resources for repair, rehabilitation, or replacement of the structures Structural health monitoring can also be useful in evaluating the life-cycle costs of structural components

The potential direct benefits of a structural health monitoring system are numerous, including real-time monitoring and reporting; reducing down time and improving safety and reliability, while reducing maintenance costs From all above reasons, structural health monitoring system was common installed in modern long-span bridge recently

2.1.2 Application of structural health monitoring for long-span bridges

2.1.2.1 Akashi Kaikyo Bridge (Japan)

Akashi Kaikyo Bridge (Figure 2.1) was opened on April 5, 1998, as the world longest bridge with 1991m of main span and 960 m of each two side spans, over the Akashi Straight between Tsurumi Ward of Kobe Prefecture and Awaji Island of Hyogo Prefecture

as a part of Kobe-Naruto route of the Honshu-Shikoku bridge construction projects Although the bridge is located in severe natural environment including strong winds,

strong tidal currents and salt air…, its service time have to be more than 100 years because

of very expensive cost (approximate 500 billion Japanese yen or U.S $3.6 billion).Therefore, the health, durability, and safety of Akashi Kaikyo Bridge in particular and long-span bridge in general in a long-term service period are now attracting a lot of scientists and engineers

In Japan where natural hazards such as typhoon and earthquake, occur frequently, monitoring of the peak displacements and real stresses of the main cables and their anchorages are the important subjects to be considered In the current long span monitoring system the above items have not been taken into account

Figure 2.1 Akashi Kaikyo Bridge

The measurement data items at Akashi Kaikyo Bridge and the layout plan of the measuring instruments are shown in Table 2.1 and Figure 2.2, respectively The special feature of the bridge is that receivers by the GPS (global positioning system) are set on stiffening girders,

at the top of the tower and the top of the anchorage (mobile stations on girders and tower, and fixed stations on anchorage) The coordinates of the various parts of the bridge can be measured in real time and precisely by the real time kinematics measurement method that utilizes the GPS receivers Using this method, the coordinates of the mobile stations are

determined in the following way The fixed stations receive the data from the satellite and transmit them to the mobile stations Then, the mobile stations carry out measurement analysis by interfering the data from the fixed station with the received data of their own to determine the coordinates This method insures a horizontal accuracy of 1 cm and a vertical accuracy of 2 cm [7]

Table 2.1 Measurement items at Akashi Kaikyo Bridge

Design verification item Points of main focus Variables to be measured Earthquake characteristics Seismic motion and magnitude

Earthquake frequency characteristics

Ground characteristics Phase difference

Seismic motion input onto superstructure

Response acceleration (speed)

Displacement Response acceleration (speed)

Wind characteristics Basic wind speed

Design wind speed Variable wind speed characteristics

Intensity of turbulence Spatial correlation Power spectrum

Wind direction and wind speed

Wind dynamic response Natural frequency of

superstructure Vibration mode configuration Structural damping

Gust response

Action of main tower (TMD)

Response acceleration (velocity)

DisplacementPredominant frequency Wind velocity and response acceleration

Response displacement

* TMD: Tuned Mass Damper

Figure 2.2 Monitoring system of Akashi Kaikyo Bridge

Monitoring system of Akashi Kaikyo Bridge includes below sensors:

Seismometer:

Two seismometers were installed in 1A and 4A foundation to record the direction, duration and force of earthquake and like concussion To avoid response vibration influence of the 1A foundation to the original seismic motion, the seismometer at this point was installed at a place that has a distance about 100 meters from bridge axis Another seismometer at 4A was installed at approximately -20 meters in the granite rock near the 4A concrete block

Anemometer:

In order to determine the wind characteristics of the bridge structure, the distribution of directional wind speed is measured in the longitudinal and

transversal directions To investigate spatial correlation in the horizontal direction, more anemometers are installed in the middle of center span

Accelerometer:

To verify the real dynamic structural behavior due to earthquake of each foundation with design values, a three-component accelerometer was installed in at least one location on each foundation

Velocity gauge:

To monitor the vibration response due to wind and earthquake of the girders and main tower

Global positioning system (GPS):

GPS units were installed on the tops of the 1A and 2P tower and in the middle of the center span The coordinate of 1A was fixed as original point and other measure point displacements were calculated in longitudinal, vertical and transversal deflection

Girder edge displacement gauge:

These gauges were installed on the west and east edges of the 2P center-span side and on the west side of the 3P side-span

Tuned mass damper (TMD) displacement gauge:

Since the main tower has a height approximate to 300m, it was confirmed through wind tunnel test that vortex oscillation would occur following a wind speed even lower than design wind speed For stabilization purpose, TMDs were installed inside the tower and its displacement was measured by displacement gauges

Thermometer:

Three cable thermometers were installed in order to compensate with the displacement measured by GPS, and one atmospheric thermometer was installed in the middle of the center span

(a) Wind direction

Figure 2.3 shows an example of Akashi Kaikyo Bridge monitoring time history data for ten minutes before and after the peak wind speed and the maximum recorded transversal displacement during typhoon The wind direction and wind speed measured by the anemometer near the middle of the center span are shown in Figures 4(a), (b), and the horizontal and vertical displacement of the related girder is shown in Figure 4(c), (d), respectively

2.1.2.2 Tatara Bridge (Japan)

The Tatara Bridge (Figure 2.4) is a cable-stayed bridge, with 890m of main span and 1,480m of total length, connecting the Islands of Ikuchi and Oumishima, as part of the Shimanami Motorway for the Honshu-Shikoku Bridge Project, with an overall length of 59.4km In 1990, it was decided that the bridge would be constructed with a semi-fan stay cable arrangement A steel box girder cable-stayed bridge with a main span of 890m was selected to avoid the huge anchorage blocks, which would be required for a suspension bridge The construction was started in April, 1992 and was completed and opened to traffic in April, 1999, as the world’s longest cable-stayed bridge at that time (this record has been replaced by Sutong Bridge in China with 1,088 m of main span from 2008)

Figure 2.4 Tatara Bridge

Similar to Akashi Kaikyo Bridge, Tatara Bridge was installed many sensors for monitoring work (Figure 2.5) However, the number as well as type of sensor of Tatara Bridge is fewer than that of Akashi Kaikyo Bridge (Figure 2.6)

Figure 2.5 Installed accelerometer in Tatara Bridge

Figure 2.6 Sensor system of Tatara Bridge 2.1.2.3 Tsing Ma Bridge (Hong Kong)

The Tsing Ma Bridge (opened in April 27, 1997 in Hong King, China, Figure 2.7) is the world's seventh-longest span suspension bridge, and the world's longest span suspension bridge (2.2 km of length, 1,377 m of main span) carrying both vehicle and railway traffic Tsing Ma Bridge has a double deck: the upper deck has six highway lanes for

vehicle traffic; the lower deck includes two railway tracks and two single-lane emergency roadways for maintenance and ensuring uninterrupted traffic from/to Hong Kong International Airport during typhoon when wind speed is still within an acceptable level

Figure 2.7 Tsing Ma Bridge

Figure 2.8 Sensor system of Tsing Ma Bridge

The sensors installed in this bridge include accelerometers, strain gauges, displacement transducers, level sensing stations, anemometers, temperature sensors and dynamic weight-in-motion sensors They measure everything from tarmac temperature and strains in structural members to wind speed and the deflection and rotation of the kilometers of cables and any movement of the bridge decks and towers Besides the conventional sensors, Fiber Bragg Grating sensors were installed by Photonics Research Center of the Hong

Kong Polytechnic University to measure vibration, strain distribution and suspension cable tension (Figure 2.8, 2.9, 2.10) With more than 350 sensors, the structural behavior of the bridge is measured 24 hours a day, seven days a week

These sensors are the early warning system for the bridges, providing the essential information that helps the Hong Kong Highways Department to accurately monitor the general health conditions of the bridge

Figure 2.9 Location of sensor in cross frame of Tsing Ma Bridge

Figure 2.10 Mean speed (a) and direction (b) of onset wind recorded in Tsing Ma Bridge

2.2 Damage detection for long-span bridge

2.2.1 Deterioration of long-span bridge

For bridge, both the initial and lifecycle costs, and the consequences of failure in any aspect of their operational or structural performance make them unique infrastructure components in terms of their societal impact Because bridges are aging and traffic is growing, structural damage can be the result of many reasons such as corrosion, structural fatigue, accident…, in which corrosion should be need special attention because it was usually located in severe corrosive environment (Figure 2.11)

Figure 2.11 Damage on cable by corrosion

Figure 2.12 Bridge deterioration prediction in Japan

In Japan, the prediction of deterioration of bridges is illustrated in Figure 2.12 [3] Although only 5% of the bridges are 50 years old at present, the rate will increase quickly The rate will become 29% in 20 years and become 48% in 30 years About one half of the bridges are steel bridges and the remaining are pre-stressed concrete bridges and/or

reinforced concrete bridges Most of the long-span bridges were constructed in the1990s Aside from some long-span bridges with adequate monitoring network, the remaining part

of the road extension includes enormous long-span bridges which bridge the country roads and middle roads These bridges are widely distributed and present many obstacles to maintenance management

To evaluate the deterioration of these long-span bridges, the survey for maintenance ways, decreasing their performance was conducted under designed criteria Table 2.2 shows an example of criteria for deterioration evaluation of deteriorating survey of galvanized structure for maintenance ways of Seto Bridge (Figure 2.13, a series of double deck bridge connecting Okayama and Kagawa prefectures in Japan across a series of five small islands

in the Seto Inland Sea) The survey was carried out for 1,118 panels of maintenance ways installed in both east and west sides of the Seto Bridge The obtained results showed that more than 40% of maintenance ways were observed as grade V, where most of the galvanized layer is consumed and the corrosion also has progressed on the steel body as well as different according to bridge location (Figure 2.14) [8]

Figure 2.13 Seto Bridge Table 2.2 Criteria for deterioration evaluation of galvanized structure of Seto Bridge

I Zinc layer remain

II Aging of zinc layer is progressing, and alloy layer is exposed

partially III Zinc layer are consumed, and alloy layer is exposed widely

IV Galvanized layer are consumed, and aging has progressed on steel

body

Figure 2.14 Deteriorating grade of maintenance ways of Seto Bridge

(SB, HB … is abbreviations of bridges names)

In the United States, most of the 1,100 major long-span bridges (those with spans of 100 meters or longer) are over 50 years old, and several notable ones are over 100 years old [9, 10] These bridges fall outside the Standard Specifications issued by AASHTO (1998), and there is little generic experience related to maintaining their performance especially after they age and/or following any damage Among them, more than 800 of the long-span bridges are classified as fracture critical and more than 40% of the nation’s bridges are either structurally deficient or functionally obsolete (Figure 2.15)

Figure 2.15 Health of long-span bridge in the USA

Many long-span bridge owners have already adopted or are currently in the process of adopting various intelligent transportation systems technologies in order to enhance traffic

flow or bridge operations such as toll collection There are compelling benefits, in terms of the optimal management of long-span bridges, for expanding such intelligent transportation systems investments into a broader health monitoring application for both operations and structural health The integration of operational and structural health monitoring will be demonstrated and the corresponding benefits will be quantified Many owners have expressed strong interest in research for integrated operational and maintenance management of their infrastructure

For a long-span suspension bridge, hanger system has an important role Although all hangers are protected carefully by many methods, they are common deteriorated by corrosion and fatigue Corrosion of hanger rope may be external, internal or both External corrosion is easily detected but it can occur in any part of the exposed surface of the rope

If it isn’t protected against, corrosion quickly leads to pitting of the external wires and in some cases this has been found to cause wire breaks Meanwhile, internal corrosion can be very difficult to detect It can cause significant loss in strength without showing any external signs A well designed rope will offer little chance of moisture penetration in its dead load condition but at clamps on the main cable of a suspension bridge, the rope is likely to open up and allow moisture to penetrate This can then run down the core of the rope and accumulate at the low point, which is usually the socket or termination, leading some reduction of cross section of hanger (Figure 2.16)

Figure 2.16 Broken wires and reduction of cross section of Alvsborg Bridge, Sweden [11]

On the other hand, fatigue is caused by many cycles of loading either axial, bending or both Axial loading is likely to be most significant from traffic, single heavy vehicles or combinations of heavy vehicles As the fatigue endurance of a spiral strand hanger is proportional to approximately the 4th power of the load range, the outer wires may begin to fracture quite early in the design life It is usual for the outer wires to fracture first and it should be noted that the loss in strength of the hanger is disproportionate to the number of broken wires, mainly due to cracked wires not yet fractured (Figure 2.17)

Bending fatigue may be due to longitudinal or transverse differential sway between the main cable and the deck but is more usually a result of hanger oscillation, either vortex shedding, galloping or buffeting Ice accretion can be particularly dangerous, leading to large galloping motions in longer hangers

Figure 2.17 Fatigued strand of Severn Bridge, England [12]

2.2.2 Damage detection for long-span bridge

Bridges are individual structures that have very little in common with each other Almost any new bridge is a prototype The combinations of facts, use, properties, boundary condition and geometry create a huge number of unknown aspects; therefore a uniform monitoring process is not feasible On the other hand, in structural engineering, the safety margins are higher to cover the unknowns This allows methodologies which can accept a