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Re-design of Nariamabune Bridge in Kaabong District of Karamoja, Uganda 2009 B.Eng.CBE

By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulai

Declaration

I declare that this project entitled Re-design of Nariamabune Bridge in Kaabong District of

Karamoja is my own original Project work, except as cited in the references. The project has

not been accepted for any degree and is not concurrently submitted in candidature or award of

any other degree.

Signature Date ..

OKUCU ANTHONY TWENY

(STUDENT)


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulaii

Supervisor Approval

Having supervised the student and read through the work herein presented, I do hereby consent

that the project is worth the award of the Degree of Bachelor of Engineering in Civil and

Building Engineering of Kyambogo University

Signed Date

DR. MICHAEL KYAKULA

SUPERVISOR.


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulaiii

Project Title:

The title to this project is:

RE-DESIGN OF NARIAMABUNE BRIDGE IN KAABONG DISTRICT

OF KARAMOJA, UGANDA


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulaiv

Dedication

To Jehovah God Almighty from whom all wisdom and blessings flow. Take all the praise,

glory and honour!


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulav

Acknowledgements

I must in all sincerity, thank my wife, Margaret, children: Shalom, Emmanuel, Rebecca and

Ezra who have had to endure my absence from home during the times I had to be away in order

to pursue this course of studies. From your patience and endurance I derived courage and

feeling of support.

Special thanks to my parents: Joshua J. Tweny and his dear wife Mummy Alice Tweny who

forfeited enjoyment of their meager income in order that I may be schooled.

To Charles Ayo, my very concerneduncle who rekindled my interest in studies when I was

almost giving up on academics. You picked me up and helped me reach here. And to my sister,

Dr. Atim C. Oyet, you were a challenge, an inspiration and a helping hand at the time of need.

To my former school teacher, Mr. Ahimbisiibwe Wilson, who helped me to realize my

engineering aptitude, and encouraged me to tow that line. Little did you know you were

shaping this work but youve done it.

Great appreciation to my Project Supervisor, Dr. Michael Kyakula for the guidance advice and

positive criticisms that have shaped this project and made it such a successful and

comprehensive design report. All my lecturers in the department of Civil and BuildingEngineering of Kyambogo University. The knowledge and skills you imparted was quite

helpful in this project.

To all the staff of Uganda National Roads Authority, Kotido Station for the support and

information they provided and to my course mates for criticizing positively the works

especially during the processes of analysis and design of the structures.

To all the authors and authorities whose works have been cited herein

Thank you all sincerely.

Okucu Anthony Tweny.


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulavi

Abstract

Nariamabune Bridge is found in Kaabong District, Karamoja Sub-region, located at

N03Po

P38 57 and E34 Po

P0209 along the Kaabong Kapedo Road link. The road link offers

trade opportunities to the local people with other communities and neighbouring towns,

promotes tourism industry (Kidepo Valley National Game Park) and links the Region to Kenya

and Southern Sudan. It also facilitates humanitarian assistance and security interventions, in

case of insurgencies.

The existing bridge structure comprises of a Bailey bridge (steel decking) supported on stone

masonry abutments that in turn sit on foundations observed to be sinking (a sign of settlement).

Scouring / erosion have also been observed at the abutments and embankments pointing to an

imminent failure.

The problems associated with the imminent failure of the bridge are quite adverse. There is

urgent need to prevent this imminent failure by re-designing the bridge. This project has had to

be carried out, therefore, by using the geo-soil properties of the existing bridge site,

determining the flood levels at the water crossings, carrying out the hydraulic design that will

minimize scouring, determining the significant loads for analysis of the substructure and the

superstructure and designing the decking of the bridge, the abutments (and piers) as technicallynecessary. Finally detailed drawings and appropriate recommendations will be made.


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulavii

Table of Contents

Pictures ....................................................................................................................................- 1 -

Chapter 1 .................................................................................................................................- 2 -

1. Introduction.........................................................................................................................- 2 -

1.1. Project Title:.....................................................................................................................- 2 -

1.2. Background:.....................................................................................................................- 2 -

1.2.1. Bridge: ...........................................................................................................................- 4 -

1.2.2. Bridge design:................................................................................................................- 4 -

1.4. Project Aim: .....................................................................................................................- 5 -

1.5. Project Objectives:...........................................................................................................- 5 -

1.6. Scope of the Project:........................................................................................................- 6 -

1.7. Justification:.....................................................................................................................- 6 -

Chapter 2 .................................................................................................................................- 8 -

2.1 Definition of a bridge; ......................................................................................................- 8 -

2.2.1 Bridge Materials: ...........................................................................................................- 9 -

2.2.2 Classification:...............................................................................................................- 10 -

Steel Bridges..........................................................................................................................- 10 -Classification according to structural action:....................................................................- 13 -

Simply supported span bridge.............................................................................................- 13 -

Continuous span bridge .......................................................................................................- 14 -

Cantilever bridge ..................................................................................................................- 14 -

Rigid frame bridges..............................................................................................................- 15 -

Classification according to Floor location..........................................................................- 15 -

Classification based on type of connections: ......................................................................- 17 -

Reinforced- concrete Bridges...............................................................................................- 18 -

Types of Bridges....................................................................................................................- 18 -

2.2.3 Requirements:..............................................................................................................- 20 -

2.3 Site Investigation.............................................................................................................- 21 -

2.4 Substructure and Foundations ......................................................................................- 22 -

2.4.1 Abutments ....................................................................................................................- 24 -


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulaviii

2.4.3. Design of Gravity and Cantilever Retaining Walls. ................................................- 24 -

2.4.3.1. Gravity retaining walls............................................................................................- 25 -

2.4.3.2. Cantilever Walls of reinforced cement concrete (RCC) ......................................- 25 -

Abutment design...................................................................................................................- 25 -

Reinforced concrete abutments...........................................................................................- 26 -

2.4.4 Reinforced concrete retaining walls...........................................................................- 26 -

2.4.5 Bearing Shelves............................................................................................................- 26 -

2.4.6 Piers...............................................................................................................................- 27 -

Reinforced Concrete Piers ...................................................................................................- 27 -

2.4.7 Determination of allowable bearing pressure, ..........................................................- 27 -

Non cohesive soils..................................................................................................................- 27 -

Cohesive soils.........................................................................................................................- 28 -Presumed values....................................................................................................................- 30 -

Table 2................................................................................................................................- 31 -

2.4.8. Foundations on Rock..................................................................................................- 33 -

2.4.9. Run-on Slabs ...............................................................................................................- 33 -

2.5 Superstructure ................................................................................................................- 34 -

2.6 Design Detailing..............................................................................................................- 35 -

2.6.1 Vertical Profile over the Bridge..................................................................................- 35 -

2.6.2 Shear Connectors.........................................................................................................- 35 -

2.6.3 Protective Treatment to Steelwork ............................................................................- 36 -

2.6.4 Bolts and nuts...............................................................................................................- 36 -

2.6.5 Bearings ........................................................................................................................- 36 -

2.6.6 Expansion joints...........................................................................................................- 37 -

2.6.7 Construction joints ......................................................................................................- 37 -

2.6.8 Slab Reinforcement .....................................................................................................- 37 -

2.6.9 Drainage........................................................................................................................- 38 -

2.6.10 Parapets, Surfacing and Services.............................................................................- 38 -

2.6.11 Provision for Pedestrians and Cyclists ....................................................................- 38 -

2.7 River Hydraulics and Hydraulic Design.......................................................................- 38 -


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulaix

Chapter 3 ...............................................................................................................................- 44 -

METHODOLOGY...............................................................................................................- 44 -

3.3.1 Trial pit excavation......................................................................................................- 45 -

3.3.2 Sampling.......................................................................................................................- 46 -

3.3.3 Soil Testing ...................................................................................................................- 46 -

Chapter 4 .................................................................................................................................-49-

Analysis and Design................................................................................................................-49-

Chapter 5 .................................................................................................................................-99-

Recommendations...................................................................................................................-99-

Appendices.............................................................................................................................-101-


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakulax

L ist of F igures

TFigure 1.1. Single span Simply supported BridgeT ..........................................................................- 13 -

TFigure 1.2. Simply Supported Multi- span Bridge.T .........................................................................- 13 -T

Figure 1.3 Continuous Span Bridge .................................................................................................- 14 -

Figure 1.4 Cantilever Bridge with a Suspended Span ....................................................................- 15 -

Figure 1.5 Solid Ribbed Arch............................................................................................................- 16 -

Figure1.6. Braced Ribbed Arch. .......................................................................................................- 16 -

Figure1.7. Spandrel Braced Arch Bridge.........................................................................................- 17 -

Figure 1.8. Tied Arch Bridge ............................................................................................................- 17 -

Figure 2.1 Slab Bridge .......................................................................................................................- 18 -

Figure 2.2. Deck Girder Bridge ........................................................................................................- 18 -

Figure 2.3. Composite Steel-Concrete Bridge..................................................................................- 19 -

Figure 2.4. Composite Pre-stressed Concrete Bridge ................................................................- 19 -


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By: Okucu Anthony Tweny (07/U/2032/ECE/PE) Supervised by Dr . M ichael Kyakula- 1 -

Pictures


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Chapter 1

1 Introduction

1.1 Project Title:

Re-Design of Nariamabune Bridge in Kaabong District of Karamoja.

1.2 Background:

Nariamabune Bridge is found in Kaabong District, Karamoja Sub-region, located at N03Po

P38

57 and E34Po

P0209 along the Kaabong Kapedo Road. (see Plate 1 Appendix 1)

The history of this road link was not readily available but Kaabong Kapedo Road plays an

important role of linking parts of Kaabong district to Pader, Kitgum and Lira districts fromwhere most of the merchandise is brought into Kaabong.

This road link is significant not only for linking economic activities of the local people, but

in the promotion of tourism industry. According to a UWA brochure, Kidepo Valley

National Game Park receives tourists from various countries annually. Some of the tourists

reach the National Park by way of Kampala Mbale Soroti Kotido Kaabong route,

while others use the Kampala Karuma Lira Kotido Kaabong - Kidepo Route yet

others use the Kampala Karuma Lira Pader Orom Karenga Kapedo Route. The

former two therefore needs the bridge at Nariamabune to be safe and sound for easy

passage.

Most tourists from Europe, the Americas, Australia and other parts of the world land in

Kenya and from Nairobi, they travel by road to Western Uganda to Mgahinga Gorrila

National Park, Bwindi Impenetrable National Park, Kibale National Park, Queen Elizabeth

National Park, Murchison Falls National Park, and others before embarking on moving to

Kidepo National Park and finally passing along Kapedo Kaabong Kotido Soroti

Mbale to Mt Elgon National Park from where they get back to Kenya.

Besides the trade and commercial activities, this road link is significant in fostering

humanitarian assistances to Karamoja region most of which lies in the arid climatic zone of

North-eastern Uganda, often characterized by famine.


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Of great importance is the fact that a road link is being planned to join Kapedo to New Site

in Southern Sudan

Kaabong District of Karamoja is located within Latitudes 2 P0P 30 and 4 P0P 15 South and

Longitudes 33 P0

P 30 and 35 P0

P 00 East in the north eastern part of Uganda.The Total area is13,208 kmP

2P (5.47% of Ugandas total area) of which arable land coverage is 7,268 kmP

2P and

water coverage is 0 km P2

P. Since arable land coverage is only 7,268 square km the cropproductivity rating is limited.

Forestry:

There are 12 natural forests and five plantation forests. Artificial forest coverage is 0.2 KmP

2P

with acassia, same and eucalyptus.

Wild life:

The largest Game Park in Uganda (Kideepo Valley National Game Park) has an area of

1,442 square Km.

Animals:

water bucks, Jacksons hartebeest, zebras, buffalos, elands, ribi, warthogs, bush bucks,

jackals, elephants, giraffes, lions, cheetahs, leopards, ran antelopes and ostriches. Bird life is

in abundance.

Minerals:

Gold, silver, copper, iron, and mica and crude petroleum at Kathile basin.

Sunshine and wind:

Between December and April the Northeasterly wind usually exceed 200Km per day.

Characterized by dust storms, desiccating and pulverizing the sparse vegetating cover.

Average rainfall is 519mm per annum. Rainy season falls in April August with a marked

minimum in June and marked maximum peaks in May and July. The rain is erratic in nature.


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Distinct wet and dry seasons are a prominent feature. The rainfall is unevenly distributed

and unreliable and has a significant influence on the economy and life of the district.

1.2.1. Bridge:

Bridge, in this context, shall mean a structure that permits traffic to pass over the water

crossing safely and conveniently.

1.2.2. Bridge design:

Bridge design in this context shall mean the application of scientific theories in combination

with field data and technological knowledge to come up with a safer and an economical

bridge structure that will permit traffic to pass safely, conveniently and comfortably above

the river crossing.

Almost all human activities involve problem solutions. Production of goods and services has

always resulted from almost all the human activities. Unless the goods and services

produced are distributed, exchanged and consumed the economic chain is not complete. In

order, therefore, to complete the economic chain, transportation becomes a vital factor in the

economy as the outcomes of the production process must find their ways into some market

where exchange and consumption take place.

Transportation involves the movement of factors of production like machinery, labour, raw

materials, fuels, and of finished goods or services to places of utility. Traffic of various

forms and kinds are involved in the said movement, depending on certain factors outlined

herein as: accessibility of the places of production and markets relative to the terrain,

existing infrastructure, drainage pattern and land use.

Traffic form is also dependent on the economic activities, magnitude of haulage and nature

and value of goods and services produced. In this case road transport has been considered as

it is the most feasible and is already an existing infrastructural investment in the area of the

project.


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1.3 Problem Statement:

The existing bridge structure comprises of a Bailey bridge (steel decking) supported on

stone masonry abutments that in turn sit on an earth foundation. It is observed that since its

constructions in 2004, the bridge deck level is sinking. Information available from UNRA

Kotido Station estimates the level to have sunk by about 400mm yet there are indications

that the embankments are prone to be cut by the waters due scouring and erosion noticed at

the site.

The local scour at abutments are an indication that the width of the channel is not sufficient,

hence a hydraulic design needs to be done, taking into account the scour and a preventive

measures taken to minimize the same.

Qualitative site survey has shown no protective measures taken to minimize the effects of

scour at the abutments upstream of the water crossing, and the side drains of the

embankments are extremely gullied, progressively leading to the embankments failure.

1.4 Project Aim:

The aim of this project is to design a strong and economical bridge to meet the structural and

hydraulic requirements and improve on the level of service for a design life of 40 years.

1.5 Project Objectives:

The objectives are to:

o Carry out the hydrologic and hydraulic analysis of the catchments area and the river,o Determine the flood levels at the water crossings,o Carry out the hydraulic design that will maximize flow with minimal width of

channel and scouring effects

o Determine the significant loads for analysis,o Design the decking of the bridgeo Use the existing soils parameter to Design the abutments, piers and the foundations.o Come up with the working and structural drawingso Make recommendations


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1.6 Scope of the Project:

o Obtaining Geotechnical data / soil parameter.o Hydrologic (Flow) analysis and Hydraulic Design,o Analysis and design of Superstructure,o Analysis and design of Substructure.o Detailed drawings

1.7 Justification:

Kaabong Kapedo - Karenga trunk road links Pader, Kitgum, Lira and Soroti to Kaabong

Town in the North - eastern part of Uganda. Failure of Nariamabune Bridge and therefore

has a very significant connotation in the economic life of the four towns.

Most of the goods and essential commodities consumed in Kaabong are sourced from

wholesale markets as far as Kampala, Soroti, Mbale and Lira by way of Lira Abim or

Soroti Abim routes. The same routes are used by humanitarian organizations to transport

relief food to the district

The same road is a link to Kidepo Valley National Game Park, therefore, very significant in

tourism industry. The road also links this part of Uganda to Southern Sudan and North-

western Kenya and therefore an important link for international trade. The two facts meanthe road is a foreign exchange earner in Ugandas economy.

There is a plan by the government of the republic of Uganda to connect this road link to

New Site in Southern Sudan through Kapedo, an investment that is anticipated to boost

traffic on Kaabong Kapedo road.

Kaabong District, as already noted elsewhere, is endowed with mineral resources likegold,silver, copper, iron, mica and crude petroleum at Kathile Basin. The exploitation of such

mineral resources require explicit transportation scheme that motivate deliberate investment

in the sector.

Besides the economic considerations, Karamoja region, in general, is an arid area and often

affected by famine. Food aid and humanitarian assistance will always require excellent


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mobility. Any slight break in mobility will lead to massive death due to famine. Other social

services also need transportation networks that are consistent to facilitate emergency

deliveries.

Worth mentioning is the fact that a section of the Karamojong have firearms and are

aggressive to passers-by. In case of failure of Nariamabune Bridge, such people will not

hesitate to attack, loot, kill or harm the unfortunate road users that may find themselves

trapped in the ignominy.

The problems associated with the imminent failure of the bridge are quite adverse as has

been seen. This project therefore is intended to offer solutions by preventing the failure

through redesigning the bridge.


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Chapter 2

2.0 LITERATURE REVIEW

The basic concept involved in this chapter is to examine the recent and historical significantstudies that form the basis of design of a bridge.

2.1 Definition of a bridge;

Abridge is a structure for carrying road traffic and other moving loads over a depression orgap or obstruction such as river, canal, channel, canyon, valley, road or railways (Punmia, et

al 1998). In other words, it's the structural system carrying the communication route and

includes beams, girders, stringers, arches, cables and all other components above bearing

level.

If a bridge is constructed to carry highway traffic, it is known as a highway bridge. If,

however, it is constructed to carry railway traffic, then it is known as a railway bridge. There

may be a combined highway and railway bridge to carry both the highway as well as railway

traffic. Some bridges, constructed exclusively to carry pedestrians, cycles and animals, are

known asfoot bridges while those constructed to carry canals and for pipe lines are known as

aqueduct bridges.

A bridge may be a culvert, high level bridge or submersible bridge. A culvert is a bridge

having a gross length of six metres or less between the faces of abutments or extreme

ventway boundaries, and measured at right angles there to the direction of water flow.

A high level bridge is a bridge which a carries the roadway above the highest floodlevel of the channel.

Asubmersible bridge is a bridge designed to be over topped in floods. A bridge may befixedormovable type. Afixed bridge is the one which always remains in one position.


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A movable bridge is the one which can be opened either horizontally or vertically soas to allow river or channel traffic to pass. Such bridges are constructed over a

navigable stream where the normal headway is not sufficient for the vehicles to pass

through.

A bridge may be either of deck type or through type.

A deck type bridge is the one in which the roadway/railway floor rests on the top of the

supporting structure.

A through bridge is the one where the roadway/railway floor rests on the bottom of the main

load supporting structure.

However when the floor lies between the top and bottom of the main load supporting

structure, it is known as half through type bridge,semi-through bridge orpony bridge.

2.2.1 Bridge Materials:

Bridges are made of different material such as timber, stone masonry, brick masonry,

concrete and steel. Timber bridges are constructed only over small spans and for temporary

purpose, to carry light loads. Masonry bridges are also constructed for shorter spans.

Concrete bridge, both of reinforced cement concrete (RCC) as well as of prestressed cement

concrete (PCC) are constructed over moderate to high spans, to carry all types of loads.

Concrete arch bridges have been constructed of spans up to 200 m. Similarly, steel bridges,

are constructed both over moderate to high spans as well as for heavy trafficular loads. In

India, steal bridges are commonly used for railways for all types of spans. [Pumnia, et al

(1995)]

Abridge just like any other civil engineering structures has two component parts, namely

Substructure Super structure


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2.2.2 Classification:

(i). According to end fixity a bridge can be classified as simple or continuous bridge.(ii). According to ability to slide/move a bridge can be fixed or movable bridge.

(iii).

According to purpose it may be foot railway or highway Bridge.(iv). According to location of bridge floor it may be deck, through or semi-through

bridge.

(v). According to highest flood levels it may be submersible or non-submersible.(vi). According to construction materials it may be stone and brick (Masonry), steel,

timber or concrete bridge

Steel Bridges

Classification

Steel bridges may be classified according to the following criteria:

1. Type of structural arrangement2. Structural action3. Floor location4. Type of connection5. Movement of structural parts

Classification according to structural arrangement:

Under this classification a steel bridge may be of the following types:

(i). I-girder bridge(ii). Plate girder bridge

(iii). Truss girder, or(iv). Suspension bridge.


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I-girdersmay be used for small as the main load carrying members. For this purpose, wide

flanged I-sections are used. Such structures are suitable for spanning over crossings of

moderate widths. There are however limitations in the maximum size of available I-sections,

hence for longer span bridges built up plate girders are to meet the requirements of section

modulus corresponding to the applied loads.

Plate girder bri dges:This type of are quite popular with railway bridges, the advantage

being in transportation since it can be transported in one piece. the limiting depth of plate

girders is only 3 -4m hence when the structural requirement for depth is more than this, steel

truss girder bridges are preferred.

Truss girder bridges: in this arrangement, trusses are used as the main load carrying

members. They are commonly used over spans of 20 200m

For still longer spans, steel arch bridges or suspension bridges using high strength steelcables may be used.


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Classification according to structural action:

According to the criteria of structural action, steel bridges may be of the following types:

(i).

Simply-supported-span bridges(ii). Continuous span bridges

(iii). Cantilever bridge(iv). Arch bridges(v). Rigid Frame Bridge.

Figure 1.1. Single span Simply supported Bridge

(i) Simply supported span bridge

Such types are commonly used when the width of gap is small, necessitating the use of

single span. However, even if the width of the gap to be bridged is large, the whole widthcan be subdivided into a number of individual spans, each span being simply supported

Figure 1.2. Simply Supported Multi- span Bridge.

Such an arrangement is preferred specially .is those locations where there is likelyhood of

uneven settlement of intermediate piers. The analysis of simply supported span bridge is

very simple.


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(ii)Continuous span bridgeWhen the width of gap is quite large, and where there are no chances of uneven settlements,

bridge may be continuous over two or more spans (Figure 1 (c)). Because of continuity,

moments are developed at pier supports resulting in the reduction of stresses at the inner

spans. This results in an economical design. Also, continuous span bridges require few

supports, since larger spans can be used; they can also support higher loads in comparison to

simply supported spans.

Figure 1.3 Continuous Span Bridge

(iii) Cantilever bridge

In the case of a three span continuous bridge, loaded with uniformly distributed load over all

the three spans, it is observed that these are two points of contraflexure in the central span.

Hence if the continuous beam/girder of the middle span is cut at these two points of

contraflexure, and shear resisting joints are made at these two points, the resulting

configuration will be a cantileverbridge with a central suspended span between these two

formed joints, as shown in Figure 1.4. Thus, a cantilever bridge consists of two simple

spans, one at each end, each having an overhanging or cantilever portion along with a

simple span (or suspended span) in between the two cantilever portions. The main

advantage of such an arrangement is that the B.M. diagram will now not be affected by the

settlement of supports. The suspended span is supported by the cantilever span, at each endby means of mechanical hinge. This gives rise to a statically determinate structure which can

be analyzed very easily. Because of development of moments at the pier supports, the

stresses in each end span is very much reduced. Another advantage of such bridge is that the

cantilever portion and the suspended span can be erected without the use of false work or


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staging. Such bridges are usually provided for longer span, ranging between 200 m to 500

m.

Figure 1.4 Cantilever Bridge with a Suspended Span

(i v) Arch Bridge.

For deep gorges, arch bridges are generally used, since they offer economical and aesthetic

solution. However, they require strong abutments to resist the thrust from the arches. The

arches may consist of girder sections or trusses, and may be:

Fixed arches Two hinged arches, or Three hinged arches,

The two hinged arches are more common.

The arch bridge may further be classified as:

solid ribbed arches braced rib arch spandrel braced arches or Tied arches (Figure 1.8).

Solid ribbed arch bridges are more commonly used for highways, while braced rib arch

bridges and spandrel braced bridges are commonly used for railways.

( v) Rigid frame bridges

Rigid frame bridges, comprising of single span or two to three continuous spans, are usedfor dry-over or under-crossing, for gaps between 10 to 20 m. These consist of steel columns

and steel girders with continuity at the knee. Such bridges are quite suitable for rigid

foundations.

Classification according to Floor location


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According to the location of the floor, bridges can be classified as under:

Deck-type bridge Through type bridge Half through-type bridge.

In deck type bridge the floor is placed on the top flange in the case of plate girder bridge and

on the top chord in the case of a truss bridge. No top bracing is therefore required.

In the case ofthrough type bridge, the floor" is placed at the level of lower chord of truss

type bridge, and the top chord is braced laterally.

However, in the case of plate-girder bridge, the floor of the through bridge is supported on the bottom flange. In half through bridge, also known as ponny truss bridge, the floor lies

between the top and the bottom. There are also double deck bridges constructed to carry the

traffic of both roadways and railways. Both the decks can have through floors. Alternatively,one deck may have through floor while another deck may be kept open. The requirements of

grade line and clearance of highways or railway track decide whether it should be through-

type bridge or a deck type bridge. Deck type bridges are more economical than through type

bridges, and hence they are more popular.

Figure 1.5 Solid Ribbed Arch.

Figure1.6. Braced Ribbed Arch.


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Figure1.7. Spandrel Braced Arch Bridge

Figure 1.8. Tied Arch Bridge

Classification based on type of connections:

Depending upon the type of connections of the joints, bridges can be of the following types:

(i). Riveted bridges(ii).

Welded bridges

(iii). Bolted (pin connected) bridges


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Reinforced- concrete Bridges

Types of Bridges

According to WinterandNilson (1972), reinforced concrete is particularly adaptable

for use in highway bridges because of its durability, rigidity, and economy, as well

as the comparative ease with which a pleasing architectural appearance can be

secured. For very short spans, from about 10 to 25 ft, one-way-slab bridges (Fig. 2.1)

are economical. For somewhat longer spans, concrete girder spans (Fig. 2.2) may be

used.

Probably most highway spans of medium length, from 40 to 90 ft, presently use

composite steel-concrete construction (Fig. 2.3) or composite pre-stressed-concrete

construction (Fig. 2.4).

Figure 2.1 Slab Bridge

Figure 2.2. Deck Girder Bridge


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Figure 2.3. Composite Steel-Concrete Bridge

Figure 2.4. Composite Pre-stressed Concrete Bridge

In composite construction with structural steel, the concrete deck is made to act

integrally with supporting steel stringers by means of devices called shear connectors,

welded to the top flange of the steel section and embedded in the slab. Although such a

bridge is not strictly a reinforced-concrete structure, the design of this type of bridge

will be discussed in some detail in this section because of its widespread use. Pre-

stressed-concrete bridges frequently make use of composite section characteristics also.

Commonly, the girders are pre-cast and placed into final location by crane, eliminating


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the necessity for obstructing traffic by falsework. The deck slab is then cast in place,

bonded, and tied to the pre-cast sections by steel dowels.

Hollow box girders, usually pre-stressed, are often used for intermediate and long-

span concrete bridges. Spans up to about 80 ft are pre-cast in one piece and lifted into

position. Longer spans of similar cross section, carrying two lanes of highway traffic

as well as shoulders and walkways, have either been cast in place or pre-cast in short

segments, which are post tensioned after positioning.

Bridge spans as long as 320 ft have been attained using pre-stressed girders. Other

possibilities for long-span concrete bridges are the various forms of arches, including

the barrel arch and the three-hinged arch. [Winter and Nilson (1972)]

2.2.3 Requirements:

The bridge should be efficient, effective and equitable so that;

It is economical

It's aesthetically sound It should serve the intended function with utmost convenience, comfort and

safety.

It is durable


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2.3 Site Investigation

The soils supporting the abutments and pier foundations always carry the weight of the

traffic, superstructure, abutments and piers. In order to design for the best suitable

foundations, the designer has to determine the nature and location of the different soil typesoccurring at the site of the bridge and its approaches, to depths containing strata sufficiently

strong to support the bridge and embankments without failure.

This information is obtained by analyzing samples taken from a grid of bore-holes or test pits

covering the whole of the proposed site, and by testing the samples for density, shear

strength, plasticity and penetration, in order to provide quantitative data for foundation

design.

2.3.1 Methods of site investigation

Test pits Hand auger boring Cable percussion boring Rotary drilling Geophysical surveying.

2.3.2 Sampling

The choice of sampling technique depends on the purpose for which the sample is required

and the character of the ground.

There are four main techniques for obtaining samples:

Taking disturbed samples from drill tools or from excavating equipment in the courseof boring or excavation

Drive sampling in which a tube or split tube sampler with a sharp cutting edge at itslower end is forced into the ground, either by static thrust or by dynamic impact

Rotary sampling, in which a tube with a cutter at its lower end is rotated into theground, so producing a core sample


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Taking block samples cut by hand from a trial pit, shaft or heading.Samples obtained by the last three techniques will be sufficiently intact to enable the ground

structure within the sample to be examined. However, the quality of these samples can vary

considerably, depending on the sampling technique and ground conditions, and most sampleswill exhibit some degree of disturbance. Table 2.1 indicates the mass of sample required for

identification purposes, Atterburg tests, moisture content, sieve analysis and sulphate tests.

Care should be taken to ensure that samples are as pure and undisturbed as possible.

Table1. Required Sample Mass

2.3.3 Aggressive Chemicals

The ground or ground water may contain chemicals capable of causing damage to concrete

or steel. These chemicals may emanate from nearby industrial processing or may occur

naturally. Total sulphate content of more than 0.2% by weight in soil and 300 ppm in ground

water are potentially aggressive (BRF 1981). There are often difficulties in specifying

ground condition before the excavation for constructions are complete. For this reason the

engineer should be prepared to review his plan, both during construction, if evidence is

found of unexpected soil conditions.

2.4 Substructure and Foundations

This is a support for the super structure and comprises of; abutments, piers, wing walls and

any part below bearing level i.e. the foundation.

Soil typeMass required

(kg)

Cla , silt, sand 2Fine and medium gravel 5

Coarse gravel 30


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The substructure can be made of materials such as mass concrete, reinforced concrete, steel,

timber and ifexperience is available in structural masonry it can be an economical substitute

for concrete. However, the engineer must be certain of the strength of materials used,

particularly when they are submerged in flowing water.

All concrete decks must have rigid substructures like those detailed in this booklet, because

uneven settlement of either abutment or pier can result in unacceptably high stresses in the

materials of the decks. The positioning of the abutment and pier foundations is critically

important.

Because the most likely cause of bridge failure is scour, a bridge designer should pay careful

attention to the estimation of general and local scour. Pier foundation depths are specified

according to foundation type and protection method. However when dealing with

substructures and foundation, there are two important issues to consider:-

The general scour area must not be obstructed or the flow will be impeded meaningmore scour damage.

Local scour is caused by turbulence and may be reduced by armouring the bed.Abutments also fail when the soil under the foundation is not strong enough to carry the

combined forces from the bridge structure and the embankment. It's recommended that

spread foundations be used wherever possible, but if adequate support is unavailable, a piled

foundation is required.

If a satisfactorily strong soil is found not too far below preferred foundation level, caisson

support may be considered. The technique is simple if the caissons are short, but the engineer

must take care that:-

The maximum soil reaction at the sides does not exceed the maximum passivepressure at any depth,

The soil pressure at the base remains compressive throughout and the maximumpressure does not exceed the allowable pressure.


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2.4.1 Abutments

Besides carrying the dead load of the superstructure, it's imperative that the abutments of a

bridge must:

Resist the vertical and horizontal live loads exerted on them by vehicles and otherelements.

Retain the approach embankments and the live loads (or surcharge) applied to them. Provide a smooth transition from the road surface to the deck running surface.

The essentials features of abutments are:-

A foundation slab, which transmits the weight of the abutment and the superstructureand its loads directly to the supporting soil, or which forms a capping slab to a system

of load bearing piles.

A front wall with bearing shelf that supports the superstructure and usually retains thesoil of the embankment.

Wing walls or retaining walls which may be separate from the abutments or, if theyare short, may be built integrally with them. These walls retain the road embankment

or river bank adjacent to the abutment and are usually built so as to bisect the angle

between the road and the river bank, though they can be set at any angle to the

abutments and may be built parallel to the road or perpendicular to it.

2.4.3. Design of Gravity and Cantilever Retaining Walls.

A gravity retaining wall is that which resists the lateral earth pressure by its own weight;

whereas, a cantilever retaining wall is that which resists the lateral earth pressure by bending

action. Garg (2005)


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2.4.3.1. Gravity retaining walls

Gravity retaining walls are, therefore, thicker in section, and are made of stone or brick

masonry or sometimes plain cement concrete (PCC).

2.4.3.2. Cantilever Walls of reinforced cement concrete (RCC)

The cantilever walls of reinforced cement concrete (RCC) are more economical, because the

backfill itself is employed to provide most of the dead weight to counteract the lateral thrust.

Both types of walls are liable to rotational turning or translational sliding movements, and

the lateral pressure for the design is computed by Rankines or Coulomb,s theories.

The design of retaining wall should be such that the wall as a whole must satisfy the two

basic conditions: The base pressure at the toe of the wall must not exceed the allowable bearing

capacity of the soil.

The Factor of safety against sliding between the base and the underlying soil must beadequate; value of 1.5 being usually required.

Unyielding retaining walls, such as the bridge abutments, restrained by the deck structure,

do not deform. In such cases, therefore, active or passive pressures would not be developed;

rather, the lateral pressures should be computed as equal to rest value using the coefficient

KBRB (also represented byKBOB).

This value ofKBRB is also very high when the backfill is compacted artificially; say as high as

0.8 or so. It has, thus, been noticed that in such unyielding walls, compaction caused by the

flow of sewers, etc. may lead to residual lateral pressures, considerably higher than the

corresponding values for the uncompacted soil.

(i)Abutment design

While designing the abutment, there is need to carefully analyze and take into account all the

individual characteristics of the site and the superstructure, e.g. foundation conditions, deck

thickness, expansion joints etc. Specifications for the concrete and steel are also important.


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(ii)Reinforced concrete abutments

For small span bridges of (span not exceeding 12m) abutments are provided with a standard

width of 1200mm at the top but the front and rear faces are vertical. This structure is

considerably lighter than its mass concrete counterpart. The bearing shelf is an integral partof the stem. The widths of the toe and heel, and the thickness of the foundation for various

heights, span and bearing pressures are suggested in ORN9.

2.4.4 Reinforced concrete retaining walls

Considerations here are just as in the abutments with reinforced concrete stems. If the

retaining wall is not long, weep holes can usually be ignored. These details apply to thetypical case of walls set at 45 to the abutments supporting road embankments with slopes of

1 in 2.

2.4.5Bearing Shelves

These designs are generally suitable for concrete, composite or timber decks, though the

bearing details will be specific to the deck type. The dowel and bearing pad details are

required for concrete and composite decks. Good drainage and the facility for removal ofdebris are important requirements on all bearing shelves. The road approaches should be

built to prevent water draining onto the bridge, but some water falling on the deck will

penetrate expansionjointseals and leak through to the bearing shelves. This is particularly

likely to occur when no seal at all is provided. A number of drainage configurations are

available, Hambly (1979), but the two main principles to be observed are:-

Slope horizontal surfaces to direct water away from the bearing pads. Provide good access for the removal of stones, vegetation, bird nests and other

debris.


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2.4.6 Piers

Just like abutments, piers perform a support function by transmitting vertical and horizontal

loads from the superstructure via the bearing shelf, stem and foundation slab to the

supporting soil. In most cases, piers stand on saturated soils for most or all of the year: theydo not retain soil embankments but are designed to withstand hydraulic pressures and impact

loads. Piers are often more affected by scour damage than abutments and need to be

orientated carefully with respect to flow direction. Their foundations should be located well

below maximum scour depth.

Reinforced Concrete Piers

Though piers may be built using masonry or mass concrete, reinforced concrete has severaladvantages, notably a more slender stem presenting less interference to flow and hence

causing less induced scour. Superstructure spans are sometimes designed to be simply

supported at the abutments and at the piers. Each span should have one fixed and one free

end. It is usual practice, though not essential, to provide one fixed bearing and one free

bearing on the bearing shelf of each pier. Pier foundations are even more susceptible to

damage by erosion than abutment foundations. They must be constructed on soils of well

established allowable bearing pressure

2.4.7 Determination of allowable bearing pressure,

a) Non cohesive soils

The allowable bearing pressure under foundation in non cohesive soils is governed by the

permissible settlement of the structure due to consolidation of the soils under the appliedloading.

If standard penetration tests have been performed in bore holes, the values of N can be used

to obtain allowable bearing pressure for various foundation dimensions.


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The allowable bearing pressure is that which causes 25mm of settlement under the given

breadth of foundation front to back, B BrB i.e. measured perpendicular to water flow direction on

the assumption that the water table always remains at the depth of at least B BrB below the

foundation level. If the water table can be higher than this, then the allowable pressure is

halved. At a shallow depth, the test seriously under-estimates the relative densities of

cohesionless soils thus the engineer may need to correct the standard penetration values

measured in boreholes before applying the relationships

To allow for this, a correction factor should be applied to the measured values

Where the N values of a fine or silty sand below the water table is greater than 15, the

density of the soil should be assumed to be equal to that of sand having the N value of

)15(2

115 + N

Very loose uniformly graded sands with N equal to 5 or less and subject to rapid changes of

water level are liable to suffer large settlements under load. In these circumstances, either the

sand should be dug out and thoroughly re-compacted or the foundation should be supported

on piles.

b)Cohesive soils

Most cohesive soils at the foundation level are saturated and have an angle of shearing

resistance equal to zero. Provided that no water is expelled from the soil as the load is

applied. This is accepted as the basis for calculating the ultimate bearing capacity of

foundations where the load is applied relatively quickly.

The ultimate bearing capacity of cohesive soils can be calculated from the following

formula.

Ultimate bearing capacity, qBfB = CBuBNBeB = P

Where


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Cu = untrained shear strength (kN/m P2

P)

NBeB = bearing capacity factor

P = total overburden pressure at foundation level kN/mP2

P

= D

Where

= density of soil above foundation level kN/m P3

P

D = depth of foundation level below ground surface.

Values of the bearing capacity factor N BcB for square or circular foundations can be read from

the graph inFigure 8.6 ORN9

For rectangular foundations

NBc(rectangular) B=BB cr N

L

B

+ 16.084.0

Where

BBrB = breadth of the foundation front to back

L = length of the foundation

The undrained shear strength CBuB of soft clays can be measured by means of field vane tests

but the results need to be corrected because the soil is sheared in the horizontal direction.

The value of CBuB to be used in the bearing capacity formula is the vane shear strength

multiplied by the correction factor read from the graph in figure 8.7 ORN 9. This factor is

dependent on the plasticity index of the soil.


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The allowable bearing pressure is one third of the calculated ultimate bearing capacity.

c)Presumed values

Sometimes at the preliminary stage of design there may be no measured values of soil

density or field strengths available. For purposes of estimation, Table 2.2 below lists

approximate values of allowable bearing pressures for different soil types.


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Table 2. Presumed Bearing Capacities

(a) Foundations beadng on ROCK, width not exceeding

3m and the length not more than 10 times width.

Description of rockPresumed

bearing

valuekN/mPzPMassive strong igneous and

metamorphic rocks and limestones10,000

Unweathered medium to fine grained

sandstones4,000

Schists and slates 3,000Hard shales, mudstones and soft

sandstones1,500

Soft limestones 600

(b) Foundations in non cohesive soils at a minimum

depth of 1m below ground level

Presumed bearing

value,kN/mPz

P

Foundation widthDescription of soil

1m 2m 3m

Very dense sands and gravels 600 500 400

Dense sands and gravels 500 400 300

Medium dense sands and gravels 250 200 150

Loose sands and gravels 100 75 75

(c) Foundations in cohesive soils at a minimum depth of

1m below ground level

Presumed bearing

value,kN/mPz

P

Foundation widthDescription of soil

1m 2m 3m

Hard boulder clays, hard fissured

clays, weathered shales and

weathered mudstones800 600 400

Very stiff boulder clays, very stiff

marls600 400 200

stiff boulder clays, stiff fissured clays

and stiff marls300 200 100

Firm clays 150 100 75Soft alluvial clays 75 35 0

Source: Table 8.1 ORN 9


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2.4.8. Foundations on Rock

The foundation designs presented in the preceding sections are for soils readily excavated by

hand or mechanical digger. Modifications may be required to suit individual site conditions,

particularly when bedrock is encountered. Where foundations are set on rock at ground levelor on the river bed, substantial keying will be necessary in the form of steel dowels and

notching.

2.4.9.Run-on Slabs

Almost all earth embankments are subject to settlement. The amount they settle will depend

on the height, the degree of compaction of the material and the strength of the sub-grade.

Relatively uniform settlement can be expected from most embankments until a cause of

uneven compaction is met, such as a bridge or a box culvert with little fill above it. It is

difficult to compact fully the embankment material close to the bridge abutments or the

culvert walls, and the result of poor compaction is more pronounced settlement. The

resulting longitudinal profile is uncomfortable for road users and causes impact loads on the

structure, owing to vehicle bounce. These local depressions in the carriageway close to

drainage structures may be bridged using run-on slabs. They are more easily constructed at

the same time as the structure, rather than afterwards as a remedial measure, and they span

the fill material susceptible to settlement. One end of the slab rests on a small shelf cast onto

the culvert wall or on the abutment ballast wall, while the other rests on well compacted

material several meters away.

Run-on slabs are usually between 3 and 6 meters long. The concrete and reinforcement

details may be abstracted from the culvert detailing, assuming that the slab is resting on good

support for one third of its length, i.e. a 6m slab will have similar details to a 4m wide

culvert lid. Run-on slabs are usually made wide enough to support the kerbs on the approach

roads.

Run-on slabs should not be required where efficient maintenance facilities are readily

available (Hambley 1979).


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2.5 Superstructure

2.5.1Steel/concrete Composite Superstructures

Because they are easy to erect, more durable if well maintained and are light in weight, steel

beams with concrete decking are often a good solution for structures intended to have a long

service life. Through regular maintenance painting can prevent deterioration of the beam

webs and bottom flanges. Where structures are intended to be permanent, the durability of

the steel over a service life of 50 years or more can be achieved more readily by the use of a

cast in situ concrete deck slab (ORN 9). Composite action of the slab and beams is secured

by the use of shear connectors welded to the top flanges of the beams and cast into the

concrete. As an alternative to the solid concrete slab decks almost any bridge can be

constructed of steel universal beams (UB) with a composite concrete deck slab. The main

beams and cross members can be of standard rolled carbon steel sections (yield stress

274N/mmP2

P), with deck slab reinforcement in either mild steel (MS) or high yield steel

(HYS).

However, Steel/concrete composite deck structures have the following advantages:

The deck self weight can be less than that of an equivalent all-concrete structure The off-site prefabrication of the main load carrying elements of the bridge greatly

reduces the work necessary on site, resulting in more rapid construction.

No temporary supports are required during construction of the deck slab, since thesoffit shuttering can be supported directly from the steel beams. This can be a

particular advantage at sites with poor ground conditions, steeply sloping terrain, or

with a fast stream.

Steel is a more reliable material which is supplied with guaranteed strengthproperties, enabling the production of high and consistently reliable structure.


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2.5.2. Reinforced Concrete Bridges

Reinforced concrete is particularly adoptable for use in highway bridges because of its

durability, rigidity, and economy as well as comparative ease with which a pleasing

architectural appearance can be secured. Winter (1972)

For very short spans of about 10ft (3m) to 25ft (7.5m), one way slab bridges are economical.

For somewhat longer spans, concrete girder spans may be used. Most highway spans of

medium length, from 40ft. (12m) to 90 ft. (27m), presently use steel- concrete construction

or composite pre-stressed concrete. In composite construction with structural steel, the

concrete deck is made to act integrally with supporting steel stringers by means of shear

connectors, welded to the top of thee flange of the steel section and embedded in the slab.

2.5.3 Design Standards

Among many, the design of any bridge and/or its superstructure can be in accordance with

B88110 structural use of concrete, BS 5950 structural use of steel. BSI (1979) for HA

loading and AASHTO (1985) for HS20-44 loading, Uganda Bridge Design Manual, and

Uganda Drainage Design manual

2.6 Design Detailing

2.6.1 Vertical Profile over the Bridge

The bridge should preferably be constructed either to a level profile or to a constant

longitudinal grade, if this is required by the road alignment. The steel beams can be

cambered to give some degree of hogging vertical curvature should the equipment be

available, but it may not be economical in most circumstances.

2.6.2 Shear Connectors

In order to provide more restraints to the steel beams and help attach firmly, the deck to the

tensile stress carrying steel beams, connectors are provided. These connectors are the only

links between the concrete slab, acting in compression and bending when under load, and the

steel beams, acting in tension and bending. The largest shear forces act at each end of the


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deck, where the shear connectors are spaced closer together. Welded channel shear

connectors are specified, since they can be produced from readily available material and

fixed locally. The joints, however, should be of good quality welding and be protected from

corrosion both in storage and in use.

2.6.3 Protective Treatment to Steelwork

The degree of protection which the steelwork will require depends on the local environment.

However Particular care should be provided for structures in coastal locations or where there

is significant atmospheric pollution. Types of paint used and surface preparation methods

depends on the availability of materials and equipment. The designer should try to achieve

the following standard in order to ensure a reasonable life to first maintenance:

Grit blasting to remove mill scale, loose rust, welding scale etc., and produce a cleansurface for any coat of painting especially prime;

Application of a multi-coat paint system to a total dry film thickness of 0.25mm.

At least one paint coat should be applied at site after completion of construction, so that

damage to paintwork incurred during transport, steel erection and concreting can be repaired.

2.6.4 Bolts and nuts

Ordinary bolts, grade 8.8 to ISO (1982) together with grade 8 nuts, are specified for fixing

cross members. Alternatives should match the tensile strength of 80 kg/mmP2

P with a

minimum elongation at fracture of 2%.

2.6.5 Bearings

Elastomeric bearings, are specified because they are durable, inexpensive and simple to

install. These bearings consist of discrete strips of black natural rubber, extending over the

full width of the slab soffit at the support point, with a maximum width of approximately

300mm and a maximum thickness of approximately 25mm. At the free end of the span, the

bridge deck locates by friction between the rubber strip and the concrete deck, with no


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positive mechanical means to develop resistance against transverse loadings. At the fixed

end, dowel bars passing through the pad at frequent intervals provide the necessary restraint,

both longitudinally and transversely. To allow rotations to occur and the deck to expand

laterally, the dowel bars are usually fitted with rubber caps where they pass into the concrete

deck slab.

2.6.6Expansion joints

At these relatively short deck lengths, joint movements due to temperature and live loading

are small and are readily accommodated by a simple gap joint, The joints are sealed by a

polysulphide sealant to prevent water penetration. An alternative unsealed joint, more

suitable for bridges on gravel roads, may be provided.

2.6.7Construction joints

It is always best if the deck slab can be cast in one continuous pour. If this will not be

practicable, permissible locations for construction joints should be marked on the drawings.

If a joint is unavoidable, it should be perpendicular to the centre line at a location least likely

to promote corrosion in underlying steelwork.

2.6.8Slab Reinforcement

According to Overseas Road Note 29 Section 9.2.5, the maximum length of reinforcing bar

generally available is 12m. Where a longer bar is required, e.g. for a 12m span bridge, two

bars must be lapped. The lap length should be at least 40 times the diameter of the lapped

bars and laps should be staggered both to avoid a line of weakness and to minimize

congestion of reinforcement. Main bars are positioned with the hooks at alternate ends.

Accurate positioning of the reinforcement is essential in order to maintain the minimumcover of 50mm of well compacted concrete, so that moisture and pollutants cannot penetrate

the slab to reach the reinforcing bar and corrode it.


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2.6.9 Drainage

Drainage should be provided if required, generally as detailed for concrete slab bridges down

pipes must be of sufficient length to ensure that run-off water is discharged at least 150mm

clear below the beam lower flanges.

2.6.10 Parapets, Surfacing and Services

Parapet and surfacing details are the same as for concrete slab bridges. If ducts for services

are required, they can be provided

2.5.11 Provision for Pedestrians and Cyclists

The recommendations set out on pathways for pedestrians and cyclists on concrete decks

apply equally to composite decks. The addition of a 2m wide walkway to a composite deck

of the type detailed here would also require one more ''I'' beam to match those for the road

bridge.

2.7 River Hydraulics and Hydraulic Design

2.7.1 River Hydraulics

In order to secure safety from the effects of flowing water, the engineer has to ensure that the

flowing water under the bridge can pass the structure without causing damage to its parts, the

road embankment or the surrounding land. This topic thus explains how the parameters for

safe hydraulic design can be obtained. Damage can occur in a number of ways, namely:

The river may react against obstructions such as piers and abutments, and scourbeneath them causing failure.

The approach embankments may act as a dam during high floods, sustaining damageor causing more extensive flooding upstream.

A river flowing on a shifting path may bypass a bridge and cut a new channel acrossthe highway

A river may over-top a bridge if sufficient clearance is not provided.


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The hydraulic characteristics of the river must be well understood and quantified so as to be

able to design a structure that avoids the above problems and unnecessary costs. The most

economical structure is usually one which is just wide and high enough to accommodate the

design flood, minimizing the total cost of abutments, piers, superstructure, approach

embankments, relief culverts and river training works.

The hydraulic data required for the design process detailed in the following sections relate to:

Design flood level DFL, flow volume and velocity. Maximum flood level, flow volume and velocity. Bed characteristics - particle size, vegetation. Channel shape and flood plain width. Sedimentation and meander characteristics. Navigational requirements and clearance of floating debris.

Using the above data, determine by calculations the:-

Geometry of waterway required at the bridge site. Backwater caused by the restriction of flow due to piers and abutments. (Scour

caused by the restriction.

River training works required.Calculating velocity using Manning's formula to estimate mean velocity

213

2

1S

P

A

nV

=

Where

V= velocity (m/sec)

A = Area of cross section of the flooded channel (mP2

P)

P= length of the wetted bed across the channel


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S= gradient of the surface or bed slope

n = value of rugosity coefficient taken from the table 4.1 (ORN 9)

There are many methods of obtaining flow volume but only two are discussed here.

(i). The Area velocity methodQ =A.V

Where

Q = volume of flow (m P3

P/sec)

A= cross sectional area (mP2

P)

V= mean velocity of the water (m/sec)

Area velocity method is necessary where the river tops its banks during flood.

(ii).

The Rational Formula

)/(6.3

3 smCIA

Q =

Where

Q = volume of flow (mP3

P/sec)

C= Catchment coefficient

I = Rainfall intensity (mm/hr)

A = Area of the watershed/ catchment.


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In design of any bridge structure, much as excessive rainfall is unpredictable, it should be

carefully designed for. Meteorological data should be used carefully to narrow or remove the

range of uncertainty in river hydrology.

Every designer needs to select a Design Flood Level (DFL), a design discharge or flowvolume and a design velocity, on which to base calculations of waterway geometry,

foundation depths, scour protection and vertical clearance.

The design flood is the maximum flow that can pass through the bridge without:-

Unacceptable disruptions to traffic. Endangering the pier and abutment foundations with respect to scour Damaging approach embankments Causing flood damage on the upstream side of embankment.

It should however be noted that the design flood is not necessarily the highest flood. The

highest flood is a rarely occurring flow that it's uneconomical to include in the design flood

but which may be considered when designing superstructure and piers of the bridge.

Where the river is narrow enough, it can be bridged with a single span. The abutments are

built clear of the level of the design flood and hence there is no restriction to river flow i.e.

no river training, no backwater or additional scour is expected as a result of the bridge

presence. For a wide flood channel, the superstructure is longer and will be very expensive if

piers are not used. Both pier and abutment foundations are below DFL and thus require

protection from scour.

The abutment walls and piers will sabotage the design flood. This restriction causes

backwater and additional scour of bed which must be considered while designing the

foundation.

(i) The Hydraulic Design Process.

The steps involved in the design process include:-

1. Establish the height of the infrastructure i.e. clearance above the DFL


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2. Establish trial positions for the abutments according to the bed shape at the proposedcrossing.

3. Make a provisional decision concerning the number of piers that will result in thelowest overall cost of the superstructure, piers and abutments.

4. Calculate the general and local scours due to the abutments and piers and draw theworst case profiles in the cross sections.

5. Check that the backwater caused by the restriction to flow doesn't cause damage tosurrounding land upstream of the bridge or affect the height set for the superstructure.

6. Prepare preliminary designs of abutments and piers.7. Check scour and backwater effects and make adjustments as required, reclaiming the

effects of any changes to the waterway.

8.

Calculate the cost of the superstructure once its length is decided and the cost of thesubstructures, embankments, river training works and relief culverts.

9. In order to obtain the most economical design, or to compare the cost of the structures catering for the different design floods, it may be necessary to repeat the above

procedure on the basis of alternative waterway conditions.

(ii) Bridge Height.

The waterway below the superstructure must be designed to pass the design flood and thefloating debris carried on it Table 5.1 below shows recommended vertical clearance at DFL

minimum measurements for a vertical clearance between the lowest part of the

superstructure and the DFL, taking in to account backwater effects. This clearance should be

increased on rivers with a history of unusually large floating items or navigational

requirement


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Discharge

(mP3

P/sec)

Vertical clearance

(mm)

300 1200

(iii) Positioning of Abutment

Choose a position bearing in mind the guidelines at the beginning of hydraulic design then

check for scour using flow charts and spacing of abutments and piers.

(iv). Scour

Scour is the erosive effect of water flow on the river bed or banks. Bridge works may alter

the existing scour pattern by restricting the free flow of the stream and/or causing local

changes to the current. Approximately half of all river bridge failures are due to scour alone

(ORN 9)

However, there are four types of scour and they include:

natural scour and channel shifting on alluvial rivers scour caused by changes to the river upstream or downstream of the bridge site General" scour caused by reduction in the channel width at the bridge works . "Local" scour at the base of piers, abutments and river training works where these

divert the general flow.

At the bridge site provide estimates for general and local scour.


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Chapter 3

METHODOLOGY

A bridge just like any other structure must be designed to resist all loads and forces that mayreasonably be exerted on it during its design life. The methodology used here was meant to

address and aim at achieving the objectives outlined in section 1.4 above in a way that will

appropriately solve the identified problem. And consider the design to take into account the

cost implications and environment impact of the bridge structure. In this chapter, procedures

to obtain which parameters are likely to occur and the magnitudes and combination of loads

that produce maximum stress are listed.

This project starte

design of a composite bridge deck

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