A072 the design of prestress concrete bridges

13 Box girders 369 14 Counter-cast technology for box section decks 386 15 The construction of girder bridges 414 16 The effect of scale on the method of construction 484 17 The desig

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The Design of Prestressed

Concrete Bridges

tailieuxdcd@gmail.com

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Reynolds’s Reinforced Concrete Designer’s Handbook, 11th edn

Information and ordering details:

For price, availability and ordering visit our website www.tandf.co.uk/builtenvironment Alternatively our books are available from all good bookshops

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The Design of Prestressed

Concrete Bridges

Concepts and principles

Robert Benaim

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2 Park Square, Milton Park, Abingdon, Oxon OX14 4RN

Simultaneously published in the USA and Canada

By Taylor & Francis

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British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication Data

Benaim, Robert.

The design of prestressed concrete bridges : concepts and principles /

Robert Benaim.

p cm.

Includes bibliographical references and index.

1 Bridges, Concrete–Design and construction 2 Reinforced concrete

This edition published in the Taylor & Francis e-Library, 2007.

“To purchase your own copy of this or any of Taylor & Francis or Routledge’s

collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.”

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phases of my professional life, from the initial decision to take the risk of starting my own practice, through the tensions and crises that are an integral part of the major international projects in which we were involved, to the long drawn out preparation

of this book

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Figures xiii

Contents

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3.7 The ductility of reinforced concrete 57

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6 Prestressing for continuous beams 139

6.22 Modiﬁ cation of bending stresses due to creep following

7 Articulation of bridges and the design of substructure 191

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8 The general principles of concrete deck design 238

9 The design of bridge deck components 250

11 Solid slabs, voided slabs and multi-cell box girders 327

12 Ribbed slabs 349

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13 Box girders 369

14 Counter-cast technology for box section decks 386

15 The construction of girder bridges 414

16 The effect of scale on the method of construction 484

17 The design and construction of arches 498

17.7 Longer span reinforced concrete arches supporting bridge

decks 509

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17.9 Progressive collapse of multi-span arch bridges 516

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1.1 Cross sections of slip road merging with main carriageway 10

1.6 Fish belly beams: simply supported beams of Maracaibo Bridge 15

1.12a Runnymede Bridge: original Lutyens design 21

1.12d New Runnymede Bridge: load testing the bridge model 22

2.1 Bending moment and shear force on cantilever 30

2.3 Section unsymmetrical about a horizontal axis 31

2.5 Statically determinate and indeterminate beams 33

3.3 Stress–strain curve for high yield reinforcing steel 40

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3.10 Typical heat of hydration cracking of walls 54

3.12 Alternative reinforcement of two-way slab with beam strips 58 3.13 Imposed loads and imposed deﬂ ections on a cantilever 59

3.28 Arching action in reinforced concrete ﬂ oor 79

5.5 Calculation of prestress force using kern and centre of pressure 98

5.9a Typical prestress anchors: CCL slab anchor for 6 strands 107 5.9b Typical prestress anchors: CCL anchor for 19 No 15.7 mm strands 108 5.9c Typical prestress anchors: CCL anchor for 37 No 15.7 mm strands 108

5.9e Typical prestress anchors: buried dead anchors 109

5.13 Eccentricity of cables within their ducts 113

5.15 Loss of prestress due to friction and anchor set 118 5.16 The concept of equivalent load for a ‘V’ proﬁ le cable 121

5.19 Equivalent loads; straight cable and deﬂ ected neutral axis 124

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5.20 Equivalent loads; variable thickness ﬂ ange 124

5.24 Equilibrium forces for slab loaded at its edges 133 5.25 Equilibrium forces for slab loaded at the centre 134 5.26 Potential web cracking due to prestress shear 134

6.2 Reactions due to prestress parasitic moments 143

6.9 River Nene Bridge: differential settlement 151 6.10 Rectangular beam subject to linear temperature gradient 153 6.11 Temperature gradients deﬁ ned by UK code of practice drawn to same

6.13 Bending moments caused by temperature gradients 158 6.14 Zone where bottom ﬁ bre is susceptible to cracking 159

6.23 Typical inﬂ uence line for parasitic moment 178 6.24 Modiﬁ cation of bending moment due to creep 180 6.25 Free cantilever construction: self-weight effects 182 6.26 Free cantilever construction: prestress effects 183

6.31 Increase of prestress lever arm at the ULS 190

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7.8 Bored pile options 207 7.9 Pile bending moment under the effect of a horizontal load 208

7.13 Typical articulation with mechanical bearings 214

7.16 Temporary ﬁ xity of deck built span-by-span 220

7.18 Typical example of the use of rubber bearings 222

8.1 Variation of bottom ﬂ ange width with span/depth ratio 239 8.2 Distribution of longitudinal stiffness to provide support for top slab 240 8.3 Alternative deck for the Sungai Kelantan Bridge 241

8.5 Transverse distribution of loads on beam type decks 242 8.6 Single-cell box girder subjected to an eccentric point load 243

8.8 Equivalent thickness of concrete decks for increasing span 245

9.3 Lever arms for prestressed and reinforced solid slabs 255

9.5 Transverse slab joint subjected to longitudinal bending moments 257

9.11 STAR Viaduct: stitches between precast segments 262

9.15 Slab spanning between torsionally unrestrained ribs 268 9.16 Restraint cracking in the top slab of cast-in-situ segments 269

9.18 Project for bridge with trussed bottom slab 273

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9.22 Variable-depth deck with narrow pier 276

9.25a STAR Viaduct: typical reinforcement cage for precast segment 281 9.25b East Moors Viaduct: failed pour for trial segment 281

9.28 East Moors Viaduct: face anchors in locally thickened webs 284 9.29 Use of inclined prestress to shorten web thickening 285

9.31 Viaduc des Glacières on the A40 motorway, France 288

9.38 River Lea Viaduct, Stanstead Abbotts Bypass: prestressed diaphragm 298

9.44 Custom-designed gulley in thin cantilever slab 306

10.2 Crosshead options for statically determinate beams 310

10.4 River Lea Viaduct, Stanstead Abbotts Bypass: alternative design 311

10.8 Express Rail Link, Malaysia: typical cross section 315 10.9 Transverse deﬂ ection of heavily loaded deck 315

10.11 Arrangement of slab reinforcement for progressively shorter

10.12 Concrete geometry and striking of shutters 320

10.16 GSZ Superhighway, Pearl River Delta Viaducts 323 10.17 GSZ Superhighway, Pearl River Delta Viaducts: deck details 324 10.18 GSZ Superhighway, Pearl River Delta Viaducts: main casting yard 326

11.3 Canada Water underground station: roof slab carrying bus station 331

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11.6 Resultant forces on portal foundations 335

11.11 The effect of prestress parasitic moments on portals 338 11.12 Diagram of prestressed roof for Central Station on the Hong

12.1 Doornhoek Bridge, South Africa: typical twin rib bridge 350 12.2 Contract 304 HKMTR: semi-mechanised falsework 350 12.3 Viaduc d’Incarville, Autoroute de Normandie: self-launching rig 351

12.5 Rotational restraint of webs by substructure 353 12.6 Typical inﬂ uence line for share of mid-span sagging bending

12.8 Contract 308 of the HKMTR, Tai Ho Viaduct: twin rib deck

12.9 Evolution of twin rib deck with decreasing depth 358 12.10 Viaduc d’Incarville: parallel-sided ribs with mechanically

12.11 Torque in ribs due to imbalance in transverse dead load moments 360

12.14 Effect of heat of hydration tensile stresses 364

12.16 Doornhoek Bridge: typical falsework for span-by-span construction

12.18 RVI Interchange, Bangkok: outline project 368 13.1 Alternative positions of construction joint 370

13.3 River Dee Bridge: effect of side cantilever on the appearance of a

13.5 Variable depth with trapezoidal cross section 375

13.9 Typical width of single cell box designed to UK loading 378

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13.11 Dagenham Dock Viaduct: widening at the West Abutment 383 13.12 Dagenham Dock Viaduct: introduction of third box 384 13.13 Typical detail for bifurcation of multiple box section deck 385

14.3 STAR: casting cell ready to receive reinforcement cage 390 14.4 STAR: side shutters, stop-end and core shutter beyond 390

14.6 Combination of plan curvature and crossfall 393 14.7 STAR: permanent and temporary anchorage blisters 397 14.8 STAR: temporary blister cast-on after precasting 398

14.14 East Moors Viaduct: temporary stressing blister doubling as

14.17 Weston-super-Mare railway viaduct: segment stacking 407 14.18 East Moors Viaduct: segment transport by low loader 408

14.20 Byker Viaduct: preparing ﬁ breglass steering packs 412 15.1 Autoroute A8, France: span-by-span construction of box decks 415 15.2 Cable arrangements for span-by-span construction 416 15.3 Options for the trailing prestress anchors 417 15.4 Support of falsework truss for span-by-span construction 418 15.5 Hong Kong MTR Contract 304: precast end block 420 15.6 Viaduc d’Incarville: falsework truss being launched 421

15.11a Temporary prestressing anchors: River Lea Viaduct, Stanstead Abbotts

15.11b Temporary prestressing anchors: STAR Viaduct, steel shoes 427

15.14 Typical dead load moments in deck ﬁ xed to piers 430 15.15 Travellers for River Dee Bridge, Newport, UK 431 15.16 Bhairab Bridge, Bangladesh: underslung falsework travellers 432

15.19 Typical layout of prestress tendons for cast-in-situ balanced

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15.21 East Moors Viaduct: interior of box showing temporary stressing

bars, web thickenings for face anchors and blisters for Stage 2 cables 442 15.22 Typical moments and forces due to temporary prestress 443

15.24 Typical arrangement of temporary prestress 445 15.25 Belfast Cross Harbour Bridges: spans erected on temporary

15.26 Typical arrangement of external stressing 447

15.29 Belfast Cross Harbour Bridges: the ﬂ exibility of crane erection 452 15.30 Grangetown Viaducts, Cardiff: erection by ﬁ xed crane 453

15.33 STAR Viaduct: load testing the pier head falsework 456

15.35 Byker Viaduct, Newcastle: progressive erection of decks 459 15.36 Self-weight bending moments during launching 462

15.38 Eccentricity of launch bearings beneath web 465

15.42 Kap Shui Mun Bridge, Hong Kong: launching of approach span 470

15.45 Poggio Iberna Viaduct, Italy: launching gantry 476 15.46 Poggio Iberna Viaduct, Italy: deck unit on its transporter 476 15.47 GSZ Pearl River Delta Viaduct, PRC: general arrangement of box

15.48 GSZ Pearl River Delta Viaduct, PRC: production line for

15.49 GSZ Pearl River Delta Viaduct, PRC: operation of launching gantry 480 15.50 GSZ Pearl River Delta Viaduct, PRC: mechanised mould for

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17.9 Comparison of an arch with a conventional bridge 506

17.12 Project for Station Viaduct Middlesbrough 508

17.14 Benaim entry to the Poole Harbour Crossing competition 510

17.17 Bridge over Sungai Dinding: balanced cantilever construction of arch 514 17.18 Bridge over Sungai Dinding: span-by-span erection of arches 514

18.8 Linkage between tower and deck oscillation 526

18.10 Preliminary scheme for 460 m span cable-stayed bridge 531 18.11 Typical inﬂ uence lines and bending moment envelope for bridge

18.14 Pont sur l’Elorn: balanced cantilever construction 537

18.20 Ah Kai Sha Bridge: details of trussed webs 548 18.21 Ah Kai Sha Bridge: wind tunnel model of towers 550

18.22 Self-anchored suspension/cable stayed hybrid 551

18.26 Project for North Woolwich Road Footbridge 558

18.28 North Woolwich Road Footbridge: early version 561 18.29 Effect of unsymmetrical loading on suspension bridge with cables

18.30 Examples of structures which deﬂ ect sideways under eccentric loads 563

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I would like to express my gratitude to all those who have helped me in the production

of this book

Professional help has been offered above all by Simon Bourne and Mark Raiss, my former colleagues and now directors of the Benaim Group, who in the midst of very busy schedules, found time to read attentively many of the chapters, make thoughtful, constructive comments, and check my mathematics in detail

In my quest for attributable images I have been dependent on consultants’ and contractors’ staff searching back in their records for jobs, some of which are thirty years old, and I would like to thank all those who helped me in this way In particular I would like to thank Pauline Shirley of Arup’s library for her patience and helpfulness in identifying and making available photographs of the Runnymede Bridge and the Byker Viaduct, among other projects I would also like to acknowledge the kind assistance of the librarians of the Institution of Civil Engineers

As it is hoped that this book will be read by the technically minded who are not trained as engineers, I am grateful to my son David for his ‘lay’ reading of the early chapters, and for his constructive and helpful comments

Acknowledgements

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One of the beneﬁ ts of no longer being responsible for running a practice is that I am free to say what I think about any subject, without having to consider the commercial consequences It should be clear that all views expressed in this book are my own, and

do not engage my former practice, the Benaim Group

Disclaimer

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Concrete has been in use as a primary building material since Roman times As it is strong in compression but weak in tension, it was used in arches, vaults and walls where it is stressed principally in compression

In the mid-nineteenth century, it was discovered that iron and later steel bars could

be embedded in the concrete, effectively giving it tensile strength This allowed it to be used in beams and slabs, where it worked in bending Buildings, bridges, retaining walls and many other structures were made in this reinforced concrete However, although it

is one of the principal building materials in the world, it has shortcomings Reinforced concrete beams and slabs defl ect signifi cantly under load, requiring stocky sections to provide adequate stiffness; as it defl ects it cracks which spoils its appearance and leaves the reinforcing bars vulnerable to corrosion; the large number of bars required to give the necessary strength to long span beams in bridges and buildings make it diffi cult to cast the concrete; it is labour intensive and slow to build

In the 1930s, Eugène Freyssinet invented prestressed concrete High tensile steel cables were substituted for the bars These cables were tensioned by jacks and were then locked to the concrete Thus they compressed the concrete, ridding it of its cracks, improving both its appearance and its resistance to deterioration The cables could be designed to counter the deﬂ ections of beams and slabs, allowing much more slender structures to be built As the cables were some four times stronger than the bars, many fewer were necessary, reducing the congestion within the beams, making them quicker

to build and less labour intensive

Most concrete bridges, except for small or isolated structures, now use prestressing

It is also being used ever more widely in buildings where the very thin fl at slabs it allows afford minimum interference to services and in some circumstances make it possible to increase the number of fl oors within a defi ned envelope

Despite its manifest advantages and widespread use in bridges, outside a minority of expert engineers, prestressing is not well understood by the profession, and is not well taught in most universities Engineers have to learn as best they can as they practice.The book has are three principal aims:

The ﬁ rst is to help improve the quality of the design of prestressed concrete bridges

Throughout my career I have been amazed by the number of grossly uneconomical and sometimes virtually unbuildable concrete bridge designs

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produced by consultants I was fortunate in this lack of competence, as it allowed

me to launch my practice by preparing alternative designs for contractors bidding for work In some cases, these alternative designs halved the materials in the bridge decks, produced very substantial savings in the cost of labour, reduced the construction programme and improved the appearance of the ﬁ nished article

A bridge must be suitable for its site and it must be of appropriate scale, it must

be designed to be built efﬁ ciently and without unnecessary risk of failure, it must

be economical and its appearance must be given a high priority These attributes depend on the quality of the conceptual design Design and analysis are often confused Design requires engineering knowledge, skill and experience combined with imagination and intuition, while analysis is a more mechanical process

I do not know of any other books that deal principally with the design of bridges as opposed to their analysis

The second aim is to explain clearly the basic concepts of prestressed concrete.Practising engineers are being pressurised to take responsibility for structures when they do not fully understand how they work They can do this by using software packages that may be well written, but are dangerous in the hands of those who are not familiar with the underlying concepts

Finally, by concentrating on the concepts and principles underlying the design

of bridges, it is hoped that this book will reinforce practising engineers’ intuitive understanding of the subject

Most textbooks on the subjects of reinforced and prestressed concrete lose the essential simplicity of the concepts in a maze of mathematics I hope this book will

be accessible not only to experienced engineers, but also to students, to architects wishing to participate more in the design of bridges and to lay people interested

in how bridges work

When running my practice, I was frequently approached by younger engineers asking for guidance on some technical matter I did not believe that my role was to tell them what to do, or how to solve a problem To do so would have limited the outcome to

my own experience and their creativity would have been sidelined Furthermore, too often the quick answers to such questions are reduced to explaining the mathematical procedure to be followed to carry out the analysis, or which software package to use Instead, I attempted to explain the underlying structural principles, and left them

to ﬁ nd out for themselves precisely how to complete the design or to carry out the analysis This book proceeds on the same principle Its intention is not to tell the reader what to do, or how to do it, but to explain the structural principles underlying any action that needs to be taken

I have put forward my best understanding of the many complex issues involved

in design My views are not always conventional, nor do they always comply with accepted wisdom Although this understanding has been used for the design of many structures over a long career, it is necessary to exercise critical judgement when using this book Speciﬁ c guidance, for instance on the spans suitable for a certain type of bridge deck or the slenderness of slabs or cantilevers, should be considered as the starting point of design, not the conclusion

The book is intended to be independent of any code of practice Although the British code has been used for some examples, this was only to give them a basis of reality; they could just as well have been based on some other code of practice Also,

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the text is intended to be jargon free; one should not need jargon to explain principles

If some has slipped in due to its familiarity making it difﬁ cult for me to distinguish it from real English, it is unintentional

The illustrations have been produced to scale, except where distortion was necessary for polemic reasons It is vital for engineers of all degrees of experience to draw and sketch to scale, particularly in the design phase of a project A distorted scale changes one’s appreciation of a problem and frequently leads to erroneous conclusions that are discovered later in the design process, wasting time, effort and credibility

As the book is based principally on my own experience, the structures used as examples are those for which I was responsible when working for Europe Etudes or Arup, or were designed by the practice that I founded in 1980 and ran for 20 years This practice was initially called Robert Benaim and Associates, or derivations of that name appropriate to the countries in which we had ofﬁ ces It started as a one man band, and gradually expanded to over a hundred staff with ofﬁ ces in six countries Since my withdrawal from the practice and its purchase by the senior managers, it

is currently known as ‘Benaim Group’ All the jobs referred to in the text that were carried out by the practice are credited to ‘Benaim’

The book is organised as follows:

The meaning and nature of design as opposed to analysis is discussed in Chapter 1

Chapter 2 is an introduction to some basic structural engineering concepts and

to the specialised vocabulary used in the book It is for the convenience of engineers

non-Chapter 3 is an introduction to reinforced concrete as this is necessary to stand the later chapters

under-Chapters 4, 5 and 6 explain the principles of prestressing

Chapter 7 is concerned with the articulation of bridges and the design of structure

sub-Chapter 8 describes the logic that underpins the design of decks for girder bridges, and gives benchmarks for the material quantities that should be achieved

Chapter 9 analyses the function of each of the components of a bridge deck

Chapters 10, 11, 12 and 13 describe the different types of bridge deck

Chapters 14 and 15 are devoted to the methods of construction of bridge decks.Chapter 16 is a synthesis of the preceding chapters, describing how the scale

of a bridge project inﬂ uences the choice of the type of deck and its method of construction

Finally, Chapters 17 and 18 deal with arches and suspended decks which follow a different logic from girder decks

Cross-referencing to sections elsewhere in the text is by section numbers shown in italics

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The nature of design

1.1 Design and analysis

The origin of the word design is the Latin ‘designare’, to draw In classical times, the stability of a structure depended on its shape, which could be drawn by those with the special skills Design now has a much-widened meaning embracing the concept of anything from bridges to ﬂ oats for a carnival

In the context of bridge engineering, design means the conceptual phase, where harmony is created out of the tumult of data which includes:

the physical characteristics of the site;

the technical aspects concerned with the strength of materials and the theory of structures;

the speciﬁ ed design life of the bridge and the maintenance regime;

the various regulations that must be complied with;

the economic and time constraints that have to be met;

the form of contract under which the bridge is to be built;

the effect the new bridge will have on the community, either by its scale, its appearance, or by the changes it will make to the local environment;

the wishes of the bridge owner

An inspired designer may attain a state of grace, where original ideas combine with technical expertise and past experience to create the perfect solution that best ﬁ ts all the data, and which in hindsight appears obvious

Design must be followed by the detailed justiﬁ cation of a project, the analysis, to demonstrate that it is safe and complies with the relevant regulations This analysis

is followed by the preparation of drawings which are needed to communicate to the contractor the information required to build the structure, and the preparation of the contractual documentation Although requiring skill and care, these latter phases

of the process are different in nature to the initial conceptual design; they are more mechanical, and do not require the combination of technical expertise, aesthetic sense and imagination that are characteristic of conceptual design

However, in many cases, design is the name given to the mechanical analysis of the structure, and even to the whole process This is more than a semantic quibble Analysing structures is principally a mathematical, mechanical procedure, whereas design is largely a matter of judgement in weighing up the importance of the many

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relevant criteria If the two processes are given the same name, one tends not to notice the relative weight given to each The mediocrity of many projects that are built is the result of the shortening, or virtual absence of the conceptual, the design phase.

Design and analysis are not strictly sequential Although clearly design must start

ﬁ rst, design development continues in parallel with analysis as ideas evolve or as the analysis gives rise to further insights into the behaviour of the structure, and opens new design possibilities The analysis is an important part of understanding the structure

It needs to start with simple models which are easy to check, and only gradually build

up to the ﬁ nal veriﬁ cation of the structure as a whole As the analysis is carried out on

a model of the structure, not the structure itself, the designer must always question whether the results represent reality or whether they have been distorted by the assumptions made in preparing the model

The designer cannot delegate the analysis, he must remain in charge and needs the required knowledge and skills, in addition to his abilities to imagine, innovate and communicate

1.2 A personal view of the design process

The nature of design is uniquely personal For this reason the following description of how the author understands design may not be recognised by all readers

Design is an interactive process, with signals shuttling between three notional centres

in the brain, those responsible for appreciating beauty, for accumulating experience, and the brain’s calculator It most deﬁ nitely is not just a sketching exercise, nor is it a logical, linear process

When a client proposes a commission for the design of a new bridge, across say a river, with an outline description of the purpose of the bridge and the characteristics

of the river, the ﬁ rst engineering response is to imagine a solution that appears to

ﬁ t the facts as they have been described (often incorrectly), and is usually based on some idea that has been tried before, has been imagined or read about, or is the extrapolation of a previous idea, pushing it further towards some logical conclusion

As more information on the project becomes available, the suitability of this ﬁ rst idea is tested and then reinforced, modiﬁ ed or dropped in favour of another ‘guide’ idea This process of imagining a solution and then subsequently confronting it with the facts is

in the author’s view the essence of creative design

At the earliest stages of this design process, calculations are carried out They are

in general very simple, to compare the cost of alternatives, and to check on the sizes

of members For most bridges and other civil engineering projects, the structure can

be notionally simpliﬁ ed to the point where the bending moments and shearing forces may be estimated by simple manual means, generally to within 15–20 per cent of the correct value

Similarly, the loading on the bridge can be reduced from the pages of the code

of practice deﬁ nition to its simplest basics From these simple beginnings, the size

of members, the density of reinforcement, the intensity of prestress and the basic deﬂ ections of the structure can be calculated Of course one makes use of books with charts of bending moments and deﬂ ections for beams and portals and safe load tables for columns and of codes of practice, not to check for detailed compliance, but to remind oneself of limiting stresses, load combinations, load factors etc If one is very computer literate, simple computer models may be invaluable, as long as one can

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produce them almost automatically, without struggling to understand manuals, sign conventions etc One must at all costs not engage one’s brain into an ‘analysis mode’,

or one’s creativity will be swamped by one’s intellect

It is very important that, at this early stage, all calculations are kept conservative, so that one is not deluding oneself about the feasibility of a favoured option These initial calculations allow the designer to develop his understanding of how the structure works, of how the forces ﬂ ow They also enable him to put sizes to members, and so to gain a ﬁ rst insight into the appearance of a structure The aesthetics of structures are critically dependent on member sizes, and how size varies along a member

The diagrams of normal forces, bending moments, shear forces and torques may be drawn along the members, and stresses calculated As the structure is better understood, the logic of how it works becomes apparent Member sizes may be reﬁ ned to improve economy, to provide reserves of strength and to affect the appearance However, one must not defy the basic logic of the structure; one cannot make a member excessively slender in deﬁ ance of the structural logic, just because it looks better; there must be

a concordance between function and appearance This does not mean that the size

of members is dictated by their stress levels, but that one must not act contrary to the structural logic This usually leaves a considerable margin for discretion in sizing members For instance, two members that are equally stressed may be given different sizes for the sake of appearance

At this stage, it may become clear that the structure is evolving in a way that is not satisfactory, either technically or aesthetically When one embarks on this design process, it is frequently not clear what the nature of the destination will be An essential part of design is the readiness to tear up what one has done and start again An engineer who does not have the courage, or the time, to recognise that he is engaged in a dead end and to start again cannot pretend to be a creative designer

1.3 Teamwork in design

Design is inevitably a team exercise At its simplest, the bridge designer will have another engineer and one or two draftsmen working directly for him, while on large bridge projects the core team may include ten or more people Generally, other specialist disciplines will also be involved for part of the design period, such as geotechnical engineers, quantity surveyors and the suppliers of proprietary products such as the bearings, expansion joints etc An architect may also be involved, either as a partner in the concept or as a specialist involved in the design of ﬁ nishes, handrails, lighting and other decorative aspects Depending on the form of contract, some decisions are likely

to require input from the client or the contractor

If the design is to be anything other than banal, the team needs a leader who makes the project his own There is no aspect of a bridge design that is not capable of more than one solution, whether it is the overall concept or the type of bridge bearing There is thus great potential for diverging views and for indecision This multitude

of design decisions must be welded into a coherent project, and this can only be done successfully through one mind

This need for a ‘chief designer’ is sometimes challenged by professionals, who claim that design is the result of collective decision making, with no one dominating the process However, it is usually only necessary to consider what the effect on the design would be if each member of the team were to be substituted in turn For most of

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the members, the result would be only minor changes to the ﬁ nished design, while generally there is one team member whose substitution would change the project fundamentally.

In order to carry out his synthesising role, the designer must know enough about all the various specialities involved, so that he can understand the implications on the design as a whole of making one choice or another Clearly, the designer is not expected to be skilled in all these various disciplines, but he must be able to question the specialists, understand the reasons for their choices, challenge their decisions, take second opinions and ultimately accept responsibility for them In particular, the bridge designer should have a reasonable knowledge of soil mechanics, as decisions on foundations often determine the type of structure to be built

This concept of the chief designer and the skills he requires is not new In Chapter 1

of his ﬁ rst book, Vitruvius discusses the education of architects (which were not differentiated from engineers) in republican Rome, and puts forward the view that very few people can be expert in all the disciplines involved in construction, but that architects must deal with them all, with only imperfect knowledge He goes on to ask the reader’s forgiveness for his imperfect grammar, as he is an architect, not a gifted writer Perhaps as a civil engineer, I may ask for the same indulgence!

1.4 The specialisation of designers

It is quite clear that society requires a large number of civil engineers to design, build, administer and maintain its roads, railways, water supply, sewerage system, power stations, ports and telecommunications infrastructure among other tasks The great majority of those tasks do not require a deep and intuitive understanding of the behaviour of structures or the exercise of aesthetic judgement It is important that these engineers be well trained, as they are in positions where they can make a signiﬁ cant contribution to society, and the more able among them are likely to attain positions

of inﬂ uence in the private sector or in government Other engineers will become specialists in a wide variety of technical disciplines, such as geotechnical engineering, dynamics, information technology, wind engineering etc

A minority of those who opt to train as civil engineers will become the designers

of structures which, in addition to their utilitarian function become part of the built environment This minority requires different training from the majority They need

to develop an intuitive understanding of the behaviour of structures, a thorough understanding of the nature of the various building materials, and an appreciation of the appearance of their structures

This distinction is recognised to some extent by the profession in the United Kingdom The Institution of Structural Engineers, with a membership of chartered engineers (MIStructE and FIStructE) in the UK of approximately 9,000, caters for the minority of designers, and the Institution of Civil Engineers, with a chartered

UK membership (MICE and FICE) of approximately 36,000, represents the majority

of more general civil engineers In order to become a member of the Institution of Structural Engineers, suitably qualiﬁ ed graduates with about three years experience

in industry have to pass an examination that tests their knowledge as designers of structures, while similarly experienced graduates applying for membership of the Institution of Civil Engineers are subjected to a written assignment that tests their more general suitability to take professional responsibility However, the distinction is

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blurred, with many engineers being members of both institutions, and some designers being members of the Institution of Civil Engineers only.

The specialisation of designers is also recognised in many other countries, where engineering graduates wishing to take responsibility for the design of structures have

to pass an additional examination some years after graduation, giving them a title such

as Professional Engineer

The distinction between general civil engineers and designers remains inadequately recognised by the profession, by the universities and by society Being a designer is almost a separate profession from that of general civil engineer If one were to imagine the spectrum of skills required in the building industry, extending at one end from

an architect and at the other to a director of a civil engineering contracting ﬁ rm, the bridge designer would cover a wide range, but his centre of gravity should be closer to the architect than to the contractor

1.5 Qualities required by a bridge designer

Before the age of enlightenment engineers/architects built many splendid structures, soaring cathedrals, slender stone towers and daring arch bridges without knowledge

of modern theory of structures or of analytical soil mechanics Then in the eighteenth and nineteenth centuries, despite the primitive state of mathematics and structural theory, engineers built huge numbers of structures associated with the development

of the canals, roads and railways, some of which were daring and dramatic, many of which have survived to the present day

It is astonishing how little curiosity is shown by engineers and teachers of engineering about this huge body of successful structures which were built without the beneﬁ ts of most of what is considered essential engineering training It should make us question

what are the basic skills required for a bridge designer

The designer of engineering structures requires an understanding of how structures work and how they are to be built, an appreciation of how they look, and the commu-nication skills required to describe his ideas to others This understanding develops gradually, starting with his technical education and continuing with the feedback from completed projects, snippets of information read or overheard, back-of-the-envelope doodles or bath-time mental calculations Sometimes, some item of information acts as the missing piece of a puzzle, suddenly illuminating an issue that was previously only partly understood This process goes on throughout a career, and a creative engineer becomes progressively more creative until his faculties begin to decline Clearly, some people are more gifted than others in this domain, and have an intuitive understanding

of structures Such natural engineers learn more quickly than others less talented, and make better use of their experience

A designer’s appreciation of beauty depends in part on his talent and in part on his training and experience In the UK, prospective engineers concentrate on mathematics and science from the age of 16, and the appreciation and creation of beauty are absent from the majority of engineering courses Mathematics and the other technical disciplines such as theory of structures and the properties of materials are the most tangible of the skills required by engineers, and thus are the ones that are given priority

in their education However they have become virtually the only skills that are taught, whereas the critical criterion that determines whether a structure will rise above the

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mediocre is the quality of the conceptual design This requires in addition to technical knowledge and skill, imagination, aesthetic judgement and an appreciation of the context of the structure These skills are much more difﬁ cult to teach.

Thus engineering designers have to rely on any innate talent for the vital aesthetic component of their practice, or alternatively seek input from architectural specialists Enlisting the help of architects in the design of bridges is far better than simply ignoring the aesthetic component of design, but is much inferior to both aesthetic and technical components being in the mind of one person An engineering designer should have an education that is reasonably balanced between the technical and the aesthetic

1.6 Economy and beauty in design

An engineer designing a bridge has twin obligations, to his client to use his money wisely, and to society to produce a structure that will enhance the built environment

In fact, beauty in engineering design has its roots in the tension that exists between designing for economy and designing for appearance

Economy in this context is not simply saving money; it is a concept of rationality and frugality It is fundamental to engineering design that the designer is constantly planning how he can save materials, and how he can make the construction process simpler, even if many of these design decisions in isolation would not register on the overall balance sheet of a project

An example of this tension between appearance and economy is given by the design

of an access ramp to a high level bridge, Figure 1.1 The main bridge consists of a trapezoidal box section, 2.4 m deep, allowing it to span 60 m or more The access ramp must climb from ground level to merge with the main structure At the point

of merger, the ramp has the same depth and shape as the main bridge However, the 2.4 m depth would be out of scale for a deck close to ground level Consequently, the ramp is given a depth that gradually reduces to 0.7 m as it approaches the ground, with the spans shortening correspondingly This is clearly not the most economical choice,

as the formwork for the downstand webs of the ramp will be continually changing In order to mitigate this additional cost of formwork, the geometry of the ramp deck may

be deﬁ ned by keeping the length of the web shutters constant and equal to those of the main bridge, but changing their angle Thus if the ramp is built span-by-span, the side shutters of the webs may be re-used for each span This is an intellectual concept based

on an attempt to rationalise the construction method and save cost, which gives rise to

a distinctive appearance Finally this appearance must be judged on its own merits.When an engineer designs, whether it is the overall concept of a bridge or an individual member, he ﬁ rst must understand the structural behaviour, and then seek rationality and economy The search will usually leave him many options, which allows him to make choices concerning the appearance of the structure

A very simple example is the design of the bridge pier carrying a single bearing, Figure 1.2 The pier is subjected to a vertical load and to a horizontal load at the top which produces a bending moment that increases linearly to a maximum at the base

of the pier, Figure 1.2 (a) The size of the pier at the top will be limited by the size of the bridge bearing, while at the bottom it will be governed by the combined effect of the compression force and the bending moment The engineer has a choice between, for instance:

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a prismatic column of a generous size that allows minimum reinforcement to be used throughout, Figure 1.2 (b);

a smaller prismatic column that needs minimum reinforcement at the top, but heavy reinforcement at the base, Figure 1.2 (c);

a column that is as small as possible at the top and tapers uniformly to the bottom, Figure 1.2 (d);

a column which is as small as possible at the top and whose width then varies such that the minimum reinforcement may be used throughout, Figure 1.2 (e);

some combination of any of these

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His choice will be informed by other aspects of the project, for instance:

the number of similar columns in the project;

the range of heights of such columns;

the need for variations on the basic column size to cater, for instance, for bridge expansion joints, anchor piers or different length spans;

the need for a family of columns to cater for other bridges forming part of the same project;

the architectural context of the bridge

As the engineer considers the economy of the various choices to be made, he will most probably ﬁ nd that several options have costs that are within the margin of estimating error Consequently, although the search for economy is at the heart of his design, it cannot be used as an alternative to aesthetic judgement; the engineer must choose the shape he considers is best in all the circumstances

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Figure 1.4 STAR Viaduct: typical pier (Photo: Benaim)

Figure 1.5 Byker Viaduct: pier ﬁ nishes (Photo: Harry Sowden/Arup)

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Once he has made his basic choices, he then has to reﬁ ne his design, both for economy and appearance; small changes of shape can greatly affect the appearance,

as may be seen in comparing the options for a column of varying width shown in Figure 1.3 Reinforced concrete detailing considerations may also suggest minor dimensional changes, to give a rational arrangement of bars, or to make best use of standard bar lengths and minimise waste

What an engineering designer cannot do and retain the integrity of his design is to

ﬂ y in the face of rationality and economy, and design a heavily loaded column that, for instance, tapers towards the bottom, Figure 1.2 (f), creating an artiﬁ cial problem that then needs to be solved by misdirected engineering ingenuity This is true even if the additional cost as compared with a rational design is negligible

There is no reason that the column should not be decorated, with corners cut off, the sides faceted, Figure 1.4, or with ribs or other decorative ﬁ nish, Figure 1.5 (7.15.4),

as long as the cost of this decoration is reasonable in the context of the project Some aspects of such decoration may be functional, for instance to reduce the apparent bulk of the column by changing the way light reﬂ ects off it or to control water runs to improve its weathering, while some may be just to make it more attractive

Engineering design is thus driven by the simultaneous consideration of rationality, economy and appearance Designing economically alone is not enough There is no automatic linkage between economy and beauty; aesthetic judgement is required at every step of a design

Engineers have been known to put their faith in the idea that if they design honestly, and refl ect in their structure the fl ow of forces, the result will inevitably be aesthetically satisfactory, or even beautiful: the idea that ‘form follows function’ Unfortunately, this is not suffi cient Within the confi nes of honesty and economy, the engineer is left with a wide choice, which requires aesthetic judgement A useful analogy is to consider the design of the human face, which is well defi ned by its function, but which gives rise

to an inﬁ nite number of outcomes

If bridge designers are not conﬁ dent of their aesthetic ability, they should request the assistance of an architect, who should be involved from the earliest stages of the design If they are lucky, they will ﬁ nd one who understands the special quality

of engineering design, and who does not take over the project with his own, engineering taste Such collaboration can be very creative, but success depends ﬁ rstly

non-on the engineer being skilled and cnon-onfi dent in the technical domain, and secnon-ondly in the architect having a genuine interest and feeling for engineering structures Even engineers who have confi dence in their aesthetic judgement can fi nd collaboration with a talented architect very creative, with the architect questioning the engineer’s choices, and proposing different ways of seeing the design

1.7 Expressive design

An important part of engineering design is that it should be expressive of the forces

in play In the example of the bridge pier given above, all other things being equal, it

is better for the greater moment at the base to be resisted by increasing the size of the pier, and thus acknowledging the greater strength required, rather than by keeping the size constant and increasing the reinforcement that is hidden within the concrete envelope

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Figure 1.6 Fish belly beams: simply supported beams of Maracaibo Bridge

Some types of structure are more expressive than others A beam of constant depth

is singularly inexpressive of the fact that the greatest bending moment is at mid-span Various designers, including Morandi, have adopted ‘ﬁ sh belly’ beams, which do express the fact that they need greater strength at mid-span, and allow the removal

of redundant web material near the ends, Figure 1.6, despite the fact that they most probably cost slightly more than a beam of constant depth; the additional complication

of the formwork and the reinforcement is likely to outweigh any savings in the volume

of concrete The designer was justiﬁ ed in attempting to make the structure more expressive, and hence more interesting, although the ﬁ nal judgement must be whether

it works aesthetically, as well as expressing engineering values

An engineer may be justiﬁ ed in choosing a type of bridge that is not the most economical, but that is more expressive Clearly, this needs the approbation of the client and cannot be done as part of a competitive tender where no credit is given for appearance For instance, it may be established that the most economical bridge for

a site is a girder bridge on vertical supports However, the site may be just right for

an arch, Figure 1.7, with the strong foundations required, although the arch would

be more expensive Such an arch bridge design can only achieve distinction if it is

a rational choice and if the designer then strives for and achieves economy in the design as described above It could not be considered a good design if the designer had imposed an arch on the site, despite its unsuitability; for instance, if the arch required massive and expensive foundations to resist the thrust, even if these foundations could not be seen by the public

This demonstrates that good engineering design has an esoteric component only

to be appreciated by professionals A good example is the series of valley piers of the Byker Viaduct in Newcastle, which were ﬂ ared to resist the wind and centrifugal forces of the twin-track railway it carried The central part of each pier was cut away

at ground level to save materials and to reduce the impact on walkers in the park The size of these cut-outs was made just large enough to allow the precast units of the bridge deck to fi t through, in an economical and innovative construction method, Figure 1.8 This fact is only known to those who remember the bridge being built, but forms part of the intellectual justifi cation for the shape and size of the cut-out The fi nal judgement on a design must rest with the combined response of public and profession

The choice of a solution that is expressive of the ﬂ ow of forces applies to the design

of members and details of a bridge as well as to the structure as a whole

The detailed shape of the towers for the two-level Ah Kai Sha cable-stayed bridge designed by Benaim to cross the Pearl River close to Guangzhou, further illustrates the relationship between technical and aesthetic decisions, Figure 1.9 The bridge has

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