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 deflect significantly under load, requiring stocky sections to provide adequate stiffness; as it deflects 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 difficult 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 deflections 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 flat slabs it allows afford minimum interference to services and in some circumstances make it possible to increase the number of floors within a defined 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 first 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 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 finished article.
A bridge must be suitable for its site and it must be of appropriate scale, it must be designed to be built efficiently 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 find 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. Specific 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, 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 difficult 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 offices. It started as a one man band, and gradually expanded to over a hundred staff with offices 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 non-engineers.
• Chapter 3 is an introduction to reinforced concrete as this is necessary to understand the later chapters.
• Chapters 4, 5 and 6 explain the principles of prestressing.
• Chapter 7 is concerned with the articulation of bridges and the design of substructure.
• 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 influences 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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