An introduction begins with the London Millennium Bridge, which made the news when it wobbled — was subject to "synchronous lateral excitation" — and closes with the Forth Railway Bridge. In between Blockley explains how successful bridges require "firm foundations, strong structure, and effective working", involve "purpose, material and form", and are built using beams, arches, trusses and suspension. He also provides brief explanations of physics concepts such as tension, compression, shear, and degrees of freedom. (The occasional sections of technical material in Bridges are clearly presented, with good use of diagrams, but readers without at least school physics may find them difficult.)
Four chapters then look at different kinds of bridges: arch bridges, beam bridges, truss bridges, and suspension and cable-stayed bridges. Blockley describes the basic components and simple physics of each kind of bridge, introducing some additional theory as he goes. For example, strain energy calculations are covered in the chapter on truss bridges and dynamic "time" effects in the chapter on suspension bridges.
All of this is set in a historical perspective and illustrated with examples and details from a broad range of bridges from around the world (with an emphasis on the English-speaking countries), looking at notable designs, construction methods, and failures. There are scattered biographical details, covering famous figures such as Brunel, Telford and Stephenson but also lesser known ones such as bridge builder Oleg Kerensky, civil engineer David Bailey, bridge designer Santiago Calatrava, and geotechnical engineers Karl von Terzaghi and Ralph Peck. And there are short digressions on a range of other topics, from people jumping off bridges to the criteria for a bridge to be "public art".
Safety and risk management feature in all of this, but are also addressed in an additional chapter. Here Blockley considers the example of failure in a beam under bending stress and the development of "plastic theory". He touches on the use of probability theory but also its limitations: "the incompleteness of the unexpected and unintended ... makes the mathematics of probability theory inadequate as a theory of risk". And he looks at the complexities of human error and argues for "practical rigour".
Throughout Bridges Blockley uses an analogy of bridges as "books", with the equivalents of chapters, sections, paragraphs, sentences, words, grammar and so forth. This is vaguely interesting but becomes repetitive and is not actually useful. A final chapter is built on a different metaphor, of bridges as links between people. Here Blockley explains that bridges are fundamentally social, built by people for people, and explains the limits of reductionism and the need for a holistic systems approach to bridge building — and other areas of human life.
Anyone narrowly interested in the engineering aspects of bridges will probably find Bridges too digressive. But as popular science it works well, with a good variety of well-presented material, much of which was new to me. There's also a small selection of simple but effective half-tones of some of the bridges discussed.