IN
DEGREE PROJECT CIVIL ENGINEERING AND URBAN MANAGEMENT,
SECOND CYCLE, 30 CREDITS STOCKHOLM SWEDEN 2018 ,
Analysis of post-tensioned concrete box-girder bridges
A comparison of Incremental launching and Movable scaffolding system
HAMAD EL HAMAD FURKAN TANHAN
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT
Analysis of post-tensioned concrete box-girder bridges
A comparison of Incremental launching and Movable scaffolding system
Hamad El Hamad Furkan Tanhan
June 2018
TRITA-ABE-MBT-18243
ISBN 978-91-7729-896-0
c
Hamad El Hamad, Furkan Tanhan, 2018 Royal Institute of Technology (KTH)
Department of Civil and Architectural Engineering
Division of Structural Engineering and Bridges
Stockholm, Sweden, 2018
Abstract
When designing a bridge it is of high importance that the geometry for the cross section is optimized for the structure. This is partly due to the influence of the amount of material needed and its impact on the budget and environment. The influence of choosing the right amount of each material lies in the unit-price of the different material, where they can differ significantly.
The Swedish Transport Administration, Trafikverket, has ordered the construction of Stockholm Bypass which is one of Swedens largest infrastructure project and is valued to 27.6 billion SEK according to the price index of the year 2009. The infrastructure project is divided into multiple projects where one of them is assigned to Implenia and Veidekke through a joint venture (Joint venture Hjulsta, JVH) and is valued to nearly 800 MSEK. The reference bridge that is used in the analysis of the master’s thesis is a part of the project.
The aim of this masters thesis was to analyze and compare the two construction methods, mov- able scaffolding system (MSS) and incremental launching for the reference bridge with respect to amount post-tensioning and slenderness. Furthermore, an economical comparison between the two construction methods was carried out based on the obtained results.
The analysis of the MSS was carried out by modeling the reference bridge structure in the finite element software SOFiSTiK AG. The bridge was modeled with different cross section height, i.e.
different slenderness where the optimal amount of post-tension tendons could be determined by iteration until stress conditions from the Eurocode were fulfilled. For the incremental launching method, a numerical analysis was performed. The optimal amount of required post-tensioning was evaluated in the construction stages and final stages with different construction heights i.e. different values of slenderness.
A cost analysis was also performed where the aim was to analyze how the total cost of the construc- tion of the bridge would be influenced by the different slenderness of the bridge as a comparison for the two construction methods. This was done by dividing the costs into fixed costs and variable costs.
The results showed that the structural rigidity had a large influence on the required amount of prestressing steel for both construction methods. In other words, the smaller the cross section the more prestressing steel was required. Incremental launching proved to require a much greater amount of (PT) tendons compared to the MSS although the identical cross sections and properties for both methods, except for the PT. The prestressing for incremental launching is generally by centrical prestressing during the construction stages.
A intersection point was obtained in the cost analysis for the construction methods. The incremental launching was the cheaper solution for slenderness smaller than the intersection point at slenderness between 17 and 18. The MSS was cheaper than the incremental launching for slenderness larger than the intersection point.
Keywords: Movable scaffolding system, MSS, Incremental launching, post-tension, prestress, slen-
derness, box-girder, Stockholm Bypass
Sammanfattning
Vid dimensionering av tv¨ arsektioner i broar ¨ ar det av stor vikt att optimera geometrin avseende ma- terial˚ atg˚ ang d˚ a m¨ angden material har stor p˚ a verkan p˚ a ett projekts budget samt milj¨ o. Eftersom konstruktioner ofta best˚ ar av olika byggnadsmaterial g¨ aller det vid optimering att v¨ alja byggnads- materialen genom optimerad proportionalitet.
F¨ orbifart Stockholm, best¨ allt av Trafikverket, ¨ ar ett av Sveriges st¨ orsta infrastrukturprojekt och v¨ arderas till 27,6 miljarder kronor enligt 2009 ˚ ars prisniv˚ a. Infrastrukturprojektet ¨ ar uppdelat i flera mindre entreprenader eller s˚ a kallade etapper. Den entreprenad som omfattar trafikplats Hjulsta S¨ odra har blivit tilldelat till Implenia och Veidekke genom ett konsortium (Jointventure Hjulsta, JVH) och v¨arderas till cirka 800 miljoner kronor. Den f¨ orsp¨ anda betongbro som byggs i trafikplats Hjulsta ligger till grund f¨ or analysen i detta examensarbete och har anv¨ ants som referens under v˚ ar studie.
Syftet med examensarbete var att analysera och j¨ amf¨ ora tv˚ a de tv˚ a olika produktionsmetoderna, Movable scaffolding system (MSS) och etappvis lansering med h¨ ansyn till erforderlig m¨ angd f¨ orsp¨ anningsk- ablar och slankhet. Vidare, baserat p˚ a erh˚ allna resultat, utf¨ ordes en ekonomisk analys och j¨ amf¨ orelse mellan produktionsmetoderna.
Analysen av MSS utf¨ ordes genom att modellera brokonstruktionen i mjukvaruprogrammet SOFiSTiK AG som bygger p˚ a finita elementmetoder. Konstruktionen modellerades f¨ or olika slankheter, d¨ ar slankheten definieras som kvoten mellan maximala spannl¨ angden och brons tv¨ arsnittsh¨ ojd. Spannl¨ ang- den h¨ olls konstant medan tv¨ arsnittsh¨ ojden varierade f¨ or att erh˚ alla olika slankheter. Den optimala slankheten best¨ amdes genom iterering av m¨ angd f¨ orsp¨ anningskablar tills sp¨ anningsvillkoren var uppfyllda enligt Eurocode.
F¨ or analysen av etappvis lansering utf¨ ordes en numerisk analys vars den optimala m¨ angden f¨ orsp¨ anningsk- ablar utv¨ arderades i byggskedet (construction stages) samt i slutskedet (final stage). Analysen utf¨ ordes p˚ a samma s¨ att f¨ or de olika slankheterna.
Slutligen genomf¨ ordes en konstandsanalys f¨ or de olika metoderna. Syftet var att j¨ amf¨ ora hur den totala kostnaden f¨ or uppf¨ orandet av brokonstruktionen skiljde sig f¨ or de olika slankheterna.
J¨ amf¨ orelsen genomf¨ ordes genom att dela upp de olika kostnaderna i fasta kostnader samt r¨ orliga kostnader.
Resultaten fr˚ an analysen visade att den erforderliga m¨ angd f¨ orsp¨ anningskablar som beh¨ ovs i en f¨ orsp¨ and betongbro ¨ ar beroende av den strukturella styvheten i tv¨ arsektionen. En h¨ ogre slankhet, allts˚ a l¨ agre tv¨ arsnittsh¨ ojd, ger l¨ agre styvhet och d¨ arav mer erforderlig f¨ orsp¨ anningskablar. Etappvis lansering visade sig vara den metod som kr¨ avde mer m¨ angd f¨ orsp¨ anningskablar.
I resultaten f¨ or kostnadsanalysen uppm¨ attes en sk¨ arningspunkt, f¨ or en slankhet mellan 17-18, mellan de tv˚ a olika metoderna. F¨ or f¨ orsp¨ anda betongbroar med slankhet l¨ agre ¨ an sk¨ arningsupunkten vid 17-18 ¨ ar etappvis lansering det billigare alternativet. F¨ or slankheter h¨ ogre ¨ an 17-18 ¨ ar MSS det mer ekonomiskt lnsamma alternativet.
Nyckelord: Movable scaffolding system, MSS, Etappvis lansering, eftersp¨ anning, f¨ orsp¨ anning,
slankhet, box-girder, F¨ orbifart Stockholm
Preface
This masters thesis is carried out by the Department of Civil and Architectural Engineering at KTH Royal Institute of Technology in collaboration with the contractor Implenia Sverige AB.
A special thanks to our supervisor and examiner Head of Division of Structural Engineering and Bridges at KTH Professor Raid Karoumi for his collaboration in constructing the master’s thesis and for inspiring us to the field of bridge design.
We are grateful to Davy Prieur and Gernot Reismann at Implenia Sverige for giving us the op- portunity to write the master’s thesis at a exciting infrastructure project. We are also thankful for being provided with working spaces at the site of construction and the opportunity to be able to visit the site whenever we wanted which resulted in better understanding of our work.
We would also like to express our sincere gratitude towards Peter Borsboom for his continuous sup- port and sharing of his experience and extraordinary expertise in the field throughout the master’s thesis. His guidance has truly been invaluable to us. Without Peter Borsboom, this work would not have been possible.
Furthermore, we want to thank Torsten Reinhard for his time and support with the economical matters, Jos´e Escudero for helping us understand the construction in details and to Nadia Jasim for providing us with necessary documents and drawings.
We are grateful to the whole organization at Joint Venture Hjulsta-S for the great hospitality which made us feel a part of the group during our work.
And lastly but not least, we would like to thank our family and friends for all the support and patience during this master’s thesis and throughout the years at KTH.
Stockholm, 20
thof June 2018 Hamad El Hamad
Furkan Tanhan
Contents
1 Introduction 1
1.1 Background . . . . 1
1.2 Problem description . . . . 2
1.3 Aim . . . . 2
1.4 Delimitations . . . . 2
1.5 Method . . . . 3
2 Literature study 5 2.1 Box girder bridges . . . . 5
2.1.1 General . . . . 5
2.1.2 Geometry . . . . 5
2.1.3 Construction methods of a box-girder bridge . . . . 6
2.2 Movable Scaffolding System - MSS . . . . 6
2.2.1 Components and Kinematics . . . . 7
2.2.2 Risks and safety . . . . 8
2.2.3 Recommendations and conclusion . . . . 9
2.3 Incremental launching . . . . 10
2.3.1 Launching nose and bending moments . . . . 10
2.4 Prestressed concrete . . . . 12
2.4.1 General . . . . 12
2.4.2 Prestressing steel . . . . 12
2.4.3 Prestressing methods . . . . 13
2.4.4 Losses of prestress . . . . 16
2.4.5 Conclusion . . . . 18
3 Project description 19 3.1 E4 Stockholm Bypass . . . . 19
3.2 FSE501 - Building contract Hjulsta S . . . . 21
3.3 Description of the main bridges 547 and 54N . . . . 22
4 Methodology 25 4.1 Modelling procedure . . . . 25
4.2 Model description . . . . 26
4.3 Materials . . . . 29
4.3.1 Concrete . . . . 29
4.3.2 Reinforcement . . . . 29
4.3.3 Prestressing steel . . . . 30
4.4 Loads . . . . 31
4.4.1 Omitted loads . . . . 31
4.4.2 Uneven settlements . . . . 31
4.4.3 Temperature loads . . . . 32
4.4.4 Wind loads . . . . 34
CONTENTS CONTENTS
4.4.5 Traffic loads . . . . 34
4.4.6 Creep and shrinkage . . . . 38
4.4.7 Suspension loads due to the MSS . . . . 38
4.5 Post-tensioning . . . . 41
4.6 Stress conditions . . . . 42
4.7 Movable Scaffolding System - MSS . . . . 43
4.8 Incremental launching . . . . 44
4.8.1 Post-tensioning . . . . 45
4.9 Cost analysis . . . . 46
5 Results 47 6 Conclusions 49 6.1 Recommendations . . . . 51
References 52 Appendices 55 A Cross sections 57 B Incremental launching - Calculations 65 B.1 Material properties - Prestressing steel . . . . 65
B.2 Cross section properties . . . . 66
B.3 Construction stages . . . . 68
B.3.1 Loads . . . . 68
B.3.2 Bending moments . . . . 68
B.3.3 Superpositioning . . . . 69
B.3.4 Prestressing forces . . . . 70
B.4 Final stage . . . . 72
B.4.1 Prestressing forces . . . . 72
B.4.2 Bending moments due to prestressing . . . . 73
B.4.3 Decompression stresses . . . . 74
B.4.4 Superposition . . . . 75
B.4.5 Required amount of post-tensioning . . . . 81
C Cost analysis 83 C.1 Movable scaffolding system - MSS . . . . 83
C.1.1 Fixed costs . . . . 83
C.1.2 Variable costs . . . . 83
C.2 Incremental launching . . . . 85
C.2.1 Fixed costs . . . . 85
C.2.2 Variable costs . . . . 86
C.3 Total costs . . . . 87
D Tendon alignment - SOFiSTiK 89
E Movable Scaffolding System - Stresses 97
x
List of Figures
1.1 Kazarma bridge, Izmir, Turkey (World Records, 2018). . . . 1
2.1 Multi-Cell Box girder cross section(Federal Highway Administration, 2016). . . . 6
2.2 Single-Cell Box girder cross section(Federal Highway Administration, 2016). . . . 6
2.3 Underslung, alongside and overhead scaffolding systems (D¨ abritz, 2011) . . . . 8
2.4 Drawing of the MSS at standard stages. . . . 9
2.5 Simplified construction sequence of incremental launching. . . . 10
2.6 The launching nose reaches the forward pier. . . . 11
2.7 The concrete bridge deck reaches the forward pier. . . . 11
2.8 The moment envelope. . . . 11
2.9 The concept of prestressed concrete (Federal Highway Administration, 2016) . . . . 12
2.10 Typical stress-strain relationship for prestressing steel and ribbed reinforcing steel (Engstr¨ om, 2011) . . . . 13
2.11 Prestressing units: a) prestressing wire, with and without a rivet head, b) prestressing strand (7-wires), ordinary and compacted types, (Engstr¨ om, 2011) . . . . 14
2.12 The profile of the prestressing steel in pre-tensioned concrete elements: a) straight profile, b) harped profile (Engstr¨ om, 2011) . . . . 15
2.13 Relaxation of prestressed steel under constant strain, (Engstr¨ om, 2011) . . . . 18
3.1 Overview of planned route for Stockholm Bypass (Trafikverket, 2017c). . . . . 20
3.2 Overview of contract FSE501 (Overview drawing by Trafikverket). . . . 21
3.3 Overview of FSE501 (Trafikverket, 2017b). . . . 22
3.4 Cross section of bridge 547. . . . 23
3.5 Cross section of bridge 54N. . . . 23
4.1 The bearing configuration for all supports. . . . 27
4.2 The modelled bridge in SOFiSTiK AG. . . . 28
4.3 Correlation between minimum/maximum shade air temperature (T
min/T
max) and minimum/maximum uniform bridge temperature (T
e,min/T
e,max) [EN1991-1-5, figure 6.1]. . . . 33
4.4 Bridge deck: Lane 1, 2, 3. . . . 37
4.5 Right alignment: Lane 10, 11, 12, 13. . . . 37
4.6 Left alignment: Lane 20, 21, 22, 23. . . . 37
4.7 Drawing of the MSS at a standard construction stage. . . . 38
4.8 Detail drawing of the MSS suspension. . . . 39
4.9 Alignment of the curved post-tensioning in the webs for the end spans. . . . 41
4.10 Alignment of the curved post-tensioning in the webs for the standard spans. . . . 41
4.11 Post-tensioning arrangement during the incremental launching. . . . 45
4.12 Post-tensioning arrangement in the final stage. . . . 45
5.1 PT-Slenderness for each cross section. . . . 48
5.2 Cost for each cross section. . . . 48
LIST OF FIGURES LIST OF FIGURES
6.1 PT-Slenderness for each cross section. . . . 50
B.1 Effective flange width parameters [EN1992-1-1, 5.3.2.1, Figure 5.3)]. . . . 67
B.2 Beam case according to (Byggformler och tabeller, 2018). . . . 68
B.3 Case for prestressing moments and restraining moments. . . . 73
E.1 Cross section 1: Uniaxial bottom stresses. . . . 98
E.2 Cross section 1: Decompression stresses in the bottom. . . . 99
E.3 Cross section 1: Uniaxial top stresses. . . 100
E.4 Cross section 1: Decompression stresses in the top. . . 101
E.5 Cross section 2: Uniaxial bottom stresses. . . 102
E.6 Cross section 2: Decompression stresses in the bottom. . . 103
E.7 Cross section 2: Uniaxial top stresses. . . 104
E.8 Cross section 2: Decompression stresses in the top. . . 105
E.9 Cross section 3: Uniaxial bottom stresses. . . 106
E.10 Cross section 3: Decompression stresses in the bottom. . . 107
E.11 Cross section 3: Uniaxial top stresses. . . 108
E.12 Cross section 3: Decompression stresses in the top. . . 109
E.13 Cross section 4: Uniaxial bottom stresses. . . 110
E.14 Cross section 4: Decompression stresses in the bottom. . . 111
E.15 Cross section 4: Uniaxial top stresses. . . 112
E.16 Cross section 4: Decompression stresses in the top. . . 113
E.17 Cross section 5: Uniaxial bottom stresses. . . 114
E.18 Cross section 5: Decompression stresses in the bottom. . . 115
E.19 Cross section 5: Uniaxial top stresses. . . 116
E.20 Cross section 5: Decompression stresses in the top. . . 117
E.21 Cross section 6: Uniaxial bottom stresses. . . 118
E.22 Cross section 6: Decompression stresses in the bottom. . . 119
E.23 Cross section 6: Uniaxial top stresses. . . 120
E.24 Cross section 6: Decompression stresses in the top. . . 121
E.25 Cross section 7: Uniaxial bottom stresses. . . 122
E.26 Cross section 7: Decompression stresses in the bottom. . . 123
E.27 Cross section 7: Uniaxial top stresses. . . 124
E.28 Cross section 7: Decompression stresses in the top. . . 125
E.29 Cross section 8: Uniaxial bottom stresses. . . 126
E.30 Cross section 8: Decompression stresses in the bottom. . . 127
E.31 Cross section 8: Uniaxial top stresses. . . 128
E.32 Cross section 8: Decompression stresses in the top. . . 129
xii
List of Tables
4.1 The height and corresponding slenderness for the cross sections . . . . 27
4.2 Characteristic concrete strength in accordance with [SS-EN1992-1-1, Table 3.1] . . . 29
4.3 Material properties for the post-tensioning strands for internal tendons . . . . 30
4.4 Predicted settlements for the bedding modulus k
s(Implenia, 2018) . . . . 32
4.5 Recommended values for k
surto account for different surfacing thickness [EN1991-1- 5, Table 6.2] . . . . 34
4.6 Wind loads . . . . 35
4.7 Load Model 1: Characteristic values [SS-EN1991-1-2, Table 4.2]. . . . 35
4.8 Adjustment factors α [TRVFS 2011:12, Table 7.1]. . . . 36
4.9 Suspension loads due to the self-weight of the MSS, work loads and the weight of the trestle. . . . 39
4.10 Suspension loads due to the concrete weight of each cross section. . . . 40
4.11 Total suspension loads for each cross section. . . . 40
4.12 Recommended values of w
max(mm) . . . . 42
5.1 Height, slenderness and area for each cross section. . . . 47
B.1 Material properties for the post-tensioning strands for internal tendons . . . . 65
B.2 Geometrical properties for each cross section. . . . 66
B.3 The required prestressing force for each cross section, P
req,cs. . . . 69
B.4 Variation of stress in the tendons due to creep, shrinkage and relaxation, ∆σ
c+s+r(x), for each cross section. . . . 70
B.5 Calculated prestressing forces, required number of strands and required amount of post-tensioning for the construction stages. . . . 72
B.6 Design prestressing forces for each cross section. . . . 72
B.7 Moments due to prestressing for each cross section. . . . 74
B.8 Recommended values of w
max(mm) . . . . 74
B.9 Combination factors. . . . 75
B.10 Calculation of edge stresses for cross section 1. . . . 77
B.11 Calculation of edge stresses for cross section 2. . . . 77
B.12 Calculation of edge stresses for cross section 3. . . . 78
B.13 Calculation of edge stresses for cross section 4. . . . 78
B.14 Calculation of edge stresses for cross section 5. . . . 79
B.15 Calculation of edge stresses for cross section 6. . . . 79
B.16 Calculation of edge stresses for cross section 7. . . . 80
B.17 Calculation of edge stresses for cross section 8. . . . 80
B.18 Required amount of post-tensioning tendons. . . . 81
Chapter 1
Introduction
1.1 Background
Mankind has always been a traveling race with the need of passing over obstacles such as valleys, rough terrain or bodies of water. Bridges as structures were first built with man made materials in the ancient times when first modern civilizations started rising in the Mesopotamia. As of then, knowledge and engineering of bridges started to spread and develop all over the world. The oldest datable bridge in the world still in use is Kazarma Bridge, also known as Arkadiko Bridge. It is a slab-stone single-arch bridge over the river Meles in Izmir, Turkey and dates from 850 BC (Guin- ness, 2018).
Figure 1.1: Kazarma bridge, Izmir, Turkey (World Records, 2018).
Today there are a large variety of bridge types, building materials as well as construction tech- niques. The most common known bridge ma- terials are of concrete, steel or both of them combined i.e. composite bridges, especially for longer bridges or bridges that are meant to carry heavy loads such as road or rail- way bridges. Some major categories of bridge types are suspension bridges, arch bridges, stay- cable bridges, box girder bridges and beam bridges.
Concrete has been in use as a building material since the Roman times. Concrete is a material very strong in compression but very weak in ten-
sion, thus it has been used in structures where it is stressed principally in compression such as in arches, vaults and walls. In the middle of the 19
thcentury, it was discovered that steel and iron bars could be embedded in the concrete in order to improve the materials tensile strength which would mean that concrete now could be used more frequently in structures as beams, slabs, build- ings and bridges. In the 1930s, Eugne Freyssinet invented prestressed concrete. He discovered that the steel bars could be substituted with steel cables that were tensioned by jacks and locked to the concrete (Benaim, 2007). The first prestressed concrete bridge was subsequently built in 1941.
Henceforth the development of prestressed concrete was ongoing rapidly thanks to the pioneers
of the new technology: Guyon, Freyssinet, Leonhardt, Magnel, Morandi, Mrsch and Ross, among
others (Rosignoli, 2002).
Problem description CHAPTER 1. INTRODUCTION
1.2 Problem description
The Swedish transport administration have assigned a nearly 800 MSEK worth of a project, to Implenia and Veidekke through a joint venture (Joint venture Hjulsta, JVH), that is to be a part of the Stockholm Bypass. The analysis will be focused on a post-tensioned concrete box-girder bridge that is to be built in the project.
The contractor Implenia Sverige AB is together with the German engineering consulting company SRP Schneider & Partner interested in determining a relationship between the amount of pre- stressing steel and the slenderness of the bridge, with the bridge that is to be built by JVH as a reference. The slenderness is defined as the longest bridge span divided by the cross section height.
The analysis will focus on the two different construction methods movable scaffolding system and incremental launching. The amount of required prestressing steel along with amount of concrete needed for the cross section can have a significant large impact on the amount of material needed for construction of the bridge and could in a economical point of view be decisive for the chosen construction method.
1.3 Aim
The aim of this master’s thesis is to analyze and compare the two construction methods, movable scaffolding system (MSS) and incremental launching with respect to amount post-tensioning and the slenderness of the bridge. Furthermore, an economical comparison between the two construction methods will be estimated based on the obtained results. The economical comparison will be estimated with focus on the amount of post-tensioning that is required, respectively.
1.4 Delimitations
The analysis will only be carried out on the superstructure of the reference bridge, thus no struc- tural analysis will be performed on the abutments, piers, bearings and foundations since the choice of construction methods does not significantly affect other parts than the bridge superstructure and is not decisive for the analysis of the post-tensioning according to Peter Borsboom. The mentioned structural parts will, however, be included as a consequence of being able to define proper boundary conditions more accurately.
The analyzed bridge is the same as the reference bridge with some simplifications; the cross section of the superstructure has been modeled without a horizontal inclination in order to acquire a sym- metrical cross section. The vertical and horizontal curvature of the bridge has also been neglected and a completely straight bridge is analyzed.
Conventional reinforcement has been excluded in this master’s thesis since the focus is on the longi- tudinal prestressing tendons. Some load cases have also been omitted due to the restricted amount of time of this master’s thesis, more details about the omitted loads are presented in section 4.4.1.
2
Method CHAPTER 1. INTRODUCTION
1.5 Method
The main focus will be to analyze the structural response during the construction stages of the two construction methods; Incremental launching and Movable Scaffolding System. This will be carried out by modelling the bridge structure in the FEM software SOFiSTiK. The construction stages for the MSS will be implemented in SOFiSTiK which will yield the design values of the post-tensioning.
The bridge will be modeled with different slenderness to be able to determine the required amount of prestressing steel for the cross sections, respectively.
For the incremental launching method, a numerical analysis will be performed. The optimal amount of required post-tensioning should be evaluated in the construction stages and final stages with dif- ferent construction heights i.e. different ratios of slenderness.
An economic analysis will also be performed in order to be able to acquire optimal results from
both the structural and economical aspects.
Chapter 2
Literature study
2.1 Box girder bridges
2.1.1 General
The most flexible structural form of a bridge deck is the box girder. The deck form allows a range of spans between 25 m and 150 m and may carry decks that are up to 30 m wide. It is due to its advantages the most favorable deck form for spans longer than 50 m (Benaim, 2007).
2.1.2 Geometry
Box girders are hollow sections and may be rectangular or trapezoidal. The sections may consist of single-cell or multi-cells, see Figure 2.1 and 2.2, which is determined by the amount of interior webs constructed. The importance of a good box lies within the rational balance between the width of the box and the width of the carried deck. Benaim (2007) also states that the designer should use as few webs as possible, which results in minimum redundant concrete near mid-span, hence the generally smaller stresses in shear. Rectangular sections are easier to build compared to the trape- zoidal sections. The disadvantages are though that their bottom slabs may be wider than necessary.
For a box of rectangular section with span/depth ratio deeper than approximately 1/20, gives an area of the bottom slab larger than necessary. The weight of a cross section can thus be reduced by choosing a trapezoidal cross section, hence a lighter construction can be obtained (Benaim, 2007).
The basic components of the cross section are (Federal Highway Administration, 2016).
• Top slab - the entire width of the concrete deck, including the portions between the webs and the overhangs outside of the webs.
• Overhangs (cantilever wings) - the overhanging portion of the top slab.
• Webs - vertical or inclined, exterior or interior.
• Bottom slab.
The geometry of the section provides a high structural efficiency which results in a minimized
required prestressing force to resist bending moments (Benaim, 2007). A closed cell, i.e. a
box-section, has a much greater torsional stiffness and strength compared to an open section,
which benefits when the bridge deck is curved in plane, and can thus counter act high torsional
stresses caused by eccentric loads that causes eccentric loads.
Movable Scaffolding System - MSS CHAPTER 2. LITERATURE STUDY
Figure 2.1: Multi-Cell Box girder cross section(Federal Highway Administration, 2016).
Figure 2.2: Single-Cell Box girder cross section(Federal Highway Administration, 2016).
2.1.3 Construction methods of a box-girder bridge
Concrete box girders may either be cast in situ or prefabricated and assembled. Casting the box in situ is generally a difficult task with regard to the accessibility of the bottom slab. Casting the deck in situ may be done either by casting the cross section in one pour or casting the deck in stages, with the second mentioned being the most common casting method. However, casting the deck section in stages will cause the second phase to crack due to restraint by hardened concrete in the earlier phase. Even though the section is reinforced so that it minimizes the crack width, it is not suitable for a prestressed structure to obtain cracks from permanent loads such as the dead weight (Benaim, 2007).
The properties and strengths of a box girder results in a lighter, due to the hollowness, and stronger construction which is sufficient when constructing bridges with long spans that are intended to carry heavy loads. However, box girders are difficult to maintain regarding the need of access to the delimited space inside the box.
2.2 Movable Scaffolding System - MSS
Movable scaffolding systems (MSSs) have been used since the construction of the Kettiger Hang- brucke in Germany 1959. This construction method is appropriate for span-by-span construction of cast in-situ concrete bridges.
The choice of a constriction method is besides the economical aspects, dependent on factors that affect the construction of the structure, which may be site conditions and environmental factors.
6
Movable Scaffolding System - MSS CHAPTER 2. LITERATURE STUDY
Due to the configuration and flexibility of the MSS, the construction method can be applied in sev- eral situations where the construction site faces topographic restraints or infrastructure that might interfere with the bridge that is to be built. Some factors that may influence on bridge construction and where the MSS is considered to be suitable are listed below
• Deep valley canyons
• Difficult access on the ground
• Crossing waters
• Inappropriate ground conditions
• Within existing infrastructure
• More than eight spans
• Variable curvature radius and the cross sectional height
• Span length 30 to 80 m without intermediate supports
• Low clearance over or under the bridge
• Different bridge bearing systems
Only one of the above-mentioned conditions needs to be fulfilled to justify the application of the movable scaffolding system (D¨ abritz, 2011)
2.2.1 Components and Kinematics
According to M. D¨ abritz, 2011, the movable scaffolding system consists of five fundamental com- ponents that are chosen according to requirements developed from bridge design, environmental and economic restraints. MSSs generally consists of a formwork, which the superstructure is cast in, a formwork supporting structure, main-girders, moving devices and supports. The formwork is constructed so that it can be moved in all required directions which minimizes lifting equipment and labor. The formwork can either be of steel or wood. Formwork out of steel is most common for sections with constant geometry and low variations. Wooden formwork, on the other hand, is more flexible and adjustable and appropriate when the geometry of the cross section varies.
There are several different types of MSSs which can be used for different preferences and can be separated by the placement of the main-girders in relation to the superstructure.
The most common type of MSS is the underslung system where the main-girders are placed be-
neath the cross section. Both underslung and along-side system allows access to material supply
from above, e.g. by a tower crane. With the underslung system, bridges with various cross sec-
tions can be constructed without interfering with the construction parts. Overhead systems, on the
other hand, are constructed in the way that the formwork supporting structure is located above
the bridge and is hence suitable for bridges with a limited area under the spans. With the MSS
positioned above, material feeding is no longer possible with tower cranes. The choice of suitable
scaffolding system for the specific bridge construction is mainly based on the requirements of bridge
cross section, curvature and other site conditions. The three different types of scaffolding systems
are presented in the figure 2.3 below.
Movable Scaffolding System - MSS CHAPTER 2. LITERATURE STUDY
Figure 2.3: Underslung, alongside and overhead scaffolding systems (D¨ abritz, 2011)
M. D¨ abritz explains further that for prestressed continuous beam bridges a bridge span and a cantilever is generally produced in a single construction stage. Once the cross section is fabricated in the formwork and subsequently prestressed, the formwork is lowered down, opened and then moved transversally under the bridge and subsequently launched forward to the next construction stage where the formwork is closed and prepared for the fabrication of the next bridge span and cantilever. The dead load is partly introduced in conjunction with the construction stages for each span and cantilever of the bridge. Once the fabrication of the end span is accomplished and the MSS is lowered, the dead load for the entire bridge is introduced.
Launching noses in front and back of the MSS are installed on the main-girders to prevent the MSS from overturning before reaching the next pier during the construction stages. The total elongation of the MSS is over the double of the maximum span length of the bridge. The main-girders can either be of truss-girder type or box-girder type. The box-girder benefits with torsional rigidity and more flexibility in positioning the load introduction points. In comparison with the truss girders, they are significantly larger and heavier. This gives rise to larger wind loads acting on the structure.
The underslung and alongside system requires two main-girders while the overhead system only needs one to operate. However, the main-girders share the same purpose and functionality in the different systems. When designing the main-girders the two main load cases that are taken into consideration is the load cases when casting the superstructure and the load case when launching the MSS.
For the MSS to be able to be launched through the construction stages, movements in all three directions are required. Movement in the transversal direction is needed for the formwork to pass the piers. The movement is possible with moving devices installed between the main-girders and pier supports. The vertical and transversal movement of the MSS and formwork is done with hydraulic jacks and double-acting hydraulic cylinders respectively. Electric engines can also be used for the transversal movements instead of hydraulic cylinders.
2.2.2 Risks and safety
The movable scaffolding system provides a higher workers safety due to the pre-assembling of the formwork and equipment in the earlier stage prior to fabrication of the superstructure, hence fewer
8
Movable Scaffolding System - MSS CHAPTER 2. LITERATURE STUDY
Figure 2.4: Drawing of the MSS at standard stages.
movements of the equipment during the construction stages. The construction method requires experienced staff that can with the designer of the MSS ensure the quality and safety of the MSS construction when changes are made during the project.
When designing the MSS there are three main critical phases that need to be considered: launching stage, relocation work of pier supports and work under incomplete MSS during assembling and dis- mantling. The risks are evaluated and estimated by a risk analysis for the assembling/dismantling operations and the usage of the MSS separately.
2.2.3 Recommendations and conclusion
M. D¨ abritz (2011) concludes in the article that the construction method offers high efficiency in
the matter of construction time and economical aspects by applying the method on bridges that
have cross sections with low variations and equal span lengths. Optimization and flexibility during
operation can also be obtained by maintaining good and stable communication between MSS de-
signer and formwork supplier. Advantages of the MSS such as the possibility to construct bridges
with various cross sections and at the meantime lowering production costs, the construction method
competes with pre-fabricated bridges.
Incremental launching CHAPTER 2. LITERATURE STUDY
2.3 Incremental launching
The bridge superstructure is casted in 15 to 30 m long segments in a stationary formwork placed behind an abutment. The hardened segments is later pushed or pulled forward with hydraulic jacks or friction launching systems along the bridge axis. Adjacent segments are casted and then stressed together successively as previous segments is being completed and launched (BBR, 2018).
Figure 2.5: Simplified construction sequence of incremental launching.
2.3.1 Launching nose and bending moments
R. Benaim (2007) indicates that bending moments increases due to the self-weight in the bridge superstructure as the bridge successively moves forward, cantilevering between the piers.
The self-weight bending moments for internal spans of a long bridge deck with equal spans are
−0.0833 pl
2at each pier and +0.0417 pl
2at each mid-span. When a deck is launched, every segment transcends from being a support section into a mid-span section. This implies however that if a segment had to cantilever from one pier to the next, the bending moments due to the self-weight would be as high as −0.5 pl
2, which corresponds to a six time higher bending moment than over the piers.
In order to retain the bending moments low, a launching nose is attached to the first segment through monolithic or stressed connections as presented in Figure 2.7. The purpose of the launching nose is consequently to shorten the cantilevering span of the bridge deck during launching and hence reduce the bending moments due to a smaller lever arm. Both R. Benaim
10
Incremental launching CHAPTER 2. LITERATURE STUDY
(2007) and M. Rosignoli (2002) states that the optimal length of the launching nose is approximately 65 percent of the span length.
Due to the flexibility of the launching nose, the bending moment in the deck successively increases over the pier as the deck is launched forwards. The bending moment will peak at its maximum value of approximately 1.3 times the typical value; which corresponds approximately to
−0.105 pl
2. As the concrete reaches the forward pier, as presented in Figure 2.7, the maximum sagging moment in the previous span will be approximately +0.07 pl
2(Benaim, 2007).
Figure 2.6: The launching nose reaches the forward pier.
Figure 2.7: The concrete bridge deck reaches the forward pier.
The final launching self-weight bending moment envelope for a deck of equal spans consists of a constant hogging moment of 0.0833 pl
2and a constant sagging moment of +0.0417 pl
2. At the front of the deck, an increase in both hogging and sagging moments is apparent as well as a decrease of the moments at the beginning of the deck as presented in Figure 2.8.
Figure 2.8: The moment envelope.
Prestressed concrete CHAPTER 2. LITERATURE STUDY
2.4 Prestressed concrete
2.4.1 General
It is well known that concrete has a low tensile strength compared to its compressive strength.
In plain, unreinforced concrete structures cracks appear normally at already low load levels. As a result of the concretes brittle behavior and weak tensile properties the cracks that appear may lead to total failure of the unreinforced structure. In reinforced concrete structures the reinforcing steel bars transfer the tensile forces across the cracks (Engstr¨ om, 2011).
A concrete structure is prestressed when a compressive force is applied to the structure in order to counteract tensile stresses obtained from other external loadings (Federal Highway Adminstration, 2016). Figure 2.9 illustrates the concept of prestressing for a simple span beam.
Figure 2.9: The concept of prestressed concrete (Federal Highway Administration, 2016)
2.4.2 Prestressing steel
The prestressing steel is made of high strength, cold-worked steel. In Figure 2.10 the stress-strain relationship for prestressing steel and ordinary ribbed reinforcing steel are compared. Unlike the reinforcing steel, the prestressing steel has no marked yield-stress. Further treatment in the pro- duction stage, for instance ’stress relieving’ or ’tempering’ may influence the material properties and can thus be improved.(Engstr¨ om, 2011)
”Strength” is usually defined as the stress where the stress-strain relationship is non-linear. Since cold-worked steel has no marked yield stress the strength can be defined by the so-called ’proof- loading’, i.e. the stress that corresponds to a strain of 0.2 % after un-loading with characteristic value f
0,2k. A new proposal to a European Standard states that the strength of prestressed steel should be defined from the 0,1 % proof-stress (Engstr¨ om, 2011)
12
Prestressed concrete CHAPTER 2. LITERATURE STUDY
p[MPa]
1800 1600 1400 1200 1000 800 600 400 200 0
0 2 4 6 8 10 12 14 16
p[10
-3 Prestressing steel
B500 ribbed reinforcing steel
Figure 3.7 Typical stress-strain relationships for prestressing steel and ribbed reinforcing steel
In case of hot-rolled reinforcing steel stresses beyond the yield stress are normally not considered in the design, because of the large deformations that would result.
However, a considerable reserve is available due to strain-hardening and it could be favourable to consider this strength and deformation reserve in design with regard to accidental loading and progressive collapse. However, for cold-worked steel it could be justified to utilise stresses above the 0,2% or 0,1% proof-stress also in ordinary design in the ultimate limit state. Hence, also the tensile strength of the steel (at rupture) is an important parameter and for prestressing steel also the characteristic tensile strength f
pukis specified.
The deformation properties are characterised by the modulus of elasticity E
pand the ultimate strain
pu. The definitions of the strength properties described above are pre- sented in Figure 3.8.
Prestressing strands have a more complex deformation behaviour, because several wires interact and are deformed and mutually displaced, when the strand is loaded. Of this reason, the modulus of elasticity, evaluated from tensile tests, is somewhat smaller for strands compared to other types of prestressing steel. It is stated in Eurocode 2 that the modulus of elasticity for prestressing wires and bars may vary between 195 and 205 GPa, while for prestressing strands it varies between 175 and 195 GPa.
Properties of various types of prestressing steel, according to the European Standard [6], are presented in Table 3.6.
Figure 2.10: Typical stress-strain relationship for prestressing steel and ribbed reinforcing steel (Engstr¨ om, 2011)
A prestressed tendon generally consists of three main prestressing steel types: prestressing wires, prestressing strands and prestressing tendons. The smallest prestressing units are the prestressing wires and are fabricated in diameters up to 10 mm. Prestressing strands is obtained by twisting several wires together. Strands with 2, 3, 7 and 19 wires are available. The most common type is the 7-wire strand and consists of 6 wires twisted around one straight, centric wire. The 7-wire strand exists in diameters between 7 and 18 mm, dependant on the manufacturer. The twist of the wires results in a good bond to the surrounding concrete. Engstr¨ om (2011) explains further that the strands may be delivered as epoxy-coated and impregnated for application where the risk for corrosion is high.
The largest prestressing unit is the so-called prestressing tendon. The prestressing tendon consists of several strands where the size of the tendon is dependant on the number of strands and may consist of up to 55 strands of 120 wires. Depending on the manufacturer the tendons exist in dif- ferent prestressing systems, where the systems are mainly distinguished in anchorage and coupling devices between the tendons.
2.4.3 Prestressing methods
2.4.3.1 The difference between pre-tensioning and post-tensioning
R. Benaim (2007) states that the term prestress means preparing a structure to receive a load by applying a load in such way that counteracts the external load. When a subject is prestressed the concrete structure is subjected to compressive stresses during the production in advance of the service loading. The prestressing methods can be divided into two main methods: pre-tensioning
13
Prestressed concrete CHAPTER 2. LITERATURE STUDY
13
2 PRESTRESSED CONCRETE STRUCTURES 2.1 Prestressing steel
It is necessary to provide a very high prestress in the prestressing steel. Otherwise the prestressing effect will not remain after long time, see example in Section 1.1.3. The concrete strain will change during time with about 0,610
-3- 1,510
-3due to creep and shrinkage. The actual value depends on the environmental conditions. With an initial prestrain, i.e. strain difference between steel and surrounding concrete, of about 510
-3- 710
-3,
the reduction due to creep and shrinkage will be about 15 - 20% which is acceptable.
In order to achieve the required prestrain, the prestressing steel is made of high strength, cold-worked steel. Three main types of prestressing units exist, prestressing wires, prestressing strands, and prestressing bars, see Figure 2.1. These types of units are specified in the European Standard, EN 10138 [6].
a)
b) c) d)
Figure 2.1 Prestressing units
a) prestressing wire, without and with a rivet head
b) prestressing strand (7-wires), ordinary and compacted types c) plain prestressing bar with a threaded end
d) ribbed prestressing bar
Figure 2.11: Prestressing units: a) prestressing wire, with and without a rivet head, b) prestressing strand (7-wires), ordinary and compacted types, (Engstr¨ om, 2011)
and post-tensioning. In pre-tension the prestressing steel cables are tensioned before the concrete is cast. In post-tensioning, the cables are installed and tensioned after the hardening of the concrete structure.
2.4.3.2 Pre-tensioning
Pre-tensioning is normally used in the production phase of the precast concrete components for relatively short span bridge decks with standard bridge beams (Benaim, 2007). The concrete com- ponents are cast in precasting plants. The bottom is provided with long tensioning beds with abutments at the ends. The prestressing steel is tensioned in the bed through a form before the concrete is to be cast in the same form. The prestressing steel is prestressed in a straight profile through the form. Special devices in the tensioning bed can be installed to hold down the steel during tensioning to obtain a ’harped’ shaped profile of the prestressed steel, see Figure 2.12. The purpose of the ’harped’ profile is to decrease the eccentricity of the prestressing force near the ends and subsequently decrease or prevent tensile stresses and the corresponding risk of flexural cracks in the top of the element after the prestressing force is released and the prestressing is applied to the concrete member.
The pre-tensioning can be applied to the concrete structure when the concrete has reached a strength of approximately 70% of the 28 days-strength. The prestressing is applied by ’detensioning’ and can be achieved through ’fast release’ i.e. cutting the prestressing units at the ends of the concrete ele- ment. An alternative method is a so-called controlled movement which is applied under controlled conditions and is less severe for the ends of the concrete element (Engstr¨ om, 2011).
14
Prestressed concrete CHAPTER 2. LITERATURE STUDY
16
made from steel composed by units in a flexible system that allows for a range of dimensions of the precast concrete elements, see Figure 2.4.
Figure 2.4 Adjustable mould for production of precast prestressed beams with I-section, according to Scott [13]
Normally the prestressing steel is tensioned in a straight profile through the moulds.
However, by special devices in the tensioning bed, it is possible to hold down the steel during tensioning, or immediately after tensioning, in one or more points between the abutments, see Figure 2.5. Hence, an more refined tendon profile can be obtained. The aim of such ‘harped’ profiles is to decrease the eccentricity of the prestressing force near the ends of the concrete elements and thereby decrease of even prevent tensile stresses and the corresponding risk of flexural cracks in the top of the element at detensioning (when the prestressing is applied to the concrete member).
Figure 2.5 The profile of the prestressing steel in pretensioned concrete elements, a) straight profile, b) harped profile
When the concrete has reached a strength of about 70% of the 28 days-strength, the prestressing can be applied. This is achieved by ‘detensioning’ of the prestressing steel, either by cutting the prestressing units at the ends of the concrete element, so called fast release, or by a controlled movement of the tensioning equipment. In the latter case a slow release can be obtained, which is less severe for the ends of the concrete element.
a)
b)
Figure 2.12: The profile of the prestressing steel in pre-tensioned concrete elements: a) straight profile, b) harped profile (Engstr¨ om, 2011)
The bond to the surrounding concrete prevents the prestressing steel to return to its original un- stressed length. The restraining of the prestressing steel creates an internal restraint in the concrete element, hence an equilibrium is obtained between the tensile force in the steel and a compressive force in the concrete.
2.4.3.3 Post-tensioning
By post-tensioning a structure the steel is tensioned after that the concrete is cast and hardened.
The prestressing tendons are inserted into the concrete element that is provided with ducts. The tendons are either pushed or pulled through the ducts before or after the concrete is cast. Once the tendons are tensioned the ducts are injected with grout. The injected grout enables steel to concrete interaction and protects the steel from corrosion. Grouting made appropriately offers full interaction between the tendons and the concrete (Engstr¨ om, 2011).
The concrete sections can be delivered to the site complete with tendons already placed in the ducts. The concrete and the steel is then placed in the mould where the steel is later tensioned.
Due to the risk of corrosion of unprotected tendons in non-grouted ducts, it is preferred to insert the prestressing steel into the ducts as late as possible before grouting.
The tensioning of the steel is made when a sufficient strength of the concrete is obtained, nor- mally about 70% of the 28 days-strength. The tensioning of the tendons is done by a hydraulic jack placed on one end against the concrete structure while the other end of the tendon is anchored, fixed to the concrete structure. The tensioning is finished once the tensile force and elongation of the tendons have reached the desired or expected values. The tendons are then locked with anchors and the ducts are filled with grout.
Compared to pre-tension the alignment of the tendons can be chosen more freely and thus adapted closely to the theoretical and actual design, normally parabolic and varies as the bending moment for self-weight.
The curved form of the tendons results in frictional forces obtained between the tendons and the
duct when tensioned and is referred as ’frictional losses’, i.e. the tendon force decreases when the
distance to the hydraulic jack is increased. A more uniformly distributed tensile force in the tendons
can be obtained by tensioning from both ends of the concrete structure. In continuous element such
as bridges, the tendons can be connected by special couplers placed at construction joints. The
Prestressed concrete CHAPTER 2. LITERATURE STUDY
next post tensioned concrete element can thus be cast and its tendons connected by the coupler of the previous cast segment.
In bridge construction ’external prestressing’ is possible. In external prestressing the tendons are placed in the hollow section of the concrete cross section without bond between the concrete and the prestressing steel. The tendons are due to their placement visible and accessible which facili- tates maintenance of the tendons during the lifetime of the bridge. The concrete sections can be fabricated independently of the tendons.
2.4.4 Losses of prestress
2.4.4.1 Friction
As mentioned in the previous section, Benaim (2007) explains that friction due to the contact be- tween the tendons and the ducts results in a decrease of the prestressing force and is referred as frictional losses. The loss of prestress due to friction is mainly dependant on the distance from the force applied by the jack and a coefficient of friction. The friction coefficient is determined by the shape of the ducts and tendons. The coefficient of friction can be determined as
• 0.1 for clean tendons in smooth plastic ducts
• 0.2 for clean tendons in steel ducts
• 0.4 for tendons bearing on concrete or rusty tendons in steel ducts
Further, the force at any point in the tendon can be determined as
P
x= P
0e
−µΣα(2.1)
where
P
xis the force at point x
P
0is the force applied by the jack at the anchorage µ is the friction coefficient
Σα is the cumulated angle turned through at point x, in radians
2.4.4.2 Shrinkage
Shrinkage is the phenomenon of when the volume decreases due to evaporation of water during drying of the concrete. Unlike creep deformation, shrinkage is independent of external loads, so called load-independent deformation (Ansell A. et al, 2014)
The prestressing effect can be utilized by fully understanding the factors that effect the prestressing
16
Prestressed concrete CHAPTER 2. LITERATURE STUDY
force. Since shrinkage is a long time effect one must apply the stress at the right time. Normally cast-in-situ concrete is stressed after between 2 and 7 days after casting, while precast concrete structures are more likely to be stressed several weeks after it is cast. The loss of prestress due to shrinkage can be well estimated by calculating the amount of shrinkage remaining after stressing (Benaim, 2007).
2.4.4.3 Creep
’Creep’ is a the result of when a material is under load for a long time and a certain increase in strain without any corresponding increase in the stress. Creep is a time-dependant deformation. After removal of loading, there is an instant recovery of the material, i.e. the initial recovery. The initial recovery is fallowed by an slower recovery, known as the creep recovery. Due to slower recovery, the material never recovers fully back to its initial state and a certain residual deformation remains, (Ansell A. et al, 2014)
2.4.4.4 Relaxation of steel
Relaxation is defined as a decrease in stress under constant strain (Brush Wellman Inc., 2009). In the case of a prestressed concrete structure relaxation results in a decrease of the prestressing effect with time. (Engstr¨ om, 2011). Relaxation is normally not significant in ordinary reinforced concrete structure. Since the purpose of the prestressing steel is to maintain a stress under a long time consideration of relaxation is of high importance.
The decrease of stress in tendons due to relaxation can be calculated as
σ
p= σ
pi− ∆σ
pr(t) (2.2)
where
∆σ
pr(t) = relaxation loss at time t (2.3) and
χ
t= ∆σ
prσ
pi(2.4)
χ
t= Relaxation factor at time t (2.5)
The initial stress, σ
pi, should for pre-tensioned tendons be determined as the stress in the tensioning
bed after immediate losses. For post-tensioned tendons, the initial stress, σ
pi, should be determined as the stress immediately after tensioning The degree of relaxation depends on the quality of the prestressing steel. As mentioned in the
previous section, the properties of the prestressing steel can be improved for instance by temper-
ing or other mechanical treatment. Information about specific prestressing steel can normally be
Prestressed concrete CHAPTER 2. LITERATURE STUDY
Figure 2.13: Relaxation of prestressed steel under constant strain, (Engstr¨ om, 2011)
specified by the supplier. Recommendations and estimation of the behavior of the relaxation are otherwise given in the Eurocode. The relaxation can be estimated by selecting an accurate class of prestressing steel and a relaxation factor, χ
n, where n is the load duration in hours.
2.4.5 Conclusion
Benaim (2007) concludes that the designing of a prestressed concrete structure requires a higher degree of skill than a normal conventional reinforced concrete structure. The designer needs fully understanding about the effect of the prestressing and the factors that may somehow counteract or affect the intended prestressing influence on the structure in form of losses of prestress and also the internal and external forces in the structure due to prestressing.
When designing a conventional reinforced concrete one could by over-reinforcing overlook his lack of understanding to optimize and still manage to design a working structure. The thought of ’over- prestressing’ to increase the safety of the structure may instead result in a decrease of the safety of the structure, e.g. increase compressive stresses in the top of the concrete structure due to the greater force in the tendons which increases the risk of failure.
18
Chapter 3
Project description
3.1 E4 Stockholm Bypass
The Swedish Transport Administration, Trafikverket, has ordered the construction of Stockholm Bypass which is one of Sweden’s largest infrastructure project of all time and is valued to 27.6 billion SEK according to the price index of the year 2009. Stockholm Bypass will connect the northern and southern parts of Stockholm since the only way to travel across the city of Stockholm today, is through Essingeleden which is heavily loaded by traffic. The purpose is thus to decrease the vulnerability in the traffic system of Stockholm as well as to increase the benefits regarding national economical aspects.
The Bypass will have a total length of 21 km consisting of approximately 18 km tunnels and will
be located west of Stockholm with its start and end in H¨ aggvik north of Stockholm and Kungens
Kurva south of Stockholm. The Bypass will ascend above ground in six different locations with
traffic interchanges that connects the highway to adjacent roads. The project was started in the
planning stage in 2006 and the first construction stages were started in 2015. Stockholm Bypass is
estimated to be opened for traffic in 2026 (Trafikverket, 2017c).
E4 Stockholm Bypass CHAPTER 3. PROJECT DESCRIPTION
Figure 3.1: Overview of planned route for Stockholm Bypass (Trafikverket, 2017c).
20
FSE501 - Building contract Hjulsta S CHAPTER 3. PROJECT DESCRIPTION
3.2 FSE501 - Building contract Hjulsta S
Hjulsta S¨ odra is one of the six interchanges above ground along Stockholm Bypass. It is located north of Stockholm and will be the first place in Sweden where two European highways intersect, E4 and E18. There will be in a total of three levels of road traffic in this intersection. At the bottom level lies E18, above E18 there is a roundabout connecting E18 to public roads and at the upper level, the E4 Stockholm Bypass bridges will cross the formerly mentioned roads. Implenia and Veidekke are the contractors through a joint venture, so called Joint Venture Hjulsta-S (JVH). The contract consists of a design and construction phase for constructions listed below. The design and construction has to be in accordance with regulations and requests by the transport administration in an object technical description compiled by Trafikverket.
• Rock tunnels with a length of approximately 20.0 m connecting to the tunnels of the adjacent contract.
• Concrete tunnels (two tubes) with a total length of 300 m.
• A total of eight bridge constructions:
– Two main bridges with a total length of 629.7 m and 627.7 m respectively.
– Two ramp bridges connecting to surrounding roads with a total length of 259.7 m and 214.7 m.
– Three plate bridges connecting to surrounding roads with a total length of 69.6 m, 70.0 m and 71.0 m.
– One smaller bridge construction which is separated since it crosses over the subway tun- nels.
The constructions are, according to the contract, estimated to be finished in 2022-02-22.
Figure 3.2: Overview of contract FSE501 (Overview drawing by Trafikverket).
Description of the main bridges 547 and 54N CHAPTER 3. PROJECT DESCRIPTION
Figure 3.3: Overview of FSE501 (Trafikverket, 2017b).
3.3 Description of the main bridges 547 and 54N
The main bridges are post-tensioned concrete box girder bridges. Bridge 547 has a total length of 627.7 meter while 54N is a bit longer due to the curvatures, measuring 629.8 m. There are a total of 13 axes which implies that the bridge is divided into 12 sections with a maximum single span of 58.0 m. The bridge has a maximum height of 18.0 m above ground but varies depending on the terrain. The width of the bridge decks are 13.5 m, with additional edge beams with a width of 1.25 m on each side and one edge beam for bridge 54N of 0.4 m, the total width goes up to a total of 16.0 m and 15.2 m respectively.
22
Description of the main bridges 547 and 54N CHAPTER 3. PROJECT DESCRIPTION
6500
16065
574 4176
526 4289
13500
1250 1250
6750 6750
550550 520 520 130
700 700
450450 105° 108°
2883 6602105108 200
CENTERLINE
350 6003162295316 3211
2.5%
Figure 3.4: Cross section of bridge 547.
13500
400 1250
6750 6750
6500 15216
526 4290
574 3326
660
105°
107° 108
2106108 2874
3169 7072295166 550550 520 520 130350
700
450450 200
CENTERLINE 2.5%
