Field load experiments and computer modeling of a steel truss bridge for assessment

To ensure the normal service of the aging bridge and meet the transportation demand of citizens, a set of full-scale field load tests including static, dynamic load and short-term monito

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長岡技術科学大学 The Nagaoka University of Technology The Graduate School of Engineering Faculty of Civil and Environmental Engineering

Field Load Experiments and Computer Modeling of a Steel

Truss Bridge for Assessment

A Thesis in Civil Engineering

by TRAN DUY KHANH

Submitted in Partial Fulfillment

of the Requirements for the Degree of

Master of Science Nagaoka, Niigata July 2015

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An abstract of Field load experiments and computer modeling of a steel truss bridge for

assessment

In Japan, it can be surely predicted that in 2030s, more than 50 percent of existing bridges, which were primarily constructed during the post World War II, a rapid economic growth between 1955 and 1975, is assumed more than 50 years

of age The research investigated an in-service, multi-span old steel through truss highway bridge constructed and opened to the traffic in 1937 The bridge, also known as a city symbol, which is on the National Highway No.351, spans the Shinano River, in Nagaoka City, Japan The bridge was constructed with materials of steel for frames and concrete for the deck, consists of 13 spans composing of 2 cantilever anchored spans, and 11 suspension spans and cantilever central anchored spans alternatively connected via hinges at upper level and pins at lower level with a total length of 850 meters To ensure the normal service of the aging bridge and meet the transportation demand of citizens,

a set of full-scale field load tests including static, dynamic load and short-term monitoring experiments was performed on the bridge to identify the current health status, and characterize the response to practical load conditions The responses

of the excited bridge was recorded continuously and simultaneously at 200 sampling frequency through an array of 100 strain gages installed on 25 identical and key members, and each of 26 displacement sensors and accelerometers instrumented at each span center point The exciters used in these tests were three-axle 20-ton dump trucks for controlled tests and normal daily traffic for the short-term monitoring measurement A three-dimensional finite element model of the bridge superstructure was constructed with geometry and structural arrangement extracted from recovered drawings The model, simulated the in-situ tests, was validated manually through a comprehensive comparison of analytical results and measurement data in terms of internal forces, vertical displacements The axial force extracted from the upper chords and diagonal chords instrumented in the static loading tests was compared the computational axial

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Table of Contents

List of Figures ix

List of Tables xiii

Acknowledgments xiv

Chapter 1 Introduction 1

1.1 Background 1

1.2 Objectives of the research 5

1.3 Scopes of research 5

Chapter 2 Bridge description, field loading tests and monitoring 7

2.1 Introduction 7

2.2 Bridge description 7

2.3 Field loading tests and monitoring 13

2.3.1 Background and purpose 13

2.3.2 Test instrumentation 16

2.3.3 Static loading test 24

2.3.4 Dynamic loading test 24

2.3.5 Short-term monitoring 26

2.4 Measurement processing for developing a finite element model 27

2.4.1 Internal forces 27

2.4.2 Vertical displacement 29

2.4.3 Thermal axial stress 29

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2.4.4 Acceleration 33

Chapter 3 Finite element model validation 36

3.1 Introduction 36

3.2 Literature review 36

3.3 Finite element (FE) model construction 40

3.3.1 Input data 40

3.3.2 Boundary conditions 43

3.3.3 Other assumptions 46

3.3.4 Loading 47

3.3.5 Mesh refinement 48

3.4 Model updating 50

3.4.1 Parameter study: Boundary condition study 53

3.4.2 Model validation 55

3.4.2.1 Axial force 55

3.4.2.2 Vertical displacement 58

3.4.2.3 Thermal induced axial stress 59

3.4.2.4 Lateral distribution stiffness 59

3.4.2.5 Natural frequencies 62

Chapter 4 Dynamic loading allowance 64

4.1 Introduction 64

4.2 Literature review 64

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4.2.1 Definition of dynamic loading allowance 64

4.2.2 Specification prescribed dynamic load allowance 64

4.2.2.1 Constant dynamic load allowance 65

4.2.2.2 Span length varying dynamic load allowance 65

4.2.2.3 Frequency varying dynamic load allowance 66

4.2.3 Dynamic field testing of bridges 67

4.2.4 Other dynamic load allowance studies 68

4.3 Utilized method 69

4.3.1 Determine the low-pass cut-off frequency 70

4.3.2 Determine the filtering order 72

4.3.3 Filtering by Matlab 73

4.3.4 Controlled dynamic load test data 75

4.3.5 Short-term monitoring data 78

4.3.5.1 Impact factor versus strain measurement 78

4.3.5.2 Impact factor versus displacement measurement 81

Chapter 5 Bridge Assessment by AASHTO Code 85

5.1 Introduction 85

5.2 Load rating procedure 85

5.3 FE model analysis 88

5.4 Load rating results 90

Chapter 6 Conclusions and recommendations 93

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6.1 Conclusions 93

6.2 Recommendations 94

References 95

Appendix A 99

Appendix B 106

Appendix C 114

Appendix D 138

Appendix E 139

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List of Figures

Figure 1-1 Composition of bridge’s age to the total number of bridge in Japan 2

Figure 1-2 Total number of highway bridges and construction year in Japan 3

Figure 2-1 Location of the target study bridge 7

Figure 2-2 A view of Chyosei Bridge-Niigata, Japan 8

Figure 2-3 Cross-section of the bridge at mid-span and support 9

Figure 2-4 Elevation of the Chyosei Bridge with condition of hinges and bearings as designed 9

Figure 2-5 Typical cross-sections 11

Figure 2-6 Upper (a) and lower (b) bracing systems 12

Figure 2-7 Location of sensors in the field test 17

Figure 2-8 Instrumentation in the field tests: a) Strain gages in Upper chord; b) Strain gages in Diagonal member; c) Displacement transducer in Lower chord; d) Accelerometer in Vertical member; e) Accelerometer over wheel guard of the pedestrian bridge 19

Figure 2-9 Strain gage location in instrumented members: a) Foil strain gages in Upper chord Layout; b) Foil strain gages in Diagonal member Layout; c) Detail of Upper chord Layout; d) Detail of Diagonal member Layout 20

Figure 2-10 Test truck configuration 21

Figure 2-11 Cross-section of 2 trucks utilized for loading tests 21

Figure 2-12 Load configurations used for the first load test 22

Figure 2-13 Load configuration used for the second load test 22

Figure 2-14 Dimensions of the trucks used for the load tests 23

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Figure 2-15 Patterns for static load tests 24

Figure 2-16 Location of test trucks in dynamic loading test 26

Figure 2-17 Typical strain gauges’ location and internal force notation 28

Figure 2-18 Deflections by spans in the static load tests 29

Figure 2-19 Temperature time history 30

Figure 2-20 Temperature and normal stress relation of an upper chord in the 6th span 31

Figure 2-21 Normal stress time history of an upper chord in 6th 32

Figure 2-22 Dynamic component stress history 32

Figure 2-23 Axial stress and thermal changes correlation 33

Figure 2-24 Typical frequency spectra for the truck passing (a) and the truck left (b) 35

Figure 3-1 Arbitrary cross-sections 42

Figure 3-2 Element types used in 3D-FE model 42

Figure 3-3 The studied bridge structure and its restraints 44

Figure 3-4 Gerber hinge modeled by 2-node links, SP2TR, SP2RO 45

Figure 3-5 Deck system (Left) and computer simulation (Right) 46

Figure 3-6 Bearing support computer simulation 46

Figure 3-7 Typical loading simulation of static load test in the first span 47

Figure 3-8 Division size and output values correlations 49

Figure 3-9 Meshed finite element model 50

Figure 3-10 Model validation procedure 53

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Figure 3-11 Relative deviation (%) between test and analytical results of each

static load case acting on corresponding members 57

Figure 3-12 Maximum vertical displacement for 13 static load cases 58

Figure 3-13 Positions of test truck in dynamic tests 60

Figure 3-14 Influence lines of axial force of upper chords 62

Figure 4-1 Canadian, Australian, and American DLF versus First flexural frequency 67

Figure 4-2 Flowchart of dynamic load allowance identification 70

Figure 4-3 Typical PSD in case of without vehicle presence 72

Figure 4-4 Typical PSD in case of vehicle presence 73

Figure 4-5 Typically filtered dynamic and static member strain response 74

Figure 4-6 Instrumentation configuration 75

Figure 4-7 Impact factor based on strain 76

Figure 4-8 Impact factor based on displacement 77

Figure 4-9 Impact factor by measurement type 78

Figure 4-10 Typical dynamic load factor versus maximum strain in U1 79

Figure 4-11 Dynamic load factor versus maximum strain for total samples 79

Figure 4-12 A typical histogram of dynamic load allowance versus strain response in U4 80

Figure 4-13 A typical histogram of dynamic load allowance versus strain response in U9 81

Figure 4-14 Typical dynamic load factor versus maximum deflection in span 1 82

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Figure 4-15 A typical histogram of dynamic load allowance versus maximum

deflection in span 1 83

Figure 4-16 Dynamic load factor versus deflections for total samples 83

Figure 5-1 AASHTO MBE load rating procedure 87

Figure 5-2 HL-93 truck 90

Figure 5-3 Rated members in the truss 91

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List of Tables

Table 2-1 Experimental installation details 18

Table 2-2 Test Vehicle Weights 19

Table 2-3 Details of test runs for field tests 25

Table 2-4 Natural frequency (Hz) 34

Table 3-1 Input parameters utilized in FE model of the bridge 41

Table 3-2 Material density of dead load effects 48

Table 3-3 Bearing supports as originally designed 54

Table 3-4 Gerber hinges as originally designed 55

Table 3-5 Proposed hinge stiffness in the calibrated model 56

Table 3-6 Proposed bearing types in the calibrated model 57

Table 3-7 Input parameters used in analyzing thermal load 59

Table 3-8 Temperature change induced axial stress (MPa) 59

Table 3-9 Values of k 61

Table 3-10 Natural frequency (Hz) 63

Table 4-1 AASHTO LRFD Table 3.6.2.1 65

Table 4-2 Length varying DLF (Eq 4-3-Eq 4-6 from Moghimi 2007) 66

Table 5-1 Values utilized in AASHTO MBE load ratings 88

Table 5-2 Load effect stress and Load rating factors 92

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Acknowledgments

I would like to take this space to thank everyone that has made this research a success First of all, I would like to express my appreciation to Associate Professor Takeshi MIYASHITA for his very kind guidance, assistance, and support throughout the academic research process I could go to ask the essential questions and make me accountable for my work My thesis and key skills have been improved as a result

I would like to thank and acknowledge Professor Eiji IWASAKI for valuable advice, additional assistance, and useful comments

We would like to express the thankfulness for the invaluable contributions

to the research from the Nagaoka Regional Promotion Bureau, Regional Management Department, and Maintenance Division Authority The Kozogiken Niigata Corporation and Tokyo Sokki Kenkyujyo Co., Ltd., who supported to conduct the field load tests, are gratefully appreciated

Last but not least, I would like to thank my families and my friends, my girlfriend for their love and constant support throughout the 2-year research in Japan

Nagaoka July 2015 TRAN DUY KHANH

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Chapter 1 Introduction 1.1 Background

Upon the gradual increase of aged or deteriorated bridges in Japan as well as

in the other countries all over the world, the maintenance and rehabilitation of old

or degraded bridges become a field of growing relevance in terms of impact factors on the economic growth for those nations In this context, the state-of-the-art and state-of-the-practice presented for non-destructive testing and evaluation play an essential role in a currently accurate condition assessment of the bridges, which in turn provides reliable data recorded from the field loading tests to assist the decision making process in their management, to identify the actual impact of the modifications introduced in the bridge behavior as a result of rehabilitation programs, as well as to improve these projects at the design stage, described by Bruno 2013

In recent years, due to requirements of economic growth or high transportation demand of human, higher truck loads have required that the case study bridge sustain loads in excess of those intended during its original design in the past There is concern that this increase in capacity demand will accelerate the rate of deterioration in this aged bridge Future gross vehicle weights are expected to increase, and knowledge of the actual behavior of the bridge under these increasing loads is very necessary

In Japan, it can be surely predicted that in 2030s, more than 50 percent of existing bridges, which were primarily constructed during the post World War II, rapid economic growth between 1955 and 1975 will be more than 50 years of age, see Figure 1-1 (refer to Fujino and Siringoringo 2008 ) Adding to the problem of aging bridges, the significant increase in traffic density, vehicle weight, and frequency of loading created by this high traffic volume generate many problems

in these aging in-service bridges A report of the current status of Japanese road bridges from Japan Ministry of Land, Infrastructure and Transport stated that there

is up to 39 percent of steel bridges out of the total more than 142.000 bridges exceeding 15 meters in long Most of these in-service bridges have been suffering

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from damages, deterioration, and severe environment attacks Japan is a nation surrounded by sea and consisting of many humid, high salt air areas In addition, seismic operations and other natural disasters such as earthquake, typhoons with higher and higher presence frequency and magnitude also are great contributors

to weaken existing bridges, damage occurs Therefore, factors of corrosion of steel members, damages of reinforced concrete slab can cause the strength of those bridge decreasing and influence on service capacity As a result, more and more deteriorated steel bridges influence on the daily demand of Japanese citizens, transportation, and are likely to cause sudden collapses and loss of lives Therefore, these aging and/or deteriorated bridges require prompt repairs, replacements or other remedial actions to assure very basic demands of people and maintain them in satisfactory conditions

Figure 1-1 Composition of bridge’s age to the total number of bridge in

to be highly considered to inspect and maintain periodically Therefore, the preparation of a huge budget for improving the infrastructure and transportation system becomes a burden for the Japan government There are many accidents causing loss of people and damage to economy relating to the deterioration of old bridges (Miyashita T and Nagai M 2009) The recent bridge collapse disaster of I-35W interstate bridge in Minnesota, USA has focused not only national attentions

of USA but also from other countries’ concern, is a typical example showing a shortage of validity in inspecting and rehabilitating aging and degraded steel bridges caused by corroded damages This renewed focus in conjunction with the

Numb

er

of construction (x1.000)

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age and deterioration of historic truss bridges puts these structures in serious risk

of being found structurally deficient, demolished and replaced To solve this national social and economic problem, the periodically proper maintenance and retrofit strategy demonstrates a wise solution of the bridge operator and manager The field load test is one of the necessary and famous works before retrofitting those deteriorated bridges A main factor in determining the present state of a historic bridge lies in the treatment and understanding of their complex behavior when exposed to full-scale real loadings At the beginning of the 20th

The results from tests performed to the selected case study multi-span steel truss bridge would be useful for understanding the structural response under static and dynamic loading of the aged steel through truss bridge and provide a reliable model to assist the decision making process in its management This model is

century, the construction of large steel structures as bridges with built-up truss members became a common phenomenon in the civil engineering field A steel cantilever truss bridge was one of those structural types that engineering designers selected

as a wise solution for a difficult situation in the performance of numerical calculation However, severe environmental impacts, and increased transportation demands have led to the sharp deterioration of steel structures A case study of an aged multi-span steel truss bridge was conducted for this research purpose The case study bridge carries the National Highway No.351 over the Shinano River, the longest river in Japan, between the east and west regions of Nagaoka city, located in the central part of Niigata Prefecture The bridge structure was built between 1934 and 1937 and was opened to the traffic in 1938 After more than 78 years of service, fatigue cracks, degradation, and corrosion or changes in connectivity or bearing supports had become prominent in the course of increasing traffic The structure may have been designed for a lighter loading than

is used as today, a different design code, or a stress range that is no longer applicable, or it may have lost live-load capacity as a result of aging, deterioration, damage to members, or added weight such as a new deck, or wearing surface, stringers, other equipment for services For this reason, a tool that accurately simulates the actual behavior of the bridge to help with its condition assessment is needed

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expected to provide valuable information to advise the present evaluation of the bridge and to assist the future maintenance and rehabilitation

1.2 Objectives of the research

This dissertation is concerned with field load test and bridge load rating on aging steel bridge The objectives of this study is to try to construct and validate a full-scale three-dimensional finite element model of the case study bridge through comparisons between analytical results from the computer analysis and practical results extracted from the on-site static and dynamic loading tests, evaluating dynamic load factors, and finally rating the load-carrying capacity of the bridge using finite element analysis and manual on bridge evaluation

1.3 Scopes of research

The scopes of this research mainly covers the computer simulation of the bridge based on the field load tests, obtaining the dynamic loading factors, and rating load-carrying capacity of this aging bridge The load rating in this study only focusses on the steel truss members located on the superstructure of the bridge Therefore, the analysis and field load testing performed on the case study bridge also concentrated primarily on the bridge superstructure, or truss systems The principle objectives of this research are divided into the following chapters separately, as follows:

Chapter 2 of this dissertation provides an overview description of the Chousei case study bridge, including construction materials, structure arrangement and its boundary condition This chapter also present detailed full-scale field loading tests consisting of instrumentation installation, loading procedure and preliminary load test response processing for static loading tests, dynamic loading tests, and short-term monitoring

In Chapter 3, analyses of a detailed three-dimensional finite element model of the bridge are conducted and validated via comparing procedures between computer-generated results and field load testing results The responses of the bridge structure from the practice are considered as a baseline to construct and

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modify the finite element model of the bridge In this chapter, parameter study is also conducted to investigate into bridge structure behaviors on boundary condition such as Gerber hinges and bearing support types, and concrete elastic modulus

Chapter 4 presents the dynamic loading factor of the case study bridge based

on the full-scale field dynamic loading tests and from the short-term monitoring data The impact factor will be obtained via a signal-filtering tool called Butterworth and becomes an important parameter for bridge load rating that presented in the Chapter 5

Chapter 5 illustrates the application of bridge evaluation specification for load rating of the case study bridge In this chapter, load rating procedures extracted from the Manual for Bridge Evaluation (MBE) 2013, AASHTO will be listed and applied to rate the load-carrying capacity of the target aging steel bridge

Finally, Chapter 6 presents a summary of the key findings of this research, recommendations for future research directions relating to this study and also describes implications for load rating of steel truss bridges

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Chapter 2 Bridge description, field loading tests and monitoring 2.1 Introduction

A case study bridge is chosen for the research purpose This chapter focuses

on the description of the bridge structure in detailed and full-scale field loading tests and monitoring procedure that has been performed on the bridge The geometry including member lengths and structural arrangement were taken from the recovered plans The field test measurement data recorded and stored in the files at the site were later processed into other relevant information for research purposes that presented in the final part of this chapter

2.2 Bridge description

Figure 2-1 Location of the target study bridge

The case study bridge, see Figure 2-1, is on the National Highway No.351, crossing the Shinano River, located near the town center, is one of the main entrances to the lively downtown area from the West Upon a period of design and

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construction from 1932, the bridge was completed and opened to the traffic in

1938 Today, this bridge not only stands as a significant landmark of the city, but also plays an important role in the infrastructure system of the region, being one of the large bridges that connect the city

The target bridge was originally designed as Gerber steel through cantilever truss bridge encompassing a series of anchored cantilever spans and suspended spans that linked to each other by Gerber hinges for forming a continuous bridge structure The suspended span is connected to the cantilevered arms at upper extremities by Gerber hinges and at a lower level by pins, see Figure 2-3 This system gives an advantage of the continuous beam type and the isostatic structure, refers Fernandez 2003

Figure 2-2 A view of Chyosei Bridge-Niigata, Japan

The bridge, seated on the two abutments, and 12 piers constructed of reinforced concrete material, covers 13 spans that are two anchored spans of 85-meters long each, six suspended spans of 30-meters long each and five of 100-meter central double anchored spans with a total length of 850.8 meters (see Figure 2-4) Figure 2-3 depicts a cross-section of the bridge with steel frames and

a reinforced concrete slab of 7 meters wide

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Almost elements of steel trusses are built-up sections fabricated by assembling

various plates and angles through riveted connections, cover plates, and lacing

bars at the shop The principal structural system of a couple of trusses that

employs opening box shaped built-up sections of the upper and lower chords and

a mixture of opening box and I-shaped built-up sections of the verticals and

diagonals (see Figure 2-5)

Figure 2-3 Cross-section of the bridge at mid-span and support

Figure 2-4 Elevation of the Chyosei Bridge with condition of hinges and

bearings as designed

Gerber hinges

Bearings Pins

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The original floor system arrangement comprises of five I-shaped steel stringers interconnected with those I-shaped crossbeams linked to the corresponding truss panel points at its both ends (see Figure 2-3) The underlying steel stringers were designed to connect the 150-centimeter thickness concrete deck slab by rigid shear links for composite action behavior The reinforced concrete deck is 7.00m wide, and the floor system is approximately 1.35 height The 60-milimeter asphalt layer covers the deck for traffic purpose (see Figure 2-3) The floor beams used are 8.0 meters long and riveted by steel angles forming flanges, cover plates for the top and bottom flanges, web plate strengthened with vertical stiffener All of the stringers is riveted by using double angles and a web plate and spaced 1.8m, and also braced to each other using built-up riveted double angles and gusset plates for connection in transverse and zigzag direction along the bridge The thermal expansion finger joints are placed at the discontinuous positions between the suspended span and cantilevered arms, in the same transverse plane of the hinges and pins

The lateral bracing system contributes to the lateral stability and resistance to wind provided by the K-bracing at the top level, X-bracing at the bottom level along with the sway bracing throughout the spans functioning to keep the two trusses parallel (see Figure 2-6) Moreover, they allow achieving higher load-carrying capacity without buckling under eccentrically vertical loadings

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Figure 2-5 Typical cross-sections

The truss also was designed to consist of sway bracings and struts that located

at all panel ends throughout the structure (see Figure 2-3) These members are in

a vertical plane and functions to keep the two trusses parallel and are considered secondary members These transverse elements have either two angles at the top and two angles at the bottom with a system of diagonal angles connected using riveted gusset plates These are subjected to the compressive stress caused by transverse and/or horizontal loads

The 13 spans are simply supported by movable or fixed bearings, depending

on their location along the structure, transferring the loads from the superstructure

to the substructure, and allowing rotation caused by permanent and transient deflection as well as horizontal movement of the superstructure due to expansion and contraction where the movable supports are located The arrangement of these bearings is depicted in Figure 2-4

(a) Lower chord (b) Upper chord (c) Vertical chord (d, e) Diagonal chord

(e) (d)

(b)

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Figure 2-6 Upper (a) and lower (b) bracing systems

To connect suspended spans to double cantilever spans or anchored cantilever spans, Gerber hinges and pin are utilized along the bridge The hinges consist of two vertically oriented steel plates pinned at the top and bottom to allow longitudinal movement The hinges also allow the intercalated arrangement of movable on the west end and fixed on the east end of the suspended spans At the lower level, the pins permit freedom rotational movement under vertical loads, see Figure 2-4

For a long time of service, under the effects of severe weather and increasing traffic on the bridge, fatigue cracks (described by Miyashita and Nagai 2009), and corrosion induced damage occurs The rehabilitation and maintenance

ever-of the structure were performed to satisfy the transportation demands and safe requirement to aim to extend the life of the historical and significant bridge The process of paint and anti-corrosive layer coating, bearing repairing and replacing, bridge collapse prevention device installing, 2-stringer adding, lacing bars, angles, covers and gussets, and rivet connection strengthening, asphalt pavement re-covering, bridge re-decking was executed A pedestrian bridge was constructed and seated on the same piers parallel to the truss bridge Therefore, the current

(a)

(b)

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state of the bridge structure is slightly different from the originally designed drawings in the general structural stiffness and the self-weight due to changes in structural arrangement configuration and cross-sectional areas

2.3 Field loading tests and monitoring

2.3.1 Background and purpose

Full-scale field loading tests have been used for various and different research’s purpose done by many researchers all over the world, presented by Dorton et al (1977) Bakht (1981), Bakht and Pinjarkar (1989), Bakht and Jaeger (1990), Stallings and Yoo (1993) These previous experiments conducted on bridges are mainly to identify current conditions before opening of new bridges or confirm and compare rehabilitated structures after repairing or strengthening Some field tests using dynamic load excited by moving trucks, hammers or ambient sources are to obtain dynamic properties of bridge structure for better understand actual responses under real loads or to modify finite element models Objective on load rating of aging or degraded bridges also is a goal that needs to conduct proof load tests on-site to determine highest load carrying capacity of those structures that meet daily demands of transportation and citizens with safely condition

A study on load distribution factor for simply supported steel I-girder bridges was done by Sangjin Kim et al 1997 using results of fields load tests with normal truck traffic and passes of a controlled truck of know weight and configuration Measured strain data recorded from the short-term monitoring of 2 days under daily passing trucks and the controlled tests was processed and filtered to obtain statistical parameters of the girder distribution factors The proposed factors were concluded that they are lower that suggested values by AASHTO

Laman et al 1999 performed full-scale on-site dynamic loading tests on three 60-year-old steel through truss bridges to investigate a study on a dynamic load allowance Selected members such as truss members, stringers, and floor beams become his research targets Dynamic strain data were recorded from the tests with controlled, and normal traffic conditions Based on a signal processing tool,

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the impact factors of those bridges were obtained and examined as a functions of the component type, component location, vehicle type, number of axles, vehicle speed, and vehicle direction

Bowen et al 2003 did his research on the historic steel truss bridge decks with two case study bridges The finite element analysis of the bridge model indicated that the actual forces developed in the floor members under truck loading are significantly less than predicted by a standard AASHTO load rating, and that a higher load rating for the floor members may be justified Many of the assumptions made in constructing the finite element model of the bridge deck was chosen to provide a conservative prediction of response or untrue response Therefore, the field load tests performed on those bridge in this study were to provide further insight into the structural behavior of the bridge deck system, and also to judge the accuracy level of the three-dimensional finite element model of the slab system The on-site full-scale loading experiments were carried out to measure the practical responses of the slab system subjected to known trucks of weight and configuration

Conducting static and dynamic load testing on a study bridge can be found in the research performed by Karoumi at al 2006 These field load tests were used before the opening of a large new bridge to verify the true structural response compared with that estimated by theoretical results Primary objectives of his study was try to better understand the structure’s behavior to static and dynamic loadings and to provide a footprint of the undamaged structure that can be used

as a baseline or information for other condition assessment, management in the future

To evaluate a damaged noncomposite steel girder bridge, consisting span continuous girders and carrying two-lane traffic that opened from 1960s, Sergio et al 2012 performed live-load testing to measure strains for better understanding the bridge behavior The experimental data were processed and compared with results from finite element models As a result, the load distribution characteristics of girders that had been damaged by a traveling overweight truck were evaluated The contribution of bridge parameters such as skew or partial

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restraint was also investigated based on the observed and computed responses The load-carrying capacity of the study bridge was rated based on these nondestructive load testing results to costly avoid bridge closures and detours Delicate construction procedure for bridges is always a topic that contains many factors that may influence its operation and exploitation A particular bridge, named Infante D Henrique located in Portugal, crossing the Douro River with a span of 280 meters became a study target, described by Filipe M et al 2014 An installation of a static monitoring system that had been in operation since that phase followed by a dynamic monitoring system set up on this bridge For 5 years

of monitoring, the collected database was used to investigate dynamic parameters such as long-term evolution of the natural frequencies, correlations between environmental and operational factors including temperature and traffic with the bridge modal parameters The continuous monitoring was implemented with the aim of developing a robust and automatic tool to detect eventual structural anomalies

In this study, the primary objective of the full-scale field load tests performed on

a Gerber steel through truss bridge was to obtain the on-site data that necessary

to construct and validate a structural three-dimensional finite element model capable of simulation of the structural response of the bridge under its real conditions The field load test can provide a more accurate assessment of the structural response and the strength of a bridge, and can also be used as a diagnostic tool to uncover problem areas with the bridge For the case study bridge, the purposes of the field load test were as follows:

• Develop an improved understanding of the overall behavior of the bridge under static and dynamic loading

• Validate an accurate structural model of the bridge

• Determine dynamic loading allowance

• Load rating of the case study bridge

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Two controlled full-scale field load test and a short-term monitoring were performed on the target bridge In the tests, 25 selected elements of the superstructure were instrumented with strain gauges, 13 members were chosen for attaching accelerometers along with 13 mid-span points were set-up with vertical transducers In the controlled experiments, while known weight and configuration trucks were controlled to drive over the bridge, the signal responses from the instrumented members were recorded

2.3.2 Test instrumentation

Two field load tests were conducted on the bridge The first test was on June 3,

2013, and the second one on the day after In the first test, a large number of members were instrumented to evaluate the overall response of the trusses under the applied truckload The second test was conducted to argue the data collected

in the first test maintaining the same gauging pattern In both tests four quantities were measured: (1) strains in the main truss steel structure; (2) vertical displacements in the middle of each bridge span; (3) acceleration in the middle of the bridge spans and (4) temperature at the steel surface and of the ambient air

Of the 100 strain gages used in this test, all was only installed on the downstream truss The locations of strain gages were selected to obtain data on critical truss members in order to evaluate the overall behavior of the bridge A total of thirteen upper chords located in the center of each span were instrumented with four strain gages, as well as all diagonal members located

at the ends of the six suspended spans

Moreover, vertical displacement sensors were settled in the vertical members

of the downstream truss for measuring the deflection at the middle of all the thirteen spans, almost 0.9m above the gusset plate that connects the truss members in the center In the same place just about 10cm below, accelerometers were placed with the intention of register the acceleration in the middle of all spans A resistance temperature detector was placed at the center

of the seventh span, away from the sunlight to reduce the temperature variation effect Most strain gauges mounted on the members were

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positioned away from the joints and were typically placed near the middle of the member length for eliminating any local variation of stress near the joints

Figure 2-7 Location of sensors in the field test

Because of the high possibility that the riveted gusset plate condition could cause the truss members to experience bending, all truss members were instrumented using four strain gages in order to verify if bending was occurring For the diagonal members, using a symmetric layout with two gauges on each cover plate of the riveted box beams a strain gradient could be recorded All four gauges were averaged to find the axial load in the member After subtracting out the averaged axial strain, the remainder strain on top and bottom indicated if there was any bending in the member For the upper chord members, the layout was done a little different placing two gages on the inside center of the web side facing the pedestrian bridge and other two on the upper cover plate of the built-up lattice box beams The location of strain gages on the cross-section of each member is shown in Figure 2-7 The gauges were placed at least 2 cm away from the rivet connections to avoid stress concentrations Since strain gages were not located symmetrically on the cross section of the upper chord members, the average

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strain in each member could not be calculated as the diagonal members Therefore, the average strain for each instrumented top chord member was calculated as the procedure presented in the measurement processing section The same process used to find bending in the diagonal members was used to find bending in the upper chord members

Three-axle single unit trucks were employed in each test to accomplish the load configurations, arbitrarily labeled Truck 1 (Mitsubishi) and Truck 2 (Nissan), were used to load the bridge for the two controlled loading tests These vehicles were selected based on their availability The typical detailed configuration of both trucks is shown in Figure 2-10 The trucks were loaded with gravel, having an average gross weight of 20 Tons each Axle dimensions and spacing of both load trucks had been identified prior to testing The front and tandem axles were weighed for determining the position of it load center (center of gravity) The exact vehicle axle weights of the both trucks used for the load tests are listed in Table 2-2

Table 2-1 Experimental installation details

Strain

Upper chords of all spans and outermost diagonal chords of suspended spans

Strain gauges (100)

Vertical

displacement Middle of all spans

Displacement transducers (13)

Acceleration Middle of all spans (vertical

detector (1) span

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Figure 2-8 Instrumentation in the field tests: a) Strain gages in Upper chord; b) Strain gages in Diagonal member; c) Displacement transducer in Lower chord; d) Accelerometer in Vertical member; e) Accelerometer over wheel guard of the pedestrian bridge

Table 2-2 Test Vehicle Weights

Truck

identification

WEIGHT IN TONS Front

Axle

Front Rear Axle

Back Rear Axle

Gross Weight

cb

a

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Figure 2-9 Strain gage location in instrumented members: a) Foil strain gages in Upper chord Layout; b) Foil strain gages in Diagonal member Layout; c) Detail of Upper chord Layout; d) Detail of Diagonal member Layout

a)

b)

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Figure 2-10 Test truck configuration

Figure 2-11 Cross-section of 2 trucks utilized for loading tests

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Figure 2-12 Load configurations used for the first load test

Figure 2-13 Load configuration used for the second load test

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Figure 2-14 Dimensions of the trucks used for the load tests

With the bridge temporarily closed to traffic, so that the test trucks were the only vehicles on the bridge, two types of loading scenarios were implemented in the field tests, static, and dynamic For any of them it was expected that the bridge response would remain within the linear-elastic range Since the bridge had a high daily traffic, (1995 estimated at 20,456 vehicles per day), all testing began after 11:00 pm to minimize disruptions to traffic

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2.3.3 Static loading test

In a static field test, the load is applied in fixed positions or moved at crawl speed along the bridge (pseudo-static) The load from the vehicles or weights used must be measured For the static load case, both trucks were to travel along the bridge together, oriented side by side, and as close as possible to each other

to create a symmetric load in both trusses, the set-up was as shown in Figure 2-11 The test trucks were positioned at pre-defined locations over the bridge and instrument readings were taken In the static load test, the truck center of load was positioned as directly above the center of the span truss nodes as possible, see Figure 2-15 Trucks were stationed at each load position, and the response data was recorded to calculate a useful average strain Strains were recorded to four decimal places of microstrain and deflections were recorded to two decimal places

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dynamic parameters of the structure such as forced vibration method and ambient vibration method In the first method, the bridge is excited by impulse hammers, drop weights, and electro-dynamic shakers In case of large structures like bridges, massive and costly equipment is needed to provide an excitation on a high level Therefore, this method could not be applied to this study bridge In this study, the second method (Ambient vibration) is suitable than the first method because of very large dimension In an ambient vibration test, the freely available excitation sources like wind and traffic are used The main advantage is that the no expensive artificial excitation devices are needed For the more accurate purpose,

a controlled known weight truck was utilized for this study

Table 2-3 Details of test runs for field tests

The dynamic tests consisted in only one of the trucks crossing the bridge with

a traversing speed of 30 km/h In order to appraise the lateral load distribution, the

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test truck traveled along three different paths, one coincident with the bridge

centerline, the others close to the downstream truss, see Figure 2-16 Over the

course of two days, each run was performed twice, and truck positions were never

changed between runs The strain, vertical displacement, and acceleration data

were recorded at a rate of 200 Hz

Figure 2-16 Location of test trucks in dynamic loading test

2.3.5 Short-term monitoring

With monitoring systems, the changes over time of the parameters measured

can be followed The test was to aim to record the bridge response to the normal

traffic loading excitation to assess various aspects of the studied bridge

performance Different advantages of those load tests mentioned above is not only

unnecessary to interrupt the transportation and to affect the daily ordinary life of

citizens, but also able to get other ambient data on temperature changes

Test Truck

Downstream side Upstream side

4000 mm

4000 mm

Pattern 1

Test Truck

Downstream side Upstream side

4000 mm

4000 mm

Pattern 2