Repair and rehabilitation of a cable stayed bridge

Table of Contents

Abstract

Acknowledgement

Table of Contents

Table of Tables

List of Figures

Chapter-1 INTRODUCTION
1.1 General Remarks
1.1.1 History of cable stayed bridge
1.1.2 First cable stayed bridge
1.1.3 First cable stayed bridge in U.S.A
1.1.4 Comparison with suspension bridge
1.1.5 Key advantages of cable stayed bridge
1.1.6 Types of cable stayed bridge
1.1.7 Parts of cable stayed bridge
1.1.7.1 Deck
1.1.7.2 Box girder
1.1.7.3 Prestressing concrete
1.1.7.4 Foundation
1.1.7.5 Pylon
1.1.7.6 Stay cables
1.1.8 Designing
1.1.9 Construction
1.1.10 Test on stay cables
1.1.11 Cable stayed bridge across Chambal river
1.1.12 General details of the project
LITRATURE RIVEW
1.2 General remarks
1.2.1 Review of work done in this field
1.2.3 Objective of the study

Chapter – 2 MODELING OF CABLE STAYED BRIDGE
2.1 General remark
2.2 Static modeling
2.2.1 Governing equation for deflected shape
2.2.2 Cables under its own weight
2.2.3 Cable subjected to UDL

Chapter -3 CABLE STAYED BRIDGE ACROSS CHAMBAL RIVER: CASE STUDY
3.1 Silent features of the bridge
3.1.1 General arrangement
3.1.2 Deck section
3.1.3 Pylon
3.1.4 Piers
3.1.5 Foundation
3.2 Main span erection
3.3 Bearing
3.4 Mode of construction
3.5 Comparison with general mode of construction
3.6 Comparison with span-by –span mode of construction
3.7 Bearing support during failure
3.8 Detail of collapse
3.9 Causes of collapse
3.10 View of various agencies about mechanism of collapse
3.11 View of committee

Chapter -4 REPAIR AND REHABILITAION
4.1 General Remarks
4.2 Design requirements
4.3 Factor of strength (limit state)
4.4 Cable condition rating
4.5 Rehabilitation strategy and cost rating
4.6 Replacement Cable Design
4.7 Super structure modification
4. 8 Corrosion protection
4.9 Wind load consideration
4.10 Temporary cable design
4.11 Security and antivandalism
4.12 Major construction faults in Chambal bridge

CONCLUSION

REFERENCEES

ABSTRACT

Cable stayed bridge has become one of the most frequently used bridge system throughout the world because of their aesthetic appeal, structural efficiency, enhanced stiffness compared with suspension bridge, ease of construction and small size of substructure. Over past 40 years, rapid developments have been made on modern cable stayed bridge. With main span length increasing , more shallow and slender stiffness girders used in modern cable stayed bridge, the safety of whole bridge under service loading and environmental dynamic loading such as impact , wind and earthquake loadings , presents increasingly important concern in design , construction and service

In India the first cable stayed bridge was AKKAR BRIDGE, SIKKIM (1985) Constructed by Gammon India limited. The other cable stayed bridge are Vidhya sagar Setu (1992) Kolkata, Bandra – worli sea link (Mumbai), Cable stayed bridge across Chambal river (Kota) etc.

In the present study, the failure of cable stayed bridge across Chambal River (Kota) will be discussed. The causes of its collapse and detail study of the cable stayed bridge cross Chambal River will be done. The static and dynamic modeling of cable stayed bridge is also done. At the end, the measure to repair and rehabilitation cable stayed is discussed.

ACKNOWLEDGEMENT

It is a matter of pleasure for me to express my deep feelings of gratitude and sincere thanks to my guide Dr.M.K.Shrimali (Head, Department of Structural Engineering) as his inspiring guidance, ever enthusiasm and parental care have been invaluable assets throughout my dissertation work. His constant encouragement and valuable suggestion for my improvement have been of great help in preparing this report.

Iam also especially thankful to Dr. S.D. Bharti (Associated Professor, Department of Structural Engineering) for providing me constant inspiration, enthusiasm, cooperation and valuable suggestion for my dissertation work.

Iam thankful to Mr. Anoop Kulshreshta , Project Director(National Highway Authority of India) and Gammon India Limited for providing me valuable datas.

Iam also thankful of all the faculty members of my Department for their invaluable guidance and encouraging me during my work.

Iam also thankful of all my friends for their help and suggestion during my work.

NOMENCLATURE

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List of Tables:

Table No. Particular Page No.

4.1 Damage Severity Levels

List of Figure

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1.1 The Millau Viaduct in France is the world's highest bridge
1.2 General configuration of cable stayed bridge
1.3 The Golden Gate Bridge now has an orthotropic deck
1.4 Box girder
1.5 Prestressing
1.6 Pretensioning
1.7 Stay cables
1.9 Parallel stand type cables
1.10 Load distribution
1.11 Construction practice
1.12 Cable stayed bridge across river Chambal

2.1 Governing equation for deflected shape
2.2 Elemental length of cable under its own weight
2.3 Cable carrying a uniform horizontally distributed load
2.4 Catenary cable element

3.1 Deck cross section of cable stayed bridge across Chambal river
3.2 Cross section of Pier
3.3 Foundation
3.4 Form work
3.5 Bearings
3.6 Collapsed pier
3.7 Collapsed structure
3.8 Honeycombing and exposed reinforcement in the pier while construction

4.1. Cost comparison among various repair strategies
4.2 Inspection buggy in use
4.3 PE pipe split the ultraviolet protection tape is missing
4.4. Burst and damaged protective tape
4.5. PE sheathing split not providing protection, and wire is corroded

CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW

1.1 GENERAL REMARK

Bridge is a structure providing passage over an obstacle without closing the way beneath. The required passage may be for a road , a railway, pedestrian, a canal or a pipeline. The obstacle to be crossed may be a river, a road, railway or a valley.

Cable-stayed bridge is consists of one or more columns with cables supporting the bridge deck.

There are two classes of cable-stayed bridges: Harp design, cables are made nearly parallel by attaching them to various points on the tower so that height of attachment of each cable on the tower is similar to the distance from tower along the roadway to its lower attachment. Where as in fan design, the cables all connect to or pass over the top of the tower.

Compared to other bridge the cable-stayed is optimal for span longer than typically seen in cantilever bridges, and shorter than typically requiring a suspension bridge. This is range in which cantilever spans would rapidly grow heavier if having long length and in which suspension cabling is not economical, were the span is to be shortened.

1.1.1 History of cable stayed bridge

Engineers introduced the concept of cable stayed bridge very early on, at the same time as they began developing suspension bridge; however , with the early collapse of cable supported bridges built over the river Tweed(Europe) and Saale(Germany), at the beginning of the 19th century, the idea was abandoned. A lot of bridges had been destroyed during World War II, it was necessary to rebuild those after the war. At this period of time, steel was less in amount and new bridge had to be constructed with minimum weight. With the aim of providing economy in material and cost, engineers have gone back to the concept of cable stayed bridge. The Stromsund Bridge in 1956 (Sweden) may be accepted as the first modern cable stayed bridge (Gimsing 1997).

1.1.2 First cable-stayed bridge

The Donzere-Mondragon Bridge in France was the first modern cable-stayed bridge. The towers were short and the cables were less than 45 degrees (they impart more horizontal force than vertical support).

1.1.3 First US cable-stayed bridge

Texas, bridge was the first US cable stayed bridge. The angles of support cables were low, it doesn't have the beam/girder deck needed to make it an extra dosed bridge. The bridge was in service for nine years before that was done and the original deck design was clearly not intended to have any vertical stiffness on its own.

1.1.4 Comparison with suspension bridge

A multiple-tower cable-stayed bridge looks like suspension bridge, but it is very different in principle and method of construction. In the suspension bridge, a large cable is made up by "spinning" small diameter wires between two towers, and at each end to anchorages into the ground or to a massive structure. These cables form the primary load-bearing structure for bridge deck. Before the deck is installed, the cables are under tension from only their own weight. Smaller cables or rods are then suspended from the main cable, and used to support the load of the bridge deck, which is lifted in a sections and attached to the suspender cables.When this is done the tension in the cables increases, as it does with the live load of vehicles or persons crossing the bridge. The tensions on the cables are to be transferred to the earth by the anchorages, which are sometimes difficult to construct due to poor soil conditions.

While in the cable-stayed bridge, the towers is the primary load-bearing structure. A cantilever approach is often used for support of the bridge deck near the towers, but mainly areas further from them are supported by cables running directly to the towers. This has the disadvantage, compared to the suspension bridge, that the cables pull to sides as opposed to directly up, requiring the bridge deck to be stronger to resist the resulting horizontal compression loads; but has the advantage of not requiring firm anchorages to resist a horizontal pull of the cables, as in the suspension bridge. All static horizontal forces are balanced so that the supporting tower does not tend to tilt or slide, needing only to resist such forces from the live loads.

1.1.5 Key advantages of the cable-stayed form are as follows:

- It has greater stiffness than the suspension bridge, so that the deformations of the deck under live loads are reduced.
- It can be constructed by cantilevering out from the tower - the cables act both as temporary and permanent supports to the bridge deck.
- For the symmetrical bridge (i.e. spans on either side of the tower are the same), the horizontal forces balance and large ground anchorages won’t be required.
- Any number of towers can be used. This bridge form can be as easily built with a single tower, as with a pair of towers. However, a suspension bridge is usually built only with a pair of towers.

1.1.6 Different type of cable stayed bridge:

1. Side-spar cable-stayed bridge

A side-spar cable-stayed bridge uses a central tower supported from only one side. This is not significantly different in structure from a conventional cable-stayed bridge.

2. Multiple span cable-stayed bridge

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.1 The Millau Viaduct in France is the world's highest bridge( http://www.hoax-slayer.com)

A Cable-stayed bridges with more than three spans involves significantly more challenges in designs than two -span or three-span structures. In two-span or three-span bridge, the loads from the main spans are normally anchored back near the end abutments by stays in the end spans. For more spans, this isn't the case, and the bridge structure is less stiff overall. This can create difficulties both in the design of the deck and the pylons. Examples of multiple span structures where this is the case are Ting Kau Bridge, where additional 'cross-bracing' stays are used to stabilize the pylons; Millau Viaduct and Mezcala Bridge, where twin-legged towers are used; and General Rafael Urdaneta Bridge, where very stiff multi-legged frame towers were adopted.

Extradosed bridge

Octavio Frias de Oliveira bridge, in Sao Paulo, Brazil. is an example of this type. It is the only bridge in the world that has 2 curved tracks supported by a single concrete mass. The extradosed bridge is a cable-stayed bridge but with a more substantial bridge deck that being more in terms of stiffness and stronger allows the cables to be omitted close to the tower and for the towers to be lower in proportion to the span.

3. Cable-stayed cradle system bridge

This system carries the strands within the stays from bridge deck to bridge deck, as a continuous element, eliminating anchorages in the pylons. Each epoxy-coated steel strand is carried inside the cradle in a one-inch steel tube. Each strand acts independently, allowing for removal and replacement of individual strands. The first two such bridges are the Penobscot Narrows Bridge, completed 2006, and the Veterans Glass City Skyway, completed 2007.

1.1.7 Various parts of bridge:

Parts of a cable-stayed bridge:

- Main span
- Side span or back span
- Tower or Pylon
- Anchor pier (both counterweight and tension)
- Intermediate piers in the back spans
- Stay cables (including those continuous through a saddle in the tower)
- Anchorages.

- General configuration of cable stayed bridge:

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Figure 1.4(http://cdn.wn.com)

1.1.7.1 Deck (bridge)

A bridge deck or roadbed is the roadway, or the pedestrian walkway, surface of a bridge. It is not to be confused with any deck of a ship. The deck may be of concrete, which in turn may be covered with asphalt concrete or other pavement. The concrete deck may be an integral part of the bridge structure (T-beam structure) or it may be supported with I-beams or steel girders (floor beams). The deck may also be of wood, or open steel grating.

Orthotropic deck

An orthotropic bridge or deck is one whose deck typically comprises a structural steel deck plate stiffened either longitudinally or transversely, or in both directions. This allows the deck both to directly bear vehicular loads and to contribute to the bridge structure's overall load-bearing behavior. The orthotropic deck may be integral with or supported on a grid of deck framing members such as floor beams and girders.

The same is also true for concrete slab in a composite girder bridge, but the steel orthotropic deck is considerably lighter, and therefore allows longer span bridges to be more efficiently designed. The stiffening elements can serve several functions simultaneously. They enhance the bending resistance of the plate to allow it to carry local wheel loads and distribute those loads to main girders. They also increase the total cross-sectional area of steel in the plate, which can increase its contribution to the overall bending capacity of the deck. The stiffeners increase the resistance of the plate to buckling.

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Figure 1.6 The Golden Gate Bridge now has an orthotropic

deck( http://www.visitingdc.com)

Some very large cable-supported bridges (cable-stayed bridges or suspension bridges) would not be feasible without steel orthotropic decks. Thousands of orthotropic deck bridges are in existence throughout the world.

1.1.7.2 Box girder

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Figure 1.7 Box girder(i.aliimg.com)

Girder is a support beam used in construction. Girders often have an I beam cross section for strength, but it can also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams. A girder is commonly used in the building of bridges.

The girder combines strength with economy of materials and can therefore be relatively light. Its structure consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsion straining forces, but only to tension or compression. It is an improvement over truss which uses a spacing configuration of isosceles triangles.

1.1.7.3 Prestressed concrete

Prestressed concrete is a method to overcome the natural weakness of concret in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons is used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. In early time reinforced concrete was based on the use of steel reinforcement bars rebar inside poured concrete.

Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbounded post-tensioned concrete.

1.1.7.4 Foundation in cable stayed bridge:

Caissons are most commonly adopted foundation for major cable stayed bridges. For piers and abutments of very large size used in cable stayed bridge, a large rectangular well with multiple dredge holes of square shape may be used. Structure which is sunk from the surface of either land or water to some desired depth is known as Caissons.

Type of caissons:-

a)Box Caissons
b) Open caissons (well)
c) Pneumatic Caissons.

OPEN CAISSONS (WELL) form the most common type of deep foundations for bridges in India.

Components:

Well curb and cutting edge -It is designed for supporting the weight of well with partial support at the bottom of cutting edge.

Steining -Provided so that at all stages well can be sunk under its own weight.

Bottom plug -It is designed for an upward load equal to soil pressure minus self wt. of bottom plug and filling.

Well cap -This is for uniform transfer of load to steining and later to ground

SHAPES OF WELL

a)Single circular
b)Twin circular
c)Twin hexagonal
d)Rectangular

1.1.7.5 PYLONS:

Pylon used is based on site condition, design, aesthetics and cable geometry. The ratio of pylon height above the bridge deck to center span length for a three-span structure should preferably be in the range of 0.6-0.2.For three-span asymmetrical structure the longer span may be equivalent to one-half the span of a three-span symmetrical structure. To optimize the quantity of cable steel required, it is desirable to increase the ratio of pylon height to span length to 0.3 for the radiating type and 0.4 for the harp type.

1.1.7.6 STAY CABLES:

Different Types of Stay Cable

1) Locked Coil Type

External is smooth and core is made of 5mm circular wires, positioned in a parallel form.

These are surrounded by several layers of wires of trapezoidal section followed by layers of z shaped wires between 4 and 7 mm in diameter. All wires are galvanized. This forms a very dense coil. The anchorages are fixed at the factory using molten zinc.

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Figure 1.10 Stay cables (http://cdn.wn.com)

This is the most recent system and probably the most used. Strand is made up of 6 galvanized wires wound around a central king wire, this producing a strand of 12.7 or 15mm in diameter. These are installed on site and cut to the required length. They are laid in a duct in a parallel fashion with each strand being held by jaws in the anchorage. The space between strands and the outer duct are filled with wax. More recently the individual strands have themselves been coated in wax and given an HDPE surround.

Picture of Parallel Strand

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Figure1.11 Parellel stand type cables (http://cdn.wn.com)

2) Parallel Wire Type

This type consists of wires-usually 4 – 7mm in diameter laid in a parallel manor inside an HDPE sheath and fixed into an anchorage using resin. This type is manufactured in a factory to a specified length. As these are small straight wires they are susceptible to corrosion and so the space between wires and sheath is filled with grout. These are transported to site on large drums – this sometimes causing problem to the grout. The most common name for this is “HiAm”.

Anchorage of Cables to Pylon

The cable has a lower and upper anchorage – these called fork socket and cylindrical socket respectively. The cylindrical socket is anchored under the deck and is where the stressing is carried out. The fork socket is fixed by a pin to the upper anchorage plate. This is a full plate varying in thickness between 115mm and 56mm which in our case was imported from Europe and shaped in India.

Changes in the Stresses in Concrete during Construction

During the different construction stages the concrete the bending moments vary considerably. This results in additional reinforcement being included in both top and bottom faces of the edge beam. The following slides show these changes. At the completion the bridge is very strong as this reinforcement is no longer needed.

1.1.8 Design – both initial and during construction

The design of a stay cable bridge differs from that of a convention bridge in that there is an enormous amount of Engineering Design needed in the construction part of the Contract. The Initial design at Tender stage will ensure that the completed bridge is safe but the bridge has to be checked for every stage of construction to ensure that the concrete, stays as well as the pylon are not compromised at any time during construction. At this time the actual design of cable is independently tested in this case in Germany. The samples of stay were fatigue loaded to 2 million cycles followed by tensile test to breaking.

Tension and Compression – Important !

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- Tension occurs alog the cable lines.
- The tower is responsible for absorbing and dealing with compression forces.

Figure 1.12 Force distribution (http://cdn.wn.com)

1.1.9 Construction practises:

This method involves constructing a bridge, normally continuous over several spans, progressively from one or both abutments, by attaching sections to the end of already erected portions. An anchor span is lifted or assembled in situ, and sections then cantilevered from this by either lifting from ground level, or running along the deck and lowering from the end. Cantilevering is an ideal method for erecting cable-stayed bridges, using the stays as supports for the cantilever as work progresses

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Figure 1.13 Construction practice (http://cdn.wn.com)

1.1.10 Testing of Stay Cable

1. For each Stay Cable Bridge the design requirements of cables varies. Therefore these have to be designed for each application. To verify the design a full scale test has to be conducted – this being of reduced length (approx 5m) but with all other details as designed.
2. length of cable, complete with two sockets identical to those specified for the permanent cable stays. The sockets were attached using the precise procedure to be used for the permanent sockets. For a satisfactory procedure, failure shall not occur by pulling out of the strand from the socket. After the test the two sockets were sectioned in three places to demonstrate that each wire was surrounded by zinc.
3. The mean load of the load cycle corresponded to a stress of 475 N/mm2 and the varying load corresponded to a stress range of 150 N/mm2.
4. During this test no wires should break.
5. The breaking load after this cyclic loading shall not be less then 85% of the specified minimum breaking load of the cable.
6. The tests for Bridge passed the tensile requirement by at least 11%

1.1.12 Cable stayed bridge across Chambal River: A case study

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Figure 1.14 Cable stayed bridge across Chambal river[ref.6]

1. Typical data of cable stayed bridge

Design, Construction and Maintenance of a Cable Stayed Bridge across River Chambal on Kota Bypass on NH-76 of East-West Corridor in the state of Rajasthan.

2. Type of Bridge: Single Plane cable Stayed Bridge.

3. Span of Bridge : 1400 m

- Main Bridge : 1050 m
- Access Bridge : 350 m

1.1.13 General details of the project:

Introduction

The bypass of Kota (RJ), as part of the NHDP Phase II project; NHAI has planned to make the NH-76 cross the Chambal River. Since this crossing was in the “Chambal gavial wild life sanctuary,” it was decided to cross the river width of about 300m with a single span, and hence reduce crocodile wild life disturbance by avoiding any pier in the river bed.

The solution given by the construction JV Hyundai-Gammon to achieve this goal was a concrete cable stayed bridge with a main span of 350m as projected by Systra. When completed, this bridge will be the first axial suspension cable stayed bridge, ever built in India.

Bridge Description

The selected deck consisted in a single cell box girder of 30.2m width, stiffened at every 3.5m by transverse ribs. This very large width was required to position the 6 lanes carriageway with the 3m central median. Two footpaths of 1.5m width are also located on the deck sides. The deck is supported by sliding spherical or pot bearings on all piers, except on shortest pylon pier where it is rigidly connected. Location of this pylon near the cliff has required special geotechnical and geophysical investigations, to ensure cliff stability.The stays are in a single plane with a semi harp arrangement, and are anchored every 7 m in the deck. The 80m high pylons, receive the passive anchorages of the stay cables.

All lateral piers of the cable stayed bridge are founded on, spread footing, and pylons foundations are composed of two 4.5 m diameter shafts. The structure will be cast in situ: on scaffoldings for the lateral spans and using the cantilever method for main span.

Site Investigations

Topographic investigations

The surveys were related to ground and vertical face of the cliff. The ground survey has been performed to localize bridge axis, determinate pier positions, and ground levels at foundations location. It has also confirmed that there was no major topographical anomaly on site. The survey of the cliff was required to create in 3D the shape of the cliff, in order to evaluate with accuracy the cliff stability; since one of the pylons (P5) is located at only 30 m from cliff edge.

Physical investigation

Geotechnical survey is divided majorly in three parts:

Site investigation, geophysical survey and satellite imagery.

A) Site soil investigations consisted in boreholes drilling for determination of rock parameters, by on site test and laboratory tests as detailed hereafter:

On site Tests:

- permeability
- CaCO3content
- visual identification for each run

Laboratory Tests:

- Dry density
- water absorption, porosity
- specific gravity
- point load index (pli)
- resistance tests (ucs)
- CaCO3content
- elastic modulus and Poisson ratio

In addition to the tests done above, a petro graphic analysis of the rock, and a water chemical analysis was also performed .Numbers and depth of borehole were governed by foundation type (pile, shaft ,or footing): one borehole was required for common pier foundations, and three boreholes at pylon foundations location up to 40m depth. Due to the type of tests to be performed (cross hole tests). number of borehole at the pile shafts location were required

The site geology is mainly composed of hard siliceous sandstone. Metamorphism takes place on the 10 first meters which increases the percentage of silica: sandstone transforms into quartzite due to temperature effects and climatic variation. This part generally creamish to brownish colored is more fractured. At a lower depth, the sandstone becomes more ferruginous.CaCO3tests have indicated that there was no trace of calcium carbonate in the rock samples. UCS test have shown that the rock has an important resistance: between 100 and 300 mpa. Youngs modulus is equal to 17 500 mpa for intact rock, and poisson’s ratio is equal to 0.21.Chemical analysis of water has shown that there was no presence of aggressive elements, which could affect structure durability At pylon located near the cliff (P5), there are different sets of joints which show no connectivity, except for joints with 45 angles. In addition, a thin layer of soil has been found at 10m depth, which has led us to propose a geophysical survey to verify that this layer do not extend to the cliff, and therefore do not affect cliff stability. A slope stability calculation was afterwards conduced considering network of fractures, block stability and low cohesion on sliding surfaces.

B) Geophysics survey includes cross-hole tests at pylons location and surface survey using the electrical method. Cross hole tests have allowed to measure velocity into the rock along the bore length. The tests have confirmed the presence of weak rock at 13m depth. On the remaining height values were consistent and no drastic variation is observed. This test has also confirmed the absence of cavities or channels. Surface geophysics survey has allowed to develop a complete 3D model of rock resistivity as shown in the next scheme.The geophysical investigation has concluded that the discovered weak zone (with low resistivity), do not extend up to the edge of the cliff and consequently do not affect the stability of the cliff.

C) Satellite imagery has allowed to confirm the geology of the area, and to determine river bed profile since no access was possible from the river for environmental reasons. This investigation has also confirmed that there was no cavity below water level, which could extend below the pylon P5 foundations.

General Design Features

Foundations

All lateral piers of the bridge are supported on rectangular footings, resting on safe rock. Pylon piers foundations are composed of two vertical shafts of 4.5m diameter, with a maximum length of 15m. For shafts design, friction and reduced end bearing were considered in order to reduce the settlement. The rock mass ratio (r m r) classification was used to determine the bearing parameters following the AASHTO LRFD code.

The pile caps above the shafts have a variable depth; from 2m at the extremities to 5m at pier axis, this shape has been chosen in order to optimize quantities by placing the concrete only where it is required. Concrete grade for all foundations is M47.5.Empty ducts will be provided on p5 pile cap for future active anchors, in case it becomes required.

Substructures

The piers have a rectangular shape for lateral piers and a cross shape for pylon piers. This cross shape is unusual, and has been chosen for structural reason. The main loads come from the pylon, so it is logical to put some material just below the rectangular axial pylon. The other part of the load comes from the deck webs and its diaphragm, so it is logical to put some material on a rectangle located just below the diaphragm. This leads to a cross shape. The pier P5 is rigidly connected to the deck, whereas the second pylon pier (P4) has a pier cap supporting four spherical bearings of 7000t capacity each.

Seismic Design

Kota is located in seismic zone II, where peak ground acceleration is 0.1 g. The design response spectrum was defined as per IRC code. To estimate earthquake forces on the structure, a 3D model of the bridge has been realized, including part of the access bridges in order to have the correct effects on transition piers.

Earthquake effects were evaluated for service stage and for the most unfavorable construction stage. The effects of all modes were combined together using complete quadratic combination CQC. Response modification factors were considered as per AASHTO LRFD, with distinction between wall type, and column type of piers for each horizontal direction.

Bearings

Lateral spans are supported on pot bearings with a maximum vertical capacity of 1200t in service stage. These bearings shall also resist uplift forces evaluated to a maximum of 380t at strength limit state.

The four spherical bearings located on P4, have a maximum vertical capacity of 7000 t.

Pylons

The pylons are 80m high above top of deck with a constant width of 3m, and a variable length: from 7 m to 4 m. The concrete grade used is M 60. The pylons contain a steel frame where are located the passive anchorages of stay cables. This steel frame is composed by 20 steel boxes (one for each pair of stay cable), that take the horizontal component of the stay cables. The vertical component is transmitted to the pylon through shear studs, located on the laterals sides of the steel boxes. A manhole with a minimum dimension of 800x1500 mm is provided inside the pylon.

Stay Cables

The stay cables are composed of individually sheathed strands having a triple protection: galvanization, wax filling and individual polyethylene sheath. The external cable duct has helicoids in order to eliminate rain and wind induced vibrations.

The strands have seven wires of class 1860 mpa and stay cables unities vary from 58 to 91 strands. Anti vibration devices will be provided for the longest stay cables.

Deck

The concrete deck (M60 grade), is prestressed longitudinally using internal and external tendons, and transversally using only internal tendons. The external tendons are located mainly in lateral spans that are longitudinally prestressed using only external tendons. Some external tendons are also provided for the continuity prestressing of main span. Provisions for future external prestressing are also provided. Internal tendons are used for cantilever tendons, cyclic tendons and some of the continuity tendons of main span.

Pier segment below pylon P4 has been studied using volume finite elements, as the transfer of pylon vertical load to the four spherical bearings was not easy to apprehend. This study has confirmed that a transverse prestressing at the bottom of the pier segment was required.The longitudinal analysis has been performed considering second order effects, construction stages and time dependent effects. Special load cases particular to the design of cable stayed bridges and cantilever construction were considered such as cable braking or replacement, differential temperature in deck and stay cables, accidental falling of travelling formwork.

Service life of the Structure

The identified risk for structure durability is the concrete carbonation. Based on a present CO2 concentration in the air estimated to 350 ppm, and an expected increase due to road traffic to 450 ppm, the service life of the structure has been estimated using the Papadakos & al. model. Estimation of structure design life is defined as the time required for carbonation to reach the first layer of reinforcement. Based on an external concrete cover for pylons and deck box-girder of 40 mm, the required design life of 100 years has been reached for a defined minimum concrete carbonation resistance.

Construction

The construction of the bridge was started in December 2007, and was scheduled to complete by 2011 before the accident. The lateral spans of the bridge was to be cast in situ on scaffoldings, starting by the spans near the pylons. Two sets of scaffoldings was to be used: one on each side of the river. The main span segments of 3.5 m length, was cast in situ using two sets of travelling formworks, and pylons using climbing formworks.

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