Here's a book that's perhaps only for the obsessives: Brunel's Timber Bridges and Viaducts, by Brian Lewis (Ian Allan Publishing, 2007, 144p, amazon.co.uk).
Isambard Kingdom Brunel is best known for his major works: Clifton Suspension Bridge, Royal Albert Bridge, SS Great Britain, and the Great Western Railway (GWR). Much of his other work is known only to those with a particular interest in engineering history.
I was certainly familiar with his timber viaducts for the South Devon Railway and Cornwall Railway, offshoots of the GWR: tall stone piers from which sprung outstretched fans of timber, an inverted forerunner of what is now the cable-stayed bridge. These were remarkable structures, built in timber to reduce initial cost and to allow railways to open to traffic more quickly, so that investments could be recouped. However, they were not long-lasting, generally being converted into metal girder viaducts, masonry arch viaducts, or demolished entirely.
However, timber was used much more widely than this on Brunel's railway lines, and this painstakingly thorough book attempts to describe every significant timber span on those projects. Today, not one of these bridges now survives.
The book opens with a lengthy discussion of how Brunel's timber bridges can be classified, well illustrated by Mike Jolly (who deserves a cover credit, given how much he contributes throughout the book). I found this thoroughly confusing, even on repeated reading, and as it forms key material for much of the book, I found I had to keep flipping back to this section to make sense of what follows. This is largely down to Lewis's classification system, which distinguishes between, for example, CLB2(E) structures and QTT1 bridges, rather than just fan-supported continuous laminated beams and queen post through trusses. Notwithstanding the confusion, it's a vital chapter, as most of the bridges used by Brunel and his assistants do fall into several key groups and then variations on each theme.
A second chapter discusses design, construction and maintenance. It's informative enough but sometimes lacking in depth, through no fault of the author. He tells us of William Bell's research into timber beam behaviour on Brunel's behalf, but there's not much to tell as none of the research actually survived. The discussion ranges from the use of iron castings and wrought iron straps to connect timber members, through alternative parapet details, load testing and wood preservation.
The majority of the chapters document different railway lines on which Brunel employed timber bridges, both as railway underbridges and overbridges: twelve main railway projects and several smaller or subsidiary schemes. The sheer volume in which timber construction was applied is startling, dozens if not hundreds of bridges of very different lengths and spans.
In addition to Mike Jolly's excellent diagrams, there are plenty of historic photographs, pictures, and, best of all, extracts from original construction drawings. These are, by far, my favourite thing here, many of them works of arts compared to the drawings used in modern construction, and it's a shame they haven't been reproduced at larger scale.
For me, the diagrams and drawings are the reason to buy this book, to dive into a plethora of timber detailing, "endless forms most beautiful and most wonderful" to borrow a quote from Charles Darwin. That may be apposite, as Lewis's book documents the generation and evolution of numerous related but divergent design solutions, each developed as a pragmatic response to the contingencies of construction and structural performance. Several of the bridges documented evolved even during their own lifetime, with additional strengthening members added in response to degradation.
Many of the structures depicted are prosaic but interesting, but many are quite spectacular, especially the tall timber viaducts of the Devon, Cornwall and Vale of Neath lines. It's a tremendous shame that none still survive, even if quite understandable (there are still timber viaducts surviving elsewhere on the railways, of course).
I think, on the whole, that Brunel's Timber Bridges and Viaducts will be too fastidiously detailed for most readers. It's sufficiently packed with detail to be a book to nibble rather than to try and digest whole. For my part, I found I eventually achieved a second wind, particularly in exploring the copious illustrations included, so I'm glad to add it to my bookshelf.