behavior and design of open steel box girder bridges by up-down construction method

The steel box girder with open-trapezoidal cross section, partial-length prefabricated concrete slab and double composite section by cast-in-place concrete on bottom flange is considered

공학박사학위 논문

지점의 상승하강 공법을 이용한 개구제형 강합성

거더교의 설계와 거동

Behavior and Design of Open Steel Box Girder Bridges

by Up-down Construction Method

공학박사학위 논문

지점의 상승하강 공법을 이용한 개구제형 강합성

거더교의 설계와 거동

Behavior and Design of Open Steel Box Girder Bridges

by Up-down Construction Method

2009 년 8 월

지도교수 : 구 민세

이 논문을 박사학위 논문으로 제출함

인하대학교 대학원 토목공학과 도다이탕

(Do Dai Thang)

Bs R6 00 Z

G&lb Eo++

b&{vinbA &["te=

€*t

[o

Behavior and Design of Open Steel Box Girder Bridges

by Up-down Construction Method

Department of Civil Engineering

August, 2009

ABSTRACT

This research investigates the behavior of open steel box girder bridge by up-down construction method The steel box girder with open-trapezoidal cross section, partial-length prefabricated concrete slab and double composite section by cast-in-place concrete on bottom flange is considered The three-dimensional finite element models, considering construction sequence in modeling, have been used to carry out the analysis A parametric study is used to investigate the effects of some structural characteristics on the behavior of steel box girder These parameters include the variation of lifting upward and lowering downward height, the length and depth of bottom concrete slab, the length of partial prefabricated concrete slab and the steel strength A modification of the time-dependent behavior for double composite section with difference in ages and modulus

of elasticity of top and bottom concrete slab are presented An overview of the effect of lifting upward and lowering downward to the stability design such as web buckling and top lateral bracing are given Optimization of the plate thickness of cross section by considering construction sequences is also presented as additional consideration Finally the recommendations are made for the engineers to the design and construct steel box girder bridges

by up-down construction method

TABLE OF CONTENTS

Title Page No

Abstract i

Table of Contents ii

List of Figures v

List of Tables xi

CHAPTER 1 INTRODUCTION 1.1 Introduction 1

1.2 Objectives and Scope 2

1.3 Organization 4

CHAPTER 2 LITERATURE REVIEW 2.1 Fundamental of Up-Down Construction Method 6

2.2 Up-Down Construction Method Applied for Steel Box Girder 9

2.3 Double Composite 15

2.4 Partial Length Prefabricated Deck Concrete Slab 19

2.5 Proposed Up-Down Method for Open Steel Box Girder 20

2.6 Summary 26

CHAPTER 3 ANALYTICAL STUDIES 3.1 Finite Element Model Description 28

3.2 Nonlinear Finite Element Analysis Modeling 35

3.4 Comparisons the Results 41

3.5 Summary 50

CHAPTER 4 PARAMETRIC STUDIES 4.1 Introduction 51

4.2 The Effect of Lifting-up and Lowering-down Height 52

4.3 The Effect of Bottom Concrete Slab 56

4.4 The Effect of Length of Partial Pre-fabricated Deck Concrete Slab 60

4.5 The Effect of Steel Strength 61

4.5 The Effect of the Top Flange 64

4.6 The Effect of Sequence Construction on Three, Four-Span Bridge 66

4.7 Summary 71

CHAPTER 5 TIME DEPENDENT ANALYSIS OF DOUBLE COMPOSITE SECTION 5.1 Prediction Models of Creep and Shrinkage 73

5.2 Modified Gilbert’ Method, an Accurate Time-Dependent Analysis with Consideration of Interval Time during Up-Down Method 76

5.3 Calculation of Short-term and Long-term Composite Section according to AASHTO LRFD Specification 89

5.4 Comparisons of the Result 90

5.5 Summary 96

CHAPTER 6 ADDITIONAL DESIGN CONSIDERATION

6.1 Stability Consideration 97

6.2 Suggested Construction Details 113

6.3 Optimum Cost of Steel Box-Girder by Varying Plate Thickness 117

6.4 Example Design and Evaluation 135

CHAPTER 7 SUMMARY AND CONCLUSIONS 7.1 Summary 141

7.2 Conclusions 143

7.3 Recommendations for Further Research 146

REFRENCES 148

APPENDIX A TIME DEPENDENT DESIGN OF COMPOSTIE STEEL BOX GIRDER BRIDGE A.1 Properties of Cross Section 155

A.2 Determination of Bending Moment during Construction Sequence 163

A.3 Time Dependent Analysis by Modification of Gilbert's Method 168

A.4 Time Dependent Analysis by AASHTO LRFD Provision 173

APPENDIX B DESIGN EXAMPLE B.1 Calculation Bending Stress for Model 1 176

B.2 Calculation Bending Stress for Model 2 181

ACKNOWLEDGEMENTS 188

LIST OF FIGURES

2.1 Single span 7

2.2 Two continuous spans 8

2.3 Steel box girder in two continuous spans 12

2.4 Detail install bottom slab in steel box girder applied up-down construction method 13

2.5 Double composite bridge Neuötting, (Hanswille, 2001) 17

2.6 Partial-length prefabricated steel box girder component 20

2.7 Construction sequence in two-spans continuous bridge 21

2.8 The normal stress at negative moment region 23

2.9 The normal stress at positive moment region 23

3.1 Example of Model 1 of two-span continuous bridge in ANSYS 30

3.2 Model 1: a) Girder Side view b) Cross section 32

3.3 Model 2: a) Girder Side view b) Cross section 33

3.4 Idealized stress-strain relationships for steel (Bureau et al., 1998) 35

3.5 Uniaxial compressive and tensile stress-strain curve for concrete a) Real curve relationships (Bangash 1989) b) Idealized curve relationships (Hognestad, 1951) 36

Figure Page No

3.6 Residual stress distribution for 800mm flange 39

3.7 Residual stress distribution for 1,800mm height web 39

3.8 Idealized residual stress distribution in top and bottom flange and web plates due to flame cutting and welding (AWS 2001) 41

3.9 Vertical deflection (at service stage) 42

3.10 Longitudinal stress of top flange at stage 3 (half bridge) .42

3.11 Longitudinal stress of top concrete slab at stage 5 (half bridge) 42

3.12 Longitudinal stress of bottom concrete slab at stage 3 43

3.13 Equivalent stress at midpoint of top flange 45

3.14 Equivalent stress at midpoint of bottom flange 45

3.15 Longitudinal stress of top flange at midpoint 46

3.16 Longitudinal stress of bottom flange at midpoint 47

3.17 Longitudinal stress of top concrete slab at midpoint 48

3.18 Longitudinal stress of top concrete slab at midpoint 48

3.19 Transverse stress of top slab across the width 49

3.20 Transverse stress of bottom slab across width at the support 49

3.21 Vertical deflection (at service state) 50

Figure Page No

4.1 Stress of top slab (in the case L=50m) 53

4.2 Initial compression stress of top slab 54

4.3 Upward lifting height for different span length 54

4.4 Relationship of the stress of top slab at service stage with lift-up height in unequal two span bridge of 65-90m length 55

4.5 Compression stress of bottom slab in stage 4 57

4.6 Tensile stress of top flange in stage 3 58

4.7 Compression stress of bottom flange in stage 3 58

4.8 Bending moment diaphragm and location of cast-in-place the top and bottom slab concrete 59

4.9 An example of slope for surface of bottom concrete slab 59

4.10 Parameter study result of prefabricated slab length 61

4.11 Layout of hybrid girder a) Grade 36/50 b) Grade 50/70 62

4.12 Variation of stress of top flange with respect the changing of thickness or width of top flange at stage 3 (in case of L=50m) 65

4.13 Typical changing of thickness and width of top flange near the support region .65

4.14 Bending moment in case 1 of three-span continuous bridge 68

4.15 Bending moment in case 2 of three-span continuous bridge 68

Figure Page No

4.16 Bending moment in case 3 of three-span continuous bridge 68

4.17 Bending moment in case 1 of four-span continuous bridge 70

4.18 Bending moment in case 2 of four-span continuous bridge 70

4.19 Bending moment in case 3 of four-span continuous bridge 70

5.1 Prediction of creep coefficient using different models 75

5.2 Prediction of shrinkage strain using different models 76

5.3 Notification of cross section a) Elevation; b) Composite section 0; c) Composite section 1; d) Composite section 2; d) Short-term strain 77

5.4 Effect of different creep and shrinkage prediction models on double composite section 92

6.1 Bending moment and shear force diaphragm due to lifting-up the interior support 99

6.2 Finite element model M4 101

6.3 Tension field action in web at end panel of exterior supports 102

6.4 Tension field action in web at panel adjacent to interior support 103

6.5 Tripping of top flange 103

6,6 Lateral torsional buckling 104

6.7 Cross section of a three-pan continuous bridge 106

Figure Page No

6.8 Steel box girder with longitudinal web stiffener 106 6.9 Arrangement of longitudinal stiffener in web plate 107 6.10 Transverse stiffener spacing in web plate at interior support 108 6.11 Top-lateral single-diagonal truss system 109 6.12 Torsional moment cause by shear force Rup of one-tenth step 112 6.13 Top lateral bracing of Model 1 112 6.14 Ratio of top flange stress due to lift-up and girder self-weight 113 6.15 Construction details for Double composite section 114 6.16 Detail at support for the case of two load-bearing each side 115 6.17 Detail at support for the case of one load-bearing each side 115 6.18 Detail at support for case of one load-bearing each side .116 6.19 Stress in plate diaphragm at bearing 117 6.20 Structural geometry of steel box girder 121

6.21 A half of four-span continuous beam divided into 16

segments (case L=50m, seg=8) .123 6.22 Variations of equivalent weight for the case kf/km=0 127 6.23 Variations of equivalent weight for different kf/km 127 6.24 Variations of Δf for different segment to L (w=200kN/m) 127

Figure Page No

6.25 Variations of (a) equivalent weight; (b) height; (c) b/h ratio with respect to L for different loading 129

6.26 Variations of (a) height h; (b) b/h with respect to L

(w=200kN/m) 130

6.27 Variation of thickness with respect to different segments

along the length (case: L=50m, w=200kN/m, seg=8) 131

6.28 Variation of thickness of web with respect to L or different segment (w=200kN/m) 132

6.29 Variation of the ℓ/L ratio with respect to different segments along the length (w=200kN/m, seg=8, all cases of L) 132 6.30 Bending moment diagram and resisting moment diagram 133

6.31 Result of longitudinal stress SZ in ANSYS (1) nodal

solution; (2) stress along middle top flange; (3) stress along middle bottom flange 133

6.32 Design of two-span continuous bridge a) Conventional

design T1; b) Design T2; c) Proposed bridge design T3 136

6.33 Design of closed-rectangular brdige by up-down method R4 a) Side view; b) Positive moment area; c) Negative moment area 137

6.34 Plate sizes design of steel box girder for Model 2 a)

Conventional bridge T1; b) Proposed bridge T3 .140 A.1 Notation of double composite cross section 155 A.2 a) Intermediate beam; b) Primary beam subject to external

loading, c) Primary loaded with redundant RB 163

LIST OF TABLES

2.1 Comparison of characteristic in single span 7

2.2 Types of up-down construction methods 10

2.3 Comparison of cross section height and amount of material 14

2.4 Summarized the loading and composite section during the up-down construction stage 27

3.1 Summarized cross section of Model 1 for parameter study 32

3.2 Material properties of steel 36

3.3 Material properties of concrete 37

3.4 Bending stress and total reaction comparisons 44

4.1 Unit cost of steel plate (Lwin, 2002) 62

4.2 Height of steel box girder in difference type of steel grade 63

4.3 Cost of steel material in difference type of steel grade 63

4.4 Cost ratio comparisons for the homogenous grade 50 .63

4.5 Construction sequence on three-span continuous bridge 67

4.6 Construction sequence on four-span continuous bridge 69

5.1 Input variables for time-dependent creep and shrinkage models 74

Table Page No

5.2 Section properties of composite sections 78

5.3 Construction sequence schedule and bending moment 91

5.4 Short term stress and long term stress in sagging moment at stage 5 94

5.5 Short term stress and long term stress in hogging moment at stage 5 95

5.6 Stress in hogging moment at service stage 95

5.7 Stress in sagging moment at service stage 96

6.1 List of ten first eigenvalues of buckling 101

6.2 Stress in the top lateral bracing strut system .112

6.3 Coefficient of moment for uniformly loaded continuous beam over equal spans 122

6.4 Maximum values of height and thickness of web and flanges 125 6.5 Summary of variables and constraints 125

6.6 Results of optimization 133

6.7 Comparison of closed section, open section bridge designs 138

6.8 Comparison of open section bridge designs 138

6.9 Comparison of the bending stress 139

6.10 Comparison of designs of Model 2 140

CHAPTER 1 INTRODUCTION

1.1 Introduction

Steel box girders have a proven high structural efficiency because of their large bending, torsional stiffness as well as rapid erection and therefore are used in a wide variety of structural applications However, they have comparatively big section, noise and vibration These demerits can be reduced by using up-down construction method

Up and down construction method is a new method for bridge construction

in which interior supports are lifted up and lowered down during the construction stage to improve some of the structural characteristics of the bridge system It has been used for prestressed concrete bridges, preflex bridges and steel box-girder bridges (MANSECOREA website, 2007) In this construction method, the separate beams are connected at interior joints with a continuity connection to get a continuous beam which reduces the beam deflection Double composite section is accomplished by pouring concrete into the bottom flange at the negative moment region; it improves the bending stiffness, prevents decay caused by rusting in the steel box girder and reduces the effect of absorbed noise and vibrations After lifting-

up the interior support, the top slab concrete was poured and cured, and then

the interior is lowered down; it causes initial compressive stresses in top concrete slab over the interior piers, so the top slab is considered to endure the negative bending moment

However, the existing up-down method for steel box-girder is uneconomical about material and construction time because of using closed rectangular section and full in-situ casting of concrete in the deck A steel box-girder

with open-trapezoidal cross section and partial-length prefabricated concrete

deck is proposed in this paper In order to evaluate practicability of proposed method, this study estimates analytically the elastic stresses of steel and concrete during construction stages by considering full-scale model of bridge with bracings, stiffeners and prefabricated concrete

1.2 Objectives and Scope

The purpose of this research is to investigate the behavior of steel box girder bridge during applied up-down construction method This study investigates the open-trapezoidal cross section of steel box girder, the partial-length prefabricated deck concrete slab and double composite section at both negative moment area by casting bottom concrete slab A three-dimensional full scale model are selected and developed in detail for the modeling of steel box girder bridge structures The effects of some structural characteristics on the behavior of steel box girder are considered This

research also considers the changing cross section of steel box girder due to remarkable distribution bending moment Finally, design method is also proposed The following tasks are targeted to achieve the above goals:

Task 1: Clarify the proposed up-down method applied for trapezoidal steel

box girder bridge with partial-length prefabricated deck concrete slab Evaluate the advantages of double composite by casting the bottom concrete slab and of applying up-down construction method

Task 2: Analyze the 3D models with focusing on the overall flexural elastic

behavior Verify the accuracy of the FE model

Task 3: Investigate the full nonlinear FEA solutions with some additional

important modeling considerations such as nonlinear material properties of steel and concrete, residual stresses and geometric imperfections

Task 4: Take parametric FEA investigations to capture all the important

physical attributes that may have significant influence on the strength behavior of these types of structures during construction stage such as the lift-up and lower-down height, the thickness and length of bottom slab, the length of partial-length fabricated top

slab, the steel strength and the construction sequence for bridge with more than three spans continuous

Task 5: Study the behavior of short-term and long-term of steel box girder

during construction sequence Modify the time-dependent analysis for double composite section

Task 6: In additional design consideration, carry out the buckling analysis

and discuss the stability design of steel box girder during up-down construction method Present an optimization of the plate thickness

of cross section Do examples to compare the results between the conventional and proposed designs

1.3 Organization

Chapter 2 provides the relevant theory dealing with fundamental of up-down construction method A brief fundamental of existing up-down construction method applied to the bridge structure Then, a review investigation of the benefit of using double composite section and prefabricated deck concrete slab are discussed Finally, a proposal of steel box girder bridge applied up-down construction method with double composite section is also outlined The three-dimension modeling and analysis of composite bridge systems are discussed in Chapter 3 The full scale modeling with nonlinear behavior is investigated and verified

Chapter 4 contains six parametric studies to consider significant influence

on the strength behavior during construction stage

Chapter 5 presents the time-dependent behavior for composite box girder during construction sequence

Chapter 6 presents some issues having related with the design of steel box girder They are discussion of stability design during up-down construction and the optimization of section steel box girder due to remarkable distribution of bending moment

In Chapter 7, a summary of this research as well as key observations are made This chapter closes by identifying future research needs

Appendix A presents detailed design of composite steel box girder The stresses in construction sequence, strength limit state and service state are computed by AASHTO standard Details of short-term and long-term analysis are also provided in this appendix Appendix B provides the EXCEL spreadsheet for the detailed calculation of design example

CHAPTER 2 LITERATURE REVIEW

2.1 Fundamental of Up-Down Construction Method

Up and down construction method applied for bridge superstructure is invented and developed by Professor Koo Min-Se and his students since

1997 This is innovative method of bridge construction in which supports are lifted up and lowered down during the construction stage to improve some of the structural characteristics of the bridge system It has been researched, developed, and applied over 200 bridges for prestressed concrete bridges, preflex bridges and steel box-girder bridges In order to understand the primary idea of up-down method, the comparison of single and continuous span bridge between conventional system and developed system is carried out as follows

Single span

It is cleared that in the developed system the distribution bending moment

is more reasonable than conventional system; especially the deflection is remarkably decrease in developed system as shown in table 2.1 Moreover, there are two expansion joints in the conventional system, while there is only one in the developed system because of one fixed end So the maintenance expansion joint at the abutment is easy

Table 2.1 Comparison of characteristic in single span

Characteristic Conventional system Developed system

Maximum

moment

2

wL8

2

wL8

=Δ

(at x =0.5L)

EI

wL0054

=Δ

(at x =0.4215L)

a) Conventional system b) Developed system

Fig 2.1 Single span

Continuous span

There is similar comparison in the case the continuous span In the conventional system, the separate girders are made continuous through cross beams and casting of deck concrete in the field Restraint moments

develop in the superstructure due to the continuity and time-dependent creep and shrinkage effects In the negative moment over the intermediate piers in continuous system, the crack occurs in concrete deck slab due to weak tensile stresses of concrete, it make low durability and serviceability

In developed continuous system the concrete girders are made continuous through special connection joints The constraint moments are considered

in the analysis and design Greater continuity is obtained over a support which reduces both the displacements and positive moments at the mid-span of a girder Compare with conventional system, the developed system have some advantages such as generate smaller positive moment compared

with simple beam, possible to reduce section height and increase span length, have less expansion joints at piers and abutments to maintain easier

About classification of sequence construction, there are three types of the up-down methods i.e up-down, down-down, and down-up method The type of up-down method chosen is depended on the actual condition of construction in field, such as available space for construction at the abutment, the span length, the weight of beam, the pier height, the position

to place the hydraulic jack and the control device Table 2.2 is the brief description of types of up-down construction methods

Up-down method is used for general condition, while down-up method normally applied for single span And the down-down method is more suitable for some conditions such as enough space for construction at the abutment, extremely heavy beam (if we use up-down method, it requires high pressure jack), greatly height pier (it is dangerous if we apply the upward loading), large span length, long time in construction

2.2 Up-down Construction Method Applied for Steel Box Girder

The up-down construction method is developed and applied for two continuous spans (2@45m) of steel box girder in 2003, (MANCECOREA website, 2007)

Table 2.2 Types of up-down construction methods

(3)- Pour concrete slab and cure it

(4)- Lift up the support inducing compression stress in slab concrete

Up-down

construction

method for

2-span bridge

(1)- Install girders and connect them at the interior support

(2)- Lift-up interior support causing additional compression stress to the bottom of the girder

(3)- Pour concrete slab and cure it

(4)- Lower-down interior support causing compression stress

(1)- Install girders and connect them at the interior support

(2)- Remove outside temporary supports to lower them, causing additional compression stress to bottom of the girder (3)- Pour concrete slab and cure it

(4)- Lower-down interior support causing compression stress

in slab concrete

The procedure of construction is the same with primary idea of up-down construction method; however the concrete is casted in bottom flange as double composite section before the interior support lifted-up as shown in Figs 2.3 and 2.4

a) Existing system

b) Developed system Fig 2.3 Steel box girder in two continuous spans

In the positive moment region, there are the same for both systems, i.e the concrete deck slab is taken as composite with steel girder In the negative moment region, the steel girder and top reinforcing steel of concrete deck slab is considered in existing system, while both top slab and bottom slab are taken as composite with steel girder in developed system

a) Install stud inside steel box b) Hole in top flange to install bottom slab Fig 2.4 Detail install bottom slab in steel box girder applied up-down

construction method

A series design of rectangular steel box girder is presented in MANSCOREA website (2007) There is a 6.8% decrease in cross section height and 14.9% decrease in the amount of steel for the developed system due to using double composite and applied up-down construction method The increasing 5.4% of concrete amount due to additional bottom slab concrete is negligible as summarized in Table 2.3

The up-down construction method used for steel box girder have many advantages like that used for prestressed concrete bridge, preflex bridge such as more reasonable redistribution bending moment, less remarkably the deflection, shallower section or longer span, less expansion joint at piers and abutment Moreover in negative moment region, the double composite section is designed, it improves the stiffness of cross section, creates the section having higher stiffness and more durability, reducing

section height of steel, lowing noise and vibration, obtaining more corrosion

anti-Table 2.3 Comparison of cross section height and amount of material

(MANSECOREA website, 2007)

Existing

system

Developed system

time of casting concrete in field due to in situ casting of concrete along a small length near the support Besides that, some stiffeners at the bottom of the flange can be eliminated because the filled concrete prevents any local buckling of bottom flange The details of proposed method will be presented later in section 2.5

2.3 Double Composite

The basis concept of a double composite design is to use a bottom concrete slab in the negative moment region of the span, replacing relatively costly steel with less costly concrete Top slab concrete in positive moment region and bottom slab concrete in negative moment region are resisted the compression stresses and make double composite section The concept of a double composite bridge is not a new idea, although it has seen limited application worldwide

Double composite concept is studied and applied not only for steel box girder bridge but also for plate girder bridge Saul (1992) cites three examples of double-composite bridges in Germany, completed between

1992 and 1994 The double composite type was selected in there cases for a combination of economy, reduced deflections and aesthetics Martinez (1995) indicates that the first double composite bridge anywhere in the world was Ciervana bridge in Spain in 1978, and cites several others

example of double composite bridges built in Spain in intervening years Stroh et al reports on a unique cable stayed double composite bridge in Hong Kong completed in 1995 Sen et al (1999) studied and developed the double composite action concept and several plate girder and box girder designs were carried out using LRFD Specifications and the results compared against a “conventional” design utilizing composite action for only the positive moment region Nagai (2006) indicates that the ultimate bending strength of double composite girders is expected to reach the plastic moment against positive and negative bending That is, the cross sections of double composite girders can be classified as compact sections along the entire length of the span, thereby making it possible to determine girder sections using a design concept similar to that for steel shapes When using the limit state design method, the cross-sectional areas can be decreased by more than 20% These results clearly show the superiority of double-composite I-girder bridges designed using the limit state design method Hanswille (2001) introduced a Germany biggest double-composite road bridge built close to Neuötting in the course of the new freeway A94 from Munich to Pocking with span length of 154m

The current design approach for steel girder bridges is to use a composite concrete deck The deck acts compositely with the steel girder in the positive moment region of the span, helping to resist the top flange

compressive stresses and reducing the required cross sectional area of the steel top flange of the girder, and consequently result in the cost savings For the existing system (without applied the up-down method), in the negative moment region top flange is in the tension, and following normal practice the tensile capacity of the concrete slab is neglected Consequently, the concrete deck slab has no contribution to the flexural capacity of the girder For the developed system, after applying up-down construction method, the top concrete slab in the negative moment region will be obtained initial compression strain, thus the concrete deck slab can be part

of composite section resisting bending moment

Fig 2.5 Double composite bridge Neuötting, (Hanswille, 2001)

In general and from a cost versus capacity view, concrete is a less costly means of resisting compressive stresses A simple calculation demonstrates

this statement: Say the unit cost of steel is $2.20/kg and the concrete is

$426/m3, and use Grade 345 Steel and concrete strength of 28MPa (Sen et

al 1999) The steel area to carry 1MN force is

1MN ÷ 345MPa = 0.00290m2and the resulting cost is:

0.00290m2 x 7850kg/m3 x $2.20/kg = $50.08/m

Assuming the concrete working at 0.85f’c, the concrete area to carry 1MN force is

1MN ÷ (28MPa x 0.85) = 0.0420m2and the resulting cost is:

• The elimination of some cross frames, afforded by the bracing of the bottom steel flange by the concrete slab

• Favorable redistribution of the moments within the girder due to the increased composite girder stiffened over the interior piers

One of the principal goals of this study is the evaluation of the effect of bottom slab in steel box girder applied up-down construction method and is presented in Section 4.3

2.4 Partial-Length Prefabricated Concrete Decks

Prefabricated steel box composite bridge superstructure elements showed that such a concept has been proposed several times during the past 35 years (Rigoberto and Benjamin, 2008) Often certain components of a bridge are prefabricated partial length and full length and then assembled at the job site In designing a composite bridge girder system, a decision must

be made between in-place decks and prefabricated decks Decks in-place have strong advantages They are simple to erect and easy to connect However, these deck systems may suffer high tensile strains produced by shrinkage, which are restrained by the steel beam A significant advantage of prefabrication is the reduced amount of construction work required at the bridge site, thereby decreasing onsite construction time Various benefits that can result from prefabrication are minimizing traffic disruptions, increasing work-zone safety, reducing

cast-environmental impact, improving constructability and increasing quality and lowering cost

In this study, a prefabricated steel box girder component includes a trapezoidal steel girder topped with a partial-length prefabricated concrete slab before transportation to the bridge site The concrete deck is to be prefabricated with the stud embedded in top flange There is a difference in prefabricated length for interior span or exterior span component, as shown

in Fig 2.6 The length of prefabricated concrete will be investigated in Section 4.4

open-to illustrate the proposal of up-down construction method for continuous trapezoidal steel box-girder, a two-span continuous trapezoidal steel box girder bridge with 50m length of each span is considered There are five stages in up-down method of bridge erection as shown in Fig 2.7

Steel connection

UP

DOWN

Stage 4: Casting deck concrete at top flange and then lower interior support

Stage 3: After pouring and curing bottom concrete slab, lifting up interior support

Stage 2: Carry out connections: connect steel joint

Stage 1: Install auxiliary supports, pre-fabricated components

Prefabricated concrete

Steel box girder

Concrete at bottom flange casted and cured before lifted-up

Concrete at top flangecasted and cured before lowering down

Secondary term load and traffic load

Stage 5 & 6: Add the secondary term dead load and traffic load

Fig 2.7 Construction sequence in two-spans continuous bridge

First, the separate prefabricated components are launched and supported on auxiliary supports as shown in stage 1 Secondly, they are connected with each other by welding Thirdly, the concrete is casted-in-place over

supports at bottom flange as shown in stage 3, and after curing the concrete, the interior supports are lifted upward by 0.4m Fourthly, the concrete is casted-in-place at the top flange as shown in stage 4 and after curing the concrete, interior supports are lowered to initial support level Finally, the secondary dead load and traffic load are applied (service stage) as shown in stage 5 & 6

Flexural Behavior during Construction Sequence

In order to clarify the bending behavior of beam during construction sequence, Figs 2.8 and 2.9 show the normal stress at negative and positive moment respectively At negative moment region, in stage 3 the bottom slab cause lowering of the neutral axis of the cross section, increasing moment of inertia, increases bending moment The stress of bottom flange decreases while that of top flange increases After lift-up, the increasing of the bending moment and shear force is significant, while the inertia moment and neutral axis is not changed The tensile stress of top flange and the compression stress of bottom flange and bottom slab also increase In stage 4 after lowering-down the interior support, the compressive stress is created in top slab and so top slab in negative moment region is considered

to resist loading The neutral axis rises up near to the top flange due to the considerable size of top concrete area and the moment of inertia increases noticeably

Steel box girder

NA in stage 2 Top concrete slab

NA in stage 3

Bottom concrete slab

Stage 5 (Live load) Stage 4 (Down)

Stage 3 (Up) Stage 2

Tension

Compression

NA in stage 4, 5

Fig 2.8 The normal stress at negative moment region

At negative moment region, the composite cross section included prefabricated concrete slab and steel girder is applied for all construction stage, the moment of inertia and neutral axis is not changed during construction stage, but bending moment change during construction stage

In stage 2, loading is self-weight; in stage 3, lifting up in interior support cause additional negative moment for all section, and thus positive moment will decrease; in stage 4, lowering down the interior support, bending moment return to be nearly equal to the value in stage 2; in stage 5 (live load), bending moment increase

M

Stage 5 Stage 3 Stage 2 and 4 Compression

Tension

Prefabricated concrete deck

NA Steel box girder

Fig 2.9 The normal stress at positive moment region