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The Author of Design of Prestressed Concrete Bridges
Robert Benaim is the editor of The Design of Prestressed Concrete Bridges Concepts and principles PDF Book.
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.
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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.
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 Design of Prestressed Concrete Bridges 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 Design of Prestressed Concrete Bridges 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 Design of Prestressed Concrete Bridges 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.
Design of Prestressed Concrete Bridges PDF Free Download
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 Design of Prestressed Concrete Bridges 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.
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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’.