An Introduction to Concrete Bridges

C O N C R E T E B R I D G E D E V E L O P M E N T G R O U P

The Concrere Centre

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A N INTRODUCTION TO I , ,

CONCRETE BRIDGES

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Our experience includes all forms of bridges from some of the world’s longest cable-stayed bridges and suspension bridges to multi-span viaducts, arches and moving bridges. We are experienced in all types of concrete bridge design including incrementally launched, balanced cantilever and segmental construction. Indeed, our bridge team is making connections throughout the globe.

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The Concrete Bridge Development Group

- COICRETf I R l D G f OtVf lOPYENT GROUP

The Concrete Bridge Development Group aims to promote excellence in the design, construction and management of concrete bridges.

With a membership that includes all the sectors - bridge-owners and managers, contractors, designers and suppliers - involved in the concrete bridge industry, the Group acts as a forum for debate and the exchange of new ideas.The Group provides an excellent vehicle for the industry to coodinate an effective approach and to enhance the use of concrete.

The Concrete Bridge Development Group has many set objectives including the identification and commissioning of future bridge research needs, the promotion of best practice and the provision of a focus for the bridge industry. Not least has been the specific effort to aid and support students and young engineers.

This includes:

Students design competition Provision of brochures and technical guidance Contribution to IT material (e.g.Calcrete) Support of education establishments (e.g. Second Severn Bridgevisitors Centre) Partner in Constructionarium (New on-site training programmme in conjunction with ClTB and industry)

CBDG will continue to seek ways to offer aid and support so that future generations can continue to design in and use concrete to maximum effort.

For further details please contact: Concrete Bridge Development Group Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: 01276 33777 Fax: 01 276 38899

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C O N C R E T E 0 R l D G E D E V E L O P M E N T G R O U P

The Concrete Centre’”

AN INTRODUCTION TO CONCRETE BRIDGES

An Introduction to Concrete Bridges

An Introduction to Concrete Bridges First published 2006

Q Concrete Bridge Development Group

ISBN 1 904482 26 0

Published for and on behalf of the Concrete Bridge Development Group by

The Concrete Society Riverside House 4 Meadows Business Park Station Approach Blackwater, Camberley Surrey GU17 9AB Tel:+44(0)1276607140 Fax:+44 (0)1276607141 E-mail: [email protected] k Website: www.concrete.org.uk

All rights reserved. Except as permitted under current legislation no part ofthis work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to the Concrete Bridge Development Group.

Although the Concrete Bridge Development Group (limited by guarantee) does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Group, its servants or agents.

Further copies of this title are available from the Concrete Bookshop, part of The Concrete Society, at www.concrete,org.uk and +44 (0)700 4 607777.

Printed by Cromwell Press, Trowbridge, UK.


CONTENTS

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

......................................... 5

.............................................................................................. 5

Aesthetics ...................................................................................................................

Bridge decks ...................................................................................

Loading ...................................................................

................................................................... 10

Material selection .... ................................................................. 14

Precast concrete in bridge construction ................................................................ 1 8

History of pre-tensioned concrete be

Durability and detailing ................................

Construction planning ........

....................................... 21

Inspection and maintenance ......................................................................................... 26

Health and safety .........................................................................

Future t r~nds .......................................................

Further r ....... ...................................................................... 28

Acknowledgement

This publication was kindly supported by The Concrete Centre. Please visit them a t www.concretecentre.com for further information



1. INTRODUCTION

Concrete will be found somewhere in all bridges- in thefoundations, abutments, piers, retaining walls and deck. For a bridge deck's main supporting members, there may be a choice berween in-situ or precast concrete, structural steel beams or a combination ofthe two materials - known as composite construction.

Concrete isversatile. ltcan be cast toany shapesodifficult geometrical requirements, such as a bridge with pronounced can be easily satisfied. Concrete bridges can be d span/depth ratios, so shallow decks are

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2. BRIDGETYPES

There are several basic bridge types that are usually adopted for the construction of concrete bridges with various combinations of layout used for the superstructure (deck) and substructure (supports and foundations).

2.1 Slab bridges For short spans, the simplest form of bridge deck is a concrete slab. Slab bridges can be cast in-situ in either reinforced or prestressed concrete. In longer spans, the self-weight of the slab may be reduced by using polystyrene void formers in the construction. Solid slab bridges may also be constructed from precast prestressed concrete beams - normally inverted T-shaped beams - with in-situ concrete infill and topping. This form of construction is economical for spans up to about 18m.

A series of spans Over several piers can be constructed as an'integral' bridge, without movement joints. In this type of bridge, either in- situ concrete or precast concrete beams can be used, with the joints

2.2 Beam and slab bridges Beam and slab bridges are generally constructed of precast prestressed concrete beams with an in-situ concrete slab. In-situ beam and slab construction, known as a ribbed deck, is rarely used now but can be found in older, existing bridges.

Beam and slab bridges are economical for spans from 12m to 36m, but the span may be limited by the length of beam that it is permissible to transport. In the UK this is normally 30m. Beyond this a special order is required from the Department for Transport, which permits lengths up to 40m when the beams are transported to motorway sites via the motorway network. Longer-span box beams can be cast in-situ and post-tensioned.

.%;p Figure 1: Slab bridge on A30, Bagshot, Surrey Figure 2: Beam and slab bridge at Oyster Creek, Gambia


between the precast beams filled with in-situ concrete.The deck may be supported by elastomeric bearings at the piers, and longitudinal movements are resisted by dowels or anchors. Alternatively the beams may be cast into the pier structure.This arrangement, without any movement joints, has typically been used for bridge structures with overall lengths of up to loom, although longer lengths are possible. It has become popular because of the problems caused by the penetration of water and de-icing salts through movement joints in other forms of construction.

2.3 Framed bridges Slabs and abutments are often connected monolithically to form a portal-frame.This type of construction is usually cast in-situ and can be used instead of slab, or beam and slab bridges. As with integral bridges, there are advantages to be gained by avoiding movement joints.

For short spans, a concrete culvert can be used as a simple form of framed bridge. Box-shaped reinforced concrete culverts are suitable for spans of up to 6m and the units can be precast.

2.4 Concrete arches Arched solutions are ideally suited to utilise the principal qualities of concrete working in compression as long as height clearance considerations below can be fully satisfied. The behaviour of the arch will depend on the rigidity of the foundations and the type of backfill used. In-situ cancr poured on formwork is normally

will affect the substructures and vice versa, so a full appreciation of their interactive behaviour needs to be understood.

Continuity is the structural connection of adjacent spans of a bridge to eiiminate joints in the deck between spans. Continuity is usually provided to carry imposed loads more efficiently and to avoid maintenance problems associated with expansion joints. All spans of a bridge - not only at intermediate supports but also between decks and aburments - are thus connected together longitudinally.

2.6 Long-span bridges The use of a fully supported soffit using formwork for long-span in- situ concrete bridges is expensive, and may also be difficult. These bridgesareoften constructed incrementally using travelling formwork or concrete sections cast on stationary formwork, with the bridge pushed out from the abutments - a system known as'incremental launching'. These bridges are frequently post-tensioned.

Segmental bridges are made from precast concrete units, stressed together with strands or bars. The units are normally counter-cast against each other to ensure a good fit, then glued together in- situ. Spans can be built out from the abutments and from the piers. When building out from a pier, rhe deck is often cantilevered in both directions so that the sections under construction balance each other. The n is usually cellular or box shaped, with the deck slab c out transversely on either side.

Bridges with spans of over 250m may be designed as arches, or as suspension or cable-stayed bridges. Arches have been used successfully for spans of up to 400m.

used for longer-span monolithic arches, whereas precast concrete is available for short-span two- or three-pinned arches from specialist manufacturers. Arch bridges should be an aesthetically pleasing solution where the site layout and foundation conditions permit.

Suspension and cable-aayed bridges may have concrete decks, either of in-situ concrete - constructed using travelling formwork - or precast concrete segments stressed together. In these bridges the primary means of deck support is achieved using suspension cables and hangers.

Figure 3: Arch bridge - Scammonden. M62. Yorkshire

2.5 Integral bridges and continuous construction Concern over the durability of bridges constructed using movement joints has encouraged the use of integral bridges, especially for highway structures.These are bridges built without movement joints in the carriageway surface and may also avoid the need for bearings. In the UK all highway bridges less than 60m in overall length and less than 30" skew must be built using integral principles in order to maximise their performance and durability (BA42/96 The Design of

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3. AESTHETICS

Bridge appearance is as important as economical and efficient design. Concrete is a very versatile material that can be moulded and finished in a variety of forms to give the desired effect. Bridges are often designed to last 100 years or more, so it is essential that they are integrated into the environment in a manner that complements and enhances the surroundings.

Overall appearance can be subjective but general advice is available in documents such as BD41 The Design and Appearance ofBridges published by the Highways Agency, which encourages designers to aim for slender decks in relation to the headroom, balanced span openings and minimising the bulk of the end supports.

Even with standard prestressed bridge beams, there is ample opportunity for the designer to influence the appearance of bridges utilising precast concrete components. Individuality can be

expressed in the deck support structure (bankseats, abutments, piers and crossheads), the edge of deck treatment and in the combined overall effect of structure with landscape.

Visual effects can be created, contrasting deck edges with shadow lines or by varying the ratio of deck-edge cantilever or string-course depth to overall deck thickness. Continuous decks can be designed with shallower elevations that are pleasing to the eye. Special concrete finishes and textures are also possible, especially where the public will pass close to the structure.

4. BRIDGE DECKS

In the great majority of modern bridges a concrete deck slab provides the structural support for the asphalt running surface. The thickness of the concrete slab will vary, depending upon the form of bridge deck that supports it.The deck is defined as that part of the superstructure that spans longitudinally between supports.

Reinforced concrete solid slab

Voided slab

Single cell box section

Precast beam and in-situ slab deck

Figure 5: Typical deck sections

Multi cell box section

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4.1 Deck types For short spans a solid reinforced concrete slab, cast in-situ, is the simplest and most cost-effective solution. The flat soffit of the in-situ reinforced concrete makes the formwork, fixing of reinforcement and concreting very simple with a corresponding reduction in cost.

pans increase, there will be a need to increase the reinforcement introduce some prestressing; the deadweight of the deck

itself can be reduced by introducing voids within the slab using polystyrene formers.They are usually of circular section to enable the concrete to flow under them to the deck soffit. It is however, that thesevoid formers are firmly held in po flotation and that the concrete under the voids is well compacted. Reinforced concrete voided slabs are more economical than the prestressed concrete alternative up to about 25m span. The exact change-over point depends on comparative costs of reinforcement and prestressing at the time of construction.

If the location of the bridge does not suit in-situ slab construction then precast pre-tensioned concrete beams may be used. Inverted T-beams placed side by side and infilled with concrete provide an alternative to the in-situ reinforced concrete slab.

For longer spans, beams and slab construction is used with a 200- 250mm concrete deck slab supported on precast pre-tensioned beams spaced at 1.0-2.0m centres.

Precast beam construction utilises high quality, factory-made components that can be quickly erected on particularly useful when bridging over live waterways where interruptions to traffic must be minimised. The standard beams currently in use are the M, U, Y and super Y beams which can be used for spans up to 40m. Detailed information may be obtained from the Prestressed Concrete Association (see www. britishprecast.org) or its member companies.

4.2 Construction methods for longer spans The span-by-span method of construction is used in multi-span viaducts with individual spans of up to 80m. A span plus a cantilever of about one-quarter the next span is first constructed. This is then

tressed and the falsework is moved forward and a full span length is then formed and stressed back to the previous cantilever. In-situ construction is used for smaller spans.

As the spans increase, the cost of falsework also increases. To minimise this, the weight of the concrete to be supported at any one time IS reduced by dividing the deck into transverse segments. These segments, which can be in-situ or precast, are normally erected on either side of each pier to form balanced cantilevers and then stressed together. Further segments are then added, extending the cantilever to midspan where a small closure is formed of in-situ construction to make the spans continuous. In precast construction, the segments are match-cast against one another and jointed with epoxy resin before being stressed together.

Straight or curved bridges of single radius and of constant cross- section may be built in short lengths from one end and incrementally launched. Completed sections are pushed off the casting bed, with the whole deck travelling forward and propelling the leading face towards the next support.

Cable-stayed bridges are normally built using a form of precast segmental cantilever construction, successively building out from the support towers.

Figure 6: Medway Bridge - In-situ balanced cantilever bridge

I - Figure 7: Launched deck,Taiwan High Speed Rail

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Figure 8: Segmental construction

Figure 9: Span by span construction - A16, Brebant, Holland

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$: p 7 Figure 11: Incremental launch - Pushing ram, Medway Bridge Figure 10 Incremental launch - Ceirog Viaduct, Nor th Wales

4.3 Span ranges for concrete bridges Figure 13 below illustrates the range of spans that may be achieved 1 using various types of concrete construction.

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Figure 13: Span ranges for concrete bridges

IN SITU RC solid slab

RC voided slab

Prestressed voided slab (Internal bonded)

Incremental launching

Span by span

Span by span

(Supported on launching truss)

(Supported on scaffolding)

Segmental balanced cantilever

Arches

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Inverted T beams cast into slab

M, U and Y beams with deck slab

Segmental balanced cantilever (erected by Crane)

PRECAST

Segmental balanced cantilever (erected by lifting gantry)

1 I I I I I 1 I

0 50 100 150 200 250 300 350 400

Cable stayed bridges by balanced cantilever - - Definite range - Possible range extension

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5. LOADING

Whether the bridge is carrying a road, railway, waterway or just pedestrians the deck will be subjected to various types of load:

rn Self-weight W Environmental, e.g. wind, snow, temperature effects

rn Traffic rn Accidental loads, e.g. impact rn Temporary loads, e.g. during construction or maintenance.

Bridges in the UK are generally designed in accordance with British Standard BS 5400, which gives details of the load combinations to be used for various bridge applications. Additional standards are published by the Highways Agency and Network Rail to supplement British Standards Many of these standards are being upgraded to Eurocodes and in time these will state the basic requirements for bridges in the UK and other European member states. Specific requirements will be incorporated into a National Annex.

6. ANALYSIS

The analysis of a bridge should be undertaken by a designer who has received sufficient training and experience. The method of analysis selected should be appropriate to the type of bridge being considered. On many concrete bridges the bending moments and shears resulting from the application of traffic load on a bridge deck are not necessarily carried by just the portion of bridge deck immediately under the load. When the affected area deflects, the deck bends transversely and twists, thereby spreading load to either side.The assessment of load that is shared in this way and the extent to which it is spread across the deck depends on the bending, torsion and shear stiffness of the deck in both longitudinal and transverse directions. Computer methods are generally used to analyse the load effects.The most versatile of these is the grillage analysis, which treats the deck as a two-dimensional series of beam elements in both the longitudinal and transverse directions. This method can be used for slab, beam and slab-and-voided slab decks where the cross-sectional area of voids does not exceed 60% of the area of the deck.

Box girders are now generally designed as one or two cells without any transverse diaphragms.These are usually quite stiff torsionally but can distort under load giving rise to distortional and warping stresses in the walls and slabs of the box. It is then necessary to use three- dimensional analytical methods such as 3D space frame, folded plate (for decks of uniform cross-section) or a 3D finite element method.

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7. SUBSTRUCTURES AND FOUNDATIONS

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Figure 14: Substructures - MZO, Maidstone, Kent

A bridge is supported a t the ends on abutments and may have intermediate piers. Both abutments and piers are usually constructed from reinforced concrete. The positions of the supports and the lengthsofthe spansaredetermined bythe topographyoftheground and the need to ensure unimpeded traffic under the bridge.

Piers and abutments carry high loads, and their foundations may require piling. The design and method of construction of the foundation will depend upon the ground and groundwater conditions.

The substructure of a bridge is particularly a t risk from damage caused by flooding, overflows from blocked drains, freezing and thawing weather conditions, and exposure to de-icing salts from sprayed or leaking water.The concrete in the substructure must be capable of resisting all forms of attack. Design for durability is vital.

The design of the substructure and foundation requires an understanding of the interaction between the substructure and the ground on which it is to be built and the structure to be supported. A thorough site investigation should be carried out. However, it may not be possible to obtain precise information about the soil conditions, in which case the designer must make sound judgements based on the data that can be obtained.

The cost of the substructure is often greater than that of the superstructure, and it is important to carry out the bridge design as a whole rather than allow the design of the deck to impose unnecessary restraints on the design of the substructure. Many bridges are designed to be continuous structures that are integral with the abutments: for such bridges the deck and the substructure have to be designed together.

The effects of the construction of the bridge on the progress of other parts of the work, such as earth moving, must also be taken into account. The substructure must be designed so that it can be constructed as quickly and easily as possible: the emphasis should be on simplicity and 'buildability: which will invariably contribute to economy. A t the same time, the substructure must have an attractive appearance which is in keeping with the bridge and its surroundings.

7.1 Thesite On restricted sites the choice of substructure is often controlled by the space available and the plant that can be used. In particular, large-bored piles and raking piles require a considerable amount of space. Overhead power lines can seriously restrict the use of plant. The interaction of construction with existing traffic is an essential factor in the design of the work. If it is possible to acquire additional land for construction, this may be cheaper than the cost of delay caused by extending the programme.

Groundwater conditions will affect the design: for example, it may not be possible to lower the water table due to the effect it might have on the stability of neighbouring structures. In this case it will be necessary to construct the foundation under water, and this may require the design to be in mass concrete rather than reinforced concrete.

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Figure 15: Boring rig

7.2 Site investigation The purpose of the site investigation is to provide information about the soil profile and groundwater conditions across the site. The extent of the investigation will depend upon the nature of the site and the type of structure to be built. Trial pits and deep boreholes will provide a general picture of the ground and groundwater conditions. A more detailed study of samples from boreholes and in-situ tests in trial pits will give further information, but it should be borne in mind that the precise position of the foundations may not be known at the time the survey is carried out. All ofthe information obtained must be carefully examined and interpreted into data to be used for the design.

7.3 Foundations The choice of foundation for an abutment or a pier is normally between a spread footing and piling. Where ground conditions permit, the spread footing will provide a simple and economic solution. Excavations for foundations should be left open for as short a time as possible before the concrete is placed in order to limit ground disturbance.

Piling will be needed where the ground conditions are poor and cannot be improved, the bridge is over a river or estuary, the water table is high or site restrictions prevent the construction of a spread footing. It is sometimes possible to improve the ground by consolidating, grouting or applying a surcharge by constructing the embankments well in advance of the bridge structure.

Differential settlement of foundations needs to be controlled, and the construction sequence will have an effect on settlements. In the early stages of construction, abutments may settle more than piers but piers will settle later when the deck is constructed.

7.4 Abutments The overall appearance ofa bridge structure is very much dependent on the abutments and piers.

The structural design of the abutments is closely related to that of the bridge deck, and for an integral bridge the structure must

be designed as a whole. Abutments are usually constructed of reinforced concrete but, in suitable circumstances, mass concrete without reinforcement may provide a simple and durable form of construction.

If the deck is constructed before the main excavation is carried out, contiguous bored piles or diaphragm walling can be used to form an abutment wall.The cost of this type of wall construction is high, but can be offset against savings in the amount of land required, the construction time, the cost of temporary works and by minimising the disruption to traffic. A facing of in-situ or precast concrete or blockwork will normally be required after exca earth construction may be suitable where there is an embankment behind the abutment, and here precast concrete facing is often used. Replacement of the ties during the life of the structure is difficult so the selection of appropriate ties and fixings is very important.

Where a bridge is constructed over a cutting it is usually possible to form a bankseat abutment on firm undisturbed ground. Alternatively, bankseats may be constructed on piled foundations. However, where bridges over motorways are designed to allow for future widening of the carriageway, the abutment may be taken down t that it can be exposed at a later date when the widening is carried out.

Figure 16: Skelton Bridge 12A, Cleveland, showing abutments and wingwalls

7.5 Wingwalls The design of wingwalls is determined by the topography of the site and can have a major effect on the appearance of the bridge. Wingwalls are often taken back at an angle from the face of the abutment for both economy and appearance.

On integral bridges wingwalls should be aligned parallel with the span direction and this has the benefit of minimising soil pressures.

In-situ concrete is normally used, but precast concrete retaining wall units are available from precast concrete manufacturers. Concrete crib walling is also used for the construction of wingwalls and its appearance makes it particularly suitable for rural situations. Filling

ted carefully to ensure that it does not flow out, and the fill must be properly drained.

It is important to limit the differential settlement between the abutment and the wingwalls. This problem can be overcome by cantilevering wingwalls from the abutment or by supporting the whole structure on one foundation. If movement joints are selected then detailing should either include some form of shear connection or incorporate some means of disguising relative movement.

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Figure 17: Piers - Docklands Light Railway, London

7.6 Piers The simplest and most economic bridge pier is a vertical member or group of members with a uniform cross-section Sections can be recrangular, square, circular or elliptical The shaping of piers

hetically pleasing but complex shapes will increase the cost unless considerable re-use of forms is possible Standardisation of shapes and sizes for several bridges in the same contract leads to economy The durability of concrete in the piers will be helped if the design is simple, the detailing good and the deck overhangs the pier

Raking piers and abutments may help to reduce the span for high ges but they do require expensive temporary propping and

support structures This complicates the construction pro considerably increases costs

7.7 Design considerations for substructures The design of the substructure, as for any structure, must ensure stability, structural safety and serviceability

It i s usual to assume that an acceptable amount of movement of an abutment or wingwall will occur, and this is taken into account in the design. Normally the backfill is a free-draining material and the wall has a satisfactory drainage system built inro the structure if these conditions are not satisfied then higher design pressures

must be used If fill is to be compacted behind the abutment then due allowance must be made for the pressure due to compaction Traffic loading and vibration caused by traffic must also be taken into account If bankseat abutments are used, their stability against slipping must be checked carefully The calculated resistance in front of the toe of a wall should be ignored if there is a possibility of excavation in this area for drainage or utilities

Creep, shrinkage and temperature movements in the bridge superstructure can create for the abutments, and these must be determined Differential s ent is a factor to be considered Piers and, to a lesser extent, abutments are vulnerable to impact loads from vehicles or shipping and must be designed ro resist impact or be protected from it Substructures of bridges over rivers and estuaries are subjected to scour and lateral forces due to water flow, unless properly protected

It is difficult to accurately predict bridge settlement by calculation and any predictions uld be compared with a study of case histories of structures on similar ground The design of a bridge to control differential settlement may the foundations being larger than those required solely for

The durability of the substructure will be improved by proper consideration being given to all aspects of its design and construction Careful selection of materials and mixes for the concrete, the design and detailing of the structure to prevent damage due to water and de-icing salts, and supervision and control of the qualrty of the work are all essential for durability

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8. MATERIAL SELECTION

8.1 Ready-mixed concrete 8.2 Why use ready-mixed concrete? Concrete has been a major construction material since Roman times and remains so today Originally, all the ingredients (cement, fine and coarse aggregates and water) were mixed on the building site In the 193Os, however, the idea of mixing a t a dedicated off-site plant and delivering to local sites was first originated It was the birth of an industry that developed rapidly and soon became recognised throughout the world

Established in the UK since the 19505, the ready mixed concrete industry offers nationwide coverage from approximately 1,200 batching plants All major suppliers are certified with organisations such as the Quality Scheme for Ready Mixed Concrete (QSRMC), an independent assessment organisation approved by the National Accreditation for Certification Bodies (NACB) This ensures that the customer will receive a consistent, quality product that will meet the specification and be fit for the purpose intended, providing that it is

placed, compacted, cured and protected to the required standards

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Figure 18: Typical pour - I U

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Concrete is a basic construction material consisting mainly of naturally occurring materials but i t s production in large volumes to meet rigorous modern specifications requires considerable expertise and experience While it is possible to mix it on site, ready-mixed concrete is now used in all but exceptional circumstances for the following reasons

Design options enhanced

Pre-sales advice on mix design and the concreting operation Production and technical support under the control of dedicated and experienced professionals Independent verification provides external assurance that the quality of concrete supplied conforms with that ordered

w Coordinated and flexible supply in terms of quantity, rate and back-up facilities, normally via a central despatch office that coordinates all deliveries in a defined area

w Increased site space Availability of up-to-date technology, materials and plant Increased speed of construction.

-711 I

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Figure 19: Modern ready-mixed concrete plant I

tigure LU: lypical ready-mixed concrete delivery truck

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~

LOW DENSITY E.G. FOAMED, HIGHLY AIR-ENTRAINED

MASS

PUMPED

Foundations

~~ ~~~ ~

Free-flowing concrete for non-structural uses, e.g.backfill for abutments and retailing walls

Normally low cement-content for large foundations and bases or backfill

Designed mix normally with increased fines to allow concrete to be placed by a specialist pump

Figure 21: Diagram of main bridge components

8.3 Ready-mixed concrete in bridges More than one million cubic metres of ready-mixed concrete are used every year in bridge constru n throughout the UK. They are usually high-profile structures and are very visible in many landscapes.

The aesthetic qualities of a bridge are of environmental significance both in terms of its design and the appearance of the large areas of concrete that are normally visible. Such considerations are very much in the minds of approval bodies and local planning offices when bridges are first planned and then designed. The use of concrete offers many alternatives in shape, form, colour and type of surface finish.

The use of ready-mixed concrete guarantees that these high standards can be achieved and that is why it plays such a prominent role. It has been used in all modern bridges including major structures such as Second Severn Crossing, Skye Bridge, the Dee Crossing and the new Medway Bridges on the Channel Tunnel Rail Link and A2/ M2 Motorway improvement.

Bridge exposure to all weather conditions and heavy traffic usage is the ultimate test for concrete performance. The consistent quality of ready-mixed concrete helps to provide the best solution to the variety of demands placed upon it.

Bridges are normally designed for a 120-year lifespan and the durability of the structure and all of i ts components are therefore

of paramount importance. The high compressive strength, the resistance to fire and impact, the adaptability to meet various structural and environmental demands by using specialist materials and mix designs, are all typical examples of h contribute tothe longevity ofa bridge structure.Th can also be used to good effect, when necessary, in the repair and maintenance of a bridge over its lifetime.

8.4 Specification of concrete mix The flexibility that it offers both designer and contractor is an important factor in meeting these demands. A typical mix for use throughout bridge construction (refer to BS 8500) could be C40/50 - a design mix which gives a compressive cylinder strength of 40N/mm2 and a compressive cube strength of 50N/mrn2 after 28 days. Nevertheless, different components of the bridge may need individual variations to aid placement, or to meet end-use requirements, while still meeting the specified strength, e.g.:

Piles - higher workability is required W Deck - the use of an air-entraining agent may be required to

increase frost resistance. All of these mix variations can be easily accommodated by a ready- mixed concrete supplier.

I concrete type uses in wage construction

I sections I HIGH STRENGTH b6ONIMM’) I Significantly increased span-to-depthratio allowing thinner beam I

I SELF-COMPACTING I To provide increased”flow”characteristics to ease placement in areas of dense I reinforcement or difficult access, e.g. voided deck-slab, whilst producing dense uniform concrete without any need for compaction

I LIGHTWEIGHT The use of lighter fly-ash aggregates for superstructure concrete produces I less loadina. and therefore smaller foundation are needed

19

An Introduction to Concrete Bridges ~ ~ ~~

8.5 Grouts Specially designed grouts are used within ducts in post-tensioned bridges to protect the steel strand from corrosion. These should comply with The Concrete Society Technical Report 47 (see Further Reading). The use of pre-measured and mixed materials should be the first choice for quality, but this does not exclude combinations of controlled materials on the basis that the quality of the end product is the important factor to ensure adequate protection of prestressing tendons.

8.6 Admixtures A variety of chemical admixtures can be included in concrete mixes to provide buildability benefits and to meet specific demands, for example:

Air-entraining agent - increased frost resistance Plasticiser - improved flow characteristics (easier placement)

W Accelerator - early high strength (to counter time constraints)

Colouring pigments and special aggregates can be used for aesthetic purposes

8.7 Reinforcement Most structural concrete is reinforced, normally with steel bars or fabric It is essential to ensure that such reinforcement is adequately protected by a minimum cover of good quality concrete to counter the varied climatic conditions experienced in the UK

A new generation of non-ferrous products is becoming available to replace steel with the aim of increasing the durability of concrete structures Because of their exposure to climate and de-icing salts, bridge design and construction is a t the forefront of such technology

Figure 23: Reinforcement River Leen Bridge, Nottingham

Without adequate protection, steel in bridges may corrode, particularly in countries like the UK where de-icing salts are used during the winter months Hence, careful consideration must be given to the protection of reinforcement and prestressing tendons The type of concrete must be correctly selected and the degree of exposure may demand the use of stainless steel reinforcement, especiallyon parapetedge beamsor in thevicinityofdeck movement joints Prestressing tendons may be galvanised in addition to other layers of corrosion protection

Figure 2 4 Non -ferrous reinforcement

20


9. PRE-TENSIONED A N D POST-TENSIONED CONCRETE

There are two main types of prestressed concrete

9.1 Pre-tensioned concrete Steel tendons are stressed by jacks anchored to fixed blocks in the casting yard. Concrete is then placed in moulds or casting beds

tendons When the concrete has hardened sufficiently, the tendons are released. As they try to return to their original length, large compressive forces are applied to the concrete.

This process is nearly always carried out in a factory environment and is the usual way of manufacturing precast prestressed bridge beams.

9.2 Post-tensioned concrete For this type of construction, normally associated with in-situ concrete, the tensioning forces are applied to the tendons after the concrete is placed and hardened. Ducts are incorporated into the formwork and the concrete is placed around them. After the concrete has hardened, the stressing tendons are threaded through the ducts and are stressed using jacks. A special grout is injected into the ducts around the tendons to provide bond and protection from corrosion Post-tensioning is mainly carried out on site although it has been used for special precast beams.

Traditionally, post-tensioned bonded tendons have relied on cement grouting for protection. However, inadequate detailing and workmanship in the past have led to corrosion of tendons, the condition of which cannot be monitored. Th internal unbonded tendons or external tendons, the condition of which can be monitored ny time, are currently preferred to the bonded type.

Unbonded tendons are normally protected by placing them in ducts, which are subsequently filled with grease or wax. Alternatively, external tendons can be left exposed, but cted by galvanising or epoxy coating.

Staae 1 -Tendons are tensioned and anchored

I I

Stage 2 -Concrete is placed

Stage 3 - Tendons are released and force is transferred to concrete

Stage 1 -Concrete cast with tendons in duct

Stage 2 -Tendons tensioned after concrete has hardened I

Stage 3 -Tendons secured at anchorages

x

Prestressing using pre-tensioned tendons

Prestressing using post-tensioned internal tendons

I Figure 25: Pre-tensioned and post-tensioned concrete

21

4n Introduction to Concrete Bridges

10. PRECAST CONCRETE IN BRIDGE CONSTRUCTION

10.4 Durability of prestressed bridge beams The use of precast concrete is both widespread and effective in modern bridge construction Many modern bridges are constructed with both in-situ and precast concrete.

10.1 What is precast concrete?

Joints in decks area common cause of problems in all types of bridge. A survey of 200 concrete highway bridges commissioned by the Department ofTransport highlighted the problems of reinforcement corrosion by water carrying de-icing salts leaking through joints or being splashed onto the reinforced concrete elements of decks and

Precast concrete IS manufactured away from the construction site, In an efficient factory environment to very high standards and without any concerns about adverse weather The quality ofthe concrete can be tightly controlled and the formwork and steel reinforcement or prestressing tendons can be prepared and positioned to extremely high tolerances. After it has been poured, the concrete can be cured effectively, again without interference from the weather, to maximise its performance (especially its durability) and appearance. Importantly, it can be stored and delivered to site a t precisely the right time in the construction programme

substructures.

10.2 Why use precast concrete?

The survey showed that the prestres comparison, had performed extremely we1

w Quality of construction (the required cover to reinforcement IS

more easily achieved in prestressed concrete and so there is less chance of corrosion) Prestressed beams are constructed with high-strength concrete (60-80Nlmm2 at 28 days) with low waterkement ratios. The waterkement ratio is generally specified to be less than 0.45, but values below 0.4 are frequently achieved through the use of efficient plasticisers and water-reducing agents

Guarantee of high-quality concrete and durability Exceptional standards of dimensional tolerance Excellent surface finishes Rapid construction

w More space on site Avoidance of falsework

10.5 Handling and transportation of beams Great care must be taken to ensure the prestressed concrete beams are stable during handling, transportation, storage and erection. Transporting long-span beams by road from factory to sire is a routine operation although careful planning is essential.

It is advisable to strengthen the longer SY beams and provide a supporting frame to enhance safety and stability during erection and transportation. Manufacturers will generally offer full support and advice

10.3 Where can precast concrete be used? Precast concrete can be used in almost all parts of a bridge structure. The use of precast piles is quite common and demand is growing for precast units in abutments but its most widespread use is in the deck support structure, in the form of prestressed beams, and as parapets and string courses.

~~

Figure 2 6 Precast beams in position

A

Figure 2 7 Delivery of a precast beam

22


10.6 Deck edges and parapets The edges of decks require special beams to provide a vertical or inclined edge face and to support steel or aluminium parapets. These beams are manufactured in precast concrete to match the beams used for the deck, for example UM beams.

Parapets are often manufactured in precast concrete and the high containment situation is a common requirement for railway bridges. The vertical faces can also be given an architectural appearance by using a variety of treatments and surface finishes.

10.7 Culverts and arches Precast concrete culverts and arches can be used to replace underbridges carrying minor roads, services and rivers. They can be installed speedily and economically to provide a durable option.

10.8 Replacement rail bridges Replacement bridges spanning over or carrying railway lines are ideally suited for the use of precast concrete due to the limited railway possession time available. Site operations can be reduced to a minimum as a trial erection can be carried out off site and the major components marked with guide-lines to facilitate the actual site erection.

Figure 28: Precast box culvert section

I?

i

I

Figure 29: Precast arch sections L

1 .

23


11. HISTORY OF PRE-TENSIONED CONCRETE BEAMS IN BRIDGES

The first pre-tensioned beams for use in the UK were manufactured in 1940 to meet the demand for emergency bridge construction during the Second World In the mid 19505, at the start of the major road building programme, the precast industry really began to develop, mainly through the work of the Cement and Concrete Association (C&CA) and the Prestressed Concrete Development Group.

MY BEAMS

TEE

Y 1 BEAMS

I M BEAMS

U UM

BEAMS

U U BEAMS

Figure 3 0 Typical beam sections

A major innovation was the range of prestressed inverted T-beams for 7-1 5m spans.These were often used with an in-situ concrete infill to form a solid, composite deck slab.

s, demand for longer-span bridges over two- and three- lane carriageways, spanning 15m-26m and 26m-35n-1, respectively, led to the introduction of I-beams and box sections. There were technical deficiencies with these and a clear need was identified for a standard beam for use in the 15m-29m span range. In 1969, following work by the C K A and the Ministry of Transport, the M- beam was born and it e the flagship for the next 20 years. M-beams were often U 1.0m centres in either pseudo box construction or, more simply, in beam-and-slab construction.

Further developments in rhe mid 197Os, saw the introduction of the U-beam, which was especially suitable for skew decks, the UM beam,

ge beam on M-beam decks, and the wide box beam.

These new beams catered for the increasing demand through to the Between 1965 and 1982, nearly 7,000 road bridges of which were in e and more than half of

the benefits of economy and ease-of-use of standard precast beams.

At the end of the 198 reinforcement in the re beams due to ingress of (PCA), an association addressed this problem and, in 1991, unveiled the Y beam. Not only did it cater for the same span range (15m-29m) but it also proved

in modern integral, or jointless, bridges.The YE beam o serve as an edge beam for Y-beam bridge decks.

Development continued with the SY beam cateringfor the motorway widening programme in the 199Os, which required spans of over 35m in some cases. The TY replaced the inverted T beam as it offered technical advantages such as improved shear capacity and thicker concrete cover to the reinforcement.

I

Figure 31: Typical deck section

24

An Introduction to Concrete Bridqes

12. DURABILITY AND DETAILING

The speed and cost of construction and the durability of any bridge are greatly affected by the attention paid to details. Details that are determined solely by the desire to reduce quantities of materials used, often by small amounts, may result in disproportionately large increases in construction costs and may have an adverse effect on durability.

Simplicity and standardisation are the keys to success. One example of a situation where simplicity of construction is preferred is in the use of a wider base on a level formation, to provide stability against sliding, rather than inclining the formation or providing a shear key. The excavation for a shear key or an inclined formation will increase the cost of construction and, in the case of the shear key, may well disturb the ground that is required to resist the sliding action.

Walls must be designed to allow access for concreting: details such as inclines to the faces, curvature on plan and heights that change along the length, will make it more difficult to place the concrete and achieve a good finish. Attractive concrete surfaces are often difficult to form in-situ, and the use of precast facings may provide a better quality finish.

Restrictions on sizes of pour and position of construction joints can adversely affect the programme for concreting. Such restrictions may be necessary to control movements ofthe concrete but they should suit the sizes of formwork panel and the construction procedures to be employed.

Bearing shelves at the top of abutments must be detailed with adequate drainage, on the assumption that water will get in. Drainage channels and down pipes must be accessible for cleaning, and many designers locate the drainage channels in front of the bearings for ease of access. Drainage of the backfill behind an abutment requires careful attention to detailing of the drainage system Free-draining granular material is not always available for backfill and less permeable materials may have to be used. The effects on earth pressures must be taken into account in the design, and the drainage system must be planned to suit.

Good design detailing will make construction easier, enhance durability and also permit easier inspection and maintenance.Typical examples are:

Positive drainage of all surface water m Provision of chamfers, fillets, drips in overhangs and chases for

tucking in waterproofing Use of standard details wherever possible, particularly in precast beams Use for bonded tendons of air-tight, non-metallic ducts; anchorage protection with end caps and provision for grouting anchorage recesses Provision of abutment chambers for inspection Provision for easy access into and along large and long box girders

12.1 Bridge bearings Bearings transfer the loads from an independent deck to its supports. All bridge decks deflect under load, so the bearings must be able to accommodate the small rotations at the supports. They must also accommodate the horizontal movements of the bridge deck caused by temperature changes, shrinkage and creep, and the shortening caused by prestress. Some bearings allow horizontal movement in one direction only and are restrained in th her, while others allow movement in any direction.

Elastomeric bearings, consisting of layers of steel plate embedded in rubber, can accommodate small horizontal shear movements. PTFE (polytetrafluoroethylene) bearings can be designed for unlimited free sliding between the low-friction PTFE surface and a steel plate. Pot bearings incorporate rubber discs that permit small rotations, while spherical bearings, moving on a PTFE surface, will permit larger rotations.

Mechanical bearings, such as rockers and rollers, provide either longitudinal fixity or resistance to lateral forces. Pot bearings, special guide bearings or pin bearings are often used for this purpose.

Bearingsneedto beinspected regularlyand may requiremaintenance or replacement during the lifetime of the bridge.This can be difficult and expensive, so it is important that the structure is designed to make inspection, maintenance and replacement possible.

Where access is difficult, the bearing should have the same design life as the rest of the structure.

Figure 32: Bearings on pier - Mollington Footbridge, M40

25


12.2 Expansion joints Expansion joints must allow free movement of the bridge, including movements in kerbs, verges and parapets, as well as those in the main deck, but they should not have too serious an effect on riding quality.

Leakage at joints leads to reduced durability and disfiguration of the structure below, so joints need to be waterproof or designed to allow for drainage. Joints should also be designed to require minimal maintenance during their lifetime. However, joints may not last the life of the structure, so they should be replaceable.

Small movements at expansion joints can be accommodated by compressible materials such as neoprene or rubber. These joints can be buried and covered by the surfacing, giving an undisturbed riding surface. Buried or'run-over'joints may consist simply of a gap, sufficient to accommodate the movement, covered by a galvanised

steel plate and a band of rubberised bitumen flexible binder to replace part of the surfacing. This type of joint is known as an 'asphaltic plug:

require a flexible sealing element supported by beams. Mechanical joints based on interlocking can be used for very large movements. Drainage

is used, it is likely to interrupt the

must be provided for such joints.

Whatever type of expa surface and give rise t design for long lengt joints a t frequent intervals. Longer lengths will result in larger movements at the joints, but will preserve riding quality and reduce maintenance. Integral bridges - constructed with joint-free decks - have been referred to earlier.

Road sutfacing Flexible material Tinted sand asphatt protective layer

gure 3 3 Typical section of asphaltic plug joint

26


Wearing course

NOSINGS (N) (HA Qpes 3 and 4

Extruded compression sed (Qpe 4) or sealant (Qpe 3)

I Base course

1 Waterproofing and protection layer

Monolithic concrete plinths

NEW W O W Nominal resin nosine

Wearing course n Nosing material

Abutment

Basecourse

REFURBISHMENT Full depth resin nosings

Description

In-situ rdsins or modified cementitious misturns placed either side of the bridge deck air gap to pmduce firm edges and protect the surfacing. Complete with watertight extended compmsion seal or sealant.

Movemtntr Up to 50mm with pdormed seals Up to 12mm with poured sealant to Bs5212

Thia drawing is indicative only and does not rppresent in MY way any partkYLrdedgn nor can It be used fora design of permanent works. It is copyright of the Bridge Joint Association and can only be mpresented with theirwritten pemidon

Figure 34a: Typical section of mechanical joint

27

An Introduction to Concrete Bridqes

Transition strip

REINFORCED ELASTOMERIC (RE) (HA Type 5)

Elastic carpet reinforced integrally with metal plates

! !

\ Secondary drainage New Works

Waterproofing and membrane protective layer

\ Bedding depth iacmased I 2 i

to suit I !- Length of studs increase

to suit extra bedding

Refurbishment w o w

Description

A joint prefabricated to exact widths and lengths, comprising of rubber surrounding metal elements, bearing plates and reinforcement. Placed onto flat beds with resin transition strips either side as pmtection and to provide a smoth running surface. Bolted directly to the structural concrete.

Movement Range

Up to 3SOmm. Different widths of carpet impose limitations on movement accommodation. Consult supplier.

This drawing is indicative only and does not represent in any way any particular des@ nor can it be used for a design of permanent w o r k It is copyright of the Bridge Joint A d h n and can only be represented with their written permission

I Figure 34b Typical section of mechanical joint

12.3 Waterproofing bridge decks It is a current UKrequirement that bridge decks are waterproofed with an approved system, which may be sheet, board or spray-applied liquid. The deck detailing should allow continuity of waterproofing ‘across’ central reservations, verges, service bays and under kerbs. Arrises (external corners) should be chamfered and fillets should be formed at internal angles. All waterproofing systems must be protected using a tinted asphalt layer before the final surface is laid.

Exposed surfaces, such as deck soffits, fascias, concrete parapets and parapet plinths may be contaminated with salt water carried by wind or from traffic spray. Impregnation, when new, with silane or a similar product can give protection for a limited period. However, factory- made precast pre-tensioned beams have an excellent durability record even without any such treatment.

1 1

__

Figure 35: Spray application of waterproofing membrane

28


Inthepast,masticasphaIthasbeen usedextensivelyforwaterproofing bridge decks, but it requires good weather conditions if it is to be laid satisfactorily, so is rarely used now. Preformed bituminous sheeting is less sensitive to laying conditions, but moisture trapped below the sheeting may cause subsequent lifting.

Hot-bonded heavy-duty reinforced sheet membranes, if properly laid, provide a completely watertight layer. These sheets are made in thicknesses of 3-4mm and have good puncture resistance, so it is not necessary to protect the sheet membrane from asphalt laid on top.

Sprayed acrylic and polyurethane waterproofing membranes are also used. These bond to the concrete deck surface with little or no risk of blowing or lifting. A tack coat must be applied over the membrane, and a protective asphalt layer is laid before the final surfacing is carried out.

Some bridges have relied upon the use of a dense, high-quality concrete to resist the penetration of water without an applied waterproofing layer. It can be advantageous to include silica fume or other veryfine powdered addition in the concrete.

13. CONSTRUCTION PLANNING

Communication is the key to success. It is important that the main and/or specialist concrete contractor(s) form a close relationship with the ready-mixed concrete supplier from an early stage. Regular liaison and progress meetings should be held both before and during the concreting programme to ensure a smooth and effective operation.

13.1 Concrete specifications A complete understanding of the specification is necessary to establish criteria such as compressive strength, size and type of aggregate, admixtures and workability.The requirements of BS 8500 must be followed. Also, knowledge of the relevant bridge codes, e.g. BS 5400 Part 4 and Highways Agency design standards for bridges such as BD24 and BD57 will be particularly relevant.

I A concrete pump may be used for the more inaccessible bridge sections and the larger pours; the mix design will probably need some adjustment to ensure the concrete can be pumped efficiently

BS 8500 requires that the cover to reinforcement be increased by 15mm for cast in-situ reinforced concrete members to give the desired durability for 120 years. Precast prestressed beams also need extra cover but only 5mm.

13.2 Formwork and reinforcement To ensure that the concrete i5 poured with the minimum of difficulty the size, type and position of formwork needs to be assessed together with the density of reinforcing steel (or other materials) and the depth of cover specified. This will ensure easier placement and compaction and will maximise durability.

How the concrete is to be placed into the formwork is a crucial factor to the ready-mixed supplier, for example:

W Small barrows or skips invariably mean longer discharge times and possible disruption to deliveries

Figure 36: Concrete pumped into position

13.3 Size of pour and rate of supply Plant, transport and labour requirements to meet the demandsofthe operation need to be established. This will be particularly important on the larger pours that may be required for the construction of a bridge, e.g. mass foundations, bridge deck.

Contingency plans need to be agreed to safeguard continuity of supply in the event of plant or transport failure.This will be especially

An introduction to Concrete Bridges

13.4 Weather conditions important when structural elements such as piers constructed as 'cold'joints would not be acceptable in such components.

Routes for the ready-mixed trucks to the Site and the provision of Agreement should be reached with site management on acceptable safe, sound authorities and site management.

on should be cleared with the appropriate weather conditions for the placement of concrete and the measures necessary to counter the extreme conditions which may be experienced on a bridge in exposed locations, e.g. wind, ice.

13.5 Testing and curing It is in everybody's interests to ensure that qualified personnel are available to carry out the standard testing procedures to British Standards.

Freshly placed concrete must be given adequate protection from rapid surface drying or temperature variations for as long as possible

m

L Figure 3 7 Ready-mixed concrete being delivered

-"_1

to maximise its performance and appearance. This is an important factor in achieving the high standards of durability required for a bridge. I

14. INSPECTION AND MAINTENANCE

All bridges should be inspected regularly, to ensure that they are in a satisfactory condition and to locate any potential sources of trouble. Detailed inspections, called Principal Inspections, are normally required every six years with general inspections at more frequent intervals.

Proper attention to waterproofing, joint design and detailing, and drainage from the deck, can prevent many of the problems that have caused deterioration of concrete bridges in the past. Adequate cover to reinforcement is vital, so care must be taken during design, specification and construction, to ensure that sufficient cover is provided.

Other methods are available for improving the corrosion resistance of reinforcement, and the finished concrete can be treated with sealing compounds to reduce the penetration of water and de-icing salts.

Bridges are normally designed to require minimal maintenance. However, it will be necessary to carry out routine inspections of drainage channels and down pipes, joints and bearings. Check on the movement of abutments, piers and walls should be made regularly, and foundations in water courses must be inspected for damage due to scour. Such inspections require access, and this must be anticipated in the design and detailing of the structure. A programme of inspections at regular intervals should be planned and any defects revealed must be attended to without delay.

30


15. HEALTH AND SAFETY

All bridges should be designed so that they perform safely and can be built in a manner that minimises risk to both the construction team and the public.

Safety factors are built into the design code requirements to cover the variabitrty of loading and material properties for in-service and ultimate conditions.

The Construction Design and Management (CDM) Regulations are a legal obligation for all organisations involved with the construction process: client, designer, contractors and suppliers. A Planning Supervisor needs to be appointed and a Health and Safety file provides essential information for each Organisation throughout the construction process and during the maintenance life of the structure

16. FUTURETRENDS

Today’s requirements for durable, continuous, integral bridges will lead to increased use of precast concrete in bridge structures. The bridge deck is one area that may be exploited by incorporating a new range of prestressed concreteT or even double-T beams in the design. With such decks, in-situ concrete would be required only as a ’topping’to stitch the beams together.

The trend towards private Design Build Finance and Operate (DBFO) road contracts with pay-back periods of up to 25 years, underlines the requirements for low-maintenance bridges. An examination of the likely costs over the lifespan of a bridge (whole-life costing) is

now considered an essential part of the overall equation. The use of precast concrete elements, with their advantages of quality, speed and efficient construction, will have a considerable beneficial imDact

31


17. FURTHER READING

AMERICAN CONCRETE INSTITUTE ACl 343R-88, Analysis and Design of Reinforced Concrete Bridge Structures ACI, Detroit, 1 988, 162 pp

ClRlA Report 155, Bridges - Design for lmproved Buildability CIRIA, London, 1996

ClRlA RP490: Buildability of Bridges CIRIA, London, year

CLARK L A Concrete Bridge Design to BS 5400 Construction Press, Longman, London, 1983

TECHNICAL GUIDES published by Concrete Bridge Development Group TGI: lntegral Bridges (1997), TG2: Guide to Testing and Monitoring The Durability of Concrete Structures (2002), TG3: The Use of fibre Composites in Concrete Bridges (ZOOO), TG4: The Aesthetics of Concrete Bridges (2001 ), TGS: fast Construction of Concrete Bridges (ZOOS), TG6: Guide to the Use of High Strength Concrete in Bridges (ZOOS), TG7: Guide to the Use ofself-Compacting Concrete in Bridges (2005), TG8: Guide to the Use of Lightweight Concrete in Bridges (2006)

Further tec hnica I guides in preparation TG9: Assessment of Concrete Bridges (2) (due 2006), TGIO: Design Example of Integral Bridges to fC2 (due 2006)

CONCRETE SOCIETYKONCRETE BRIDGE DEVELOPMENT GROUP Durable Post-tensioned Concrete Bridges, Technical Report 47 (Second Edition), The Concrete Society, Camberley, 2002,69 pp

The Construction (Design and Management) Regulations, SI 1994/3247, HMSO, London, 1994

HAMBLY, E C Bridge Deck Behaviour, 2nd edn E & F Spon, London, 1 9 9 1 , 3 1 3 ~ ~

HAMBLY, E C Bridge foundations and Substructures. HMSO, London,

HAMBLY, E C & NICHOLSON, B Prestressed Beam lntegral Bridges. Prestressed Concrete Association, Leicester, 1991,29 pp

LEE, D J & RICHMOND, B Bridges. Civilfngineer’s Reference Book, Ed L S Blake Chapter 18 Newnes-Butterworth, London, 1988,71 pp

LIEBENBERG, A C Bridges. Handbook of Structural Concrete, Eds F K Kong et al Chapter 36 Pitman, London, 1983,168 pp

PRESTRESSED CONCRETE ASSOCIATION Precast Bridge Beams - Product Information Sheets

PRITCHARD, B Bridge Design for Economy and Durability. Thomas Telford, London, 1992, 192 pp

READY-MIXED CONCRETE BUREAU The Essential Ingredient British Cement Association, Camberley, 1993-1 997

SOUBRY, MA ClRlA Report C543. Bridge Detailing Guide CIRIA, London, 2001

TOMLINSON, M J foundation Design and Construction. Pitman Publishing Limited, London, 1980,793 pp

1979,93 pp

WELTMAN, A.J. & HEAD, J.M. Site lnvestigation Manual. ClRlA Special Publication 25. CIRIA, London, 1983,144 pp.

BRITISH STANDARDS INSTITUTION. BS 6031: 1981. Code of Practice for Earthworks. BSI, London, 1981,86 pp.

BRITISH STANDARDS INSTITUTION. BS 5400. Steel, Concrete and Composite Bridges. Part 1 : General Statement, Part 2: Specification for Loads, Part 4 Code of Practice for Design of Concrete Bridges, Part 5: Code offractice for Design of Composite Bridges. BSI, London,

BRITISH STANDARDS INSTITUTION. BS 8500. Concrete - Complementary British Standard to BS EN 206-1, Part 1: Method of Specifying and Guidance for the Specifier, Part 2: Specification for Constituent Materials and Concrete. BSI, London, 2002.

DEPARTMENT OF TRANSPORT. Manual of Contract Documents for Highway Works.

1978-1 990.

Volume 1. Specification for Highway Works.

Volume 2. Notes for Guidance on the Specification for Highway Works.

Volume 4. Bills of Quantities for Highway Works. DOT, London.

HIGHWAYS AGENCY. Design Manual for Roads and Bridges.

Volume 1. Highway Structures - ApprovalProcedures and General Design.

BA 41, The Design and Appearance of Bridges.

BA 42, The Design of Integral Bridges.

BD 24, The Design of Concrete Highway Bridges. Use of BS 5400: Part 4: 1990.

BD 57, Design for Durability.

Many construction activities are potentially dangerous so care is needed a t all times. Current legislation requires all persons to consider the effects of their actions or lack of action on the health and safety of themselves and others. Advice on safety legislation may be obtained from any of the area offices of the Health and Safety Executive.

All advice or information from the Concrete Bridge Development Group is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all publications are subject to revision from time to time and should, therefore, ensure that they are in possession of the latest version.

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Influence surface technology is used to determine optimum loading patterns for thc appmpriote d d g n code.

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Case Study: How simple design tools from SAM save money in the field.

Vhat are the factors that affect the cost of precast re-tensioned bridge beams, and how easy is it to .esign according to an optimised cost criteria? Two xamples are presented here which demonstrate hat the answers are not always what they may seem.

Example 2 - Which of the following 2 beams costs less, the Y4 beam in Example 1 , or the Y4 beam shown next?

Again both beams are designed for the same 20m span. Both are Y4 beams, but the tirst has 25 tendons, and the second has 27 tendons. The second is clearly more expensive. The difference between the two beams however is that the first requires a concrete strength

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:xample 1 - Vhich of the following two beams costs less?

1 1 7 1 I

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1 ........... 1 :. : *. :. at transfer 10N/mm? higher-than the second. The two additional tendons could therefore enable a much lower occupancy time of the precasting bed, and consequent improved efficiency in manufacture.

The tendon optimisation algorithm in SAM was used to generate and investigate both these examples in less than an hour. Such is the effectiveness that can be achieved with the new simple and yet sophisticated concrete bridge design tools within SAM.

. . ...... .... i . ... :...:..:1 I 'he natural instinct of most bridge engineers would be 1 design for the minimum beam size. Both beams were esigned for a typical 20m span bridge with 30 units of [B load allowed. The Y3 beam has 29 tendons, and the '4 beam has 25 tendons with an additional 0.715m' of oncrete. Distributors

Bestech Systems Ltd Australia Station House. Adams Hill Malaysia Knursford. WA 16 6DN Tel: +44 (0) 1565 654 300 Singapore Web: www.bestech.co.uk UAE

New Zealand Vith no other considerations such as overall depth onstraints, the Y4 beam is likely to be more cost ffecti ve.

An Introduction to Concrete Bridges

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introductionto

british standards

prestressed

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