In 1996, London's Royal Academy of Arts hosted an exhibition titled "Living Bridges: The inhabited bridge, past, present and future". The exhibition presented a host of real and never-realised inhabited bridges both from history and more modern times, and also played host to a speculative bridge design contest, the Thames Water Habitable Bridge Competition.
It was an eye-opening exhibition, and the accompanying book (Prestel, 1996, 158pp) [amazon.co.uk] was (and is) a splendid companion. It's a thoroughly well-researched account of the inhabited bridge from mediaeval to modern times, and extremely well-illustrated.
Early inhabited bridges included fortified bridges, and also chapel bridges. The fortifications were essential to ensure that a bridge's primary purpose in enabling trade could not lead to the spans providing a too-convenient route through a city's defensive walls. The chapel often recognised that bridges were built to combine religious favour with a practical purpose. Several early inhabited bridges incorporated both features, as at London Bridge and the Pont d'Avignon.
Another common feature of early inhabited bridges was an association with businesses requiring proximity to the river, such as watermills. The Pont de Blois incorporated no less than five watermills at one time. Other businesses took advantage of the opportunity to front onto one of a city's most used thoroughfares, while residences took advantage of the direct route to the river for sewage. Including buildings on a bridge was attractive to those who built and owned the structures, as income from rent could help towards the costs of maintaining the bridge.
Living Bridges features extensive accounts of these early bridges, accompanied by period illustrations and numerous reproductions of paintings, lithographs etc. Bridges which have survived to the present day are represented with photographs. It's a goldmine of interesting information which I don't believe is covered in such depth anywhere else.
In addition to well known bridges such as the Ponte Vecchio, Ponte de Rialto and Pulteney Bridge, the book features a number of speculative proposals including John Soane's 1776 Triumphal Bridge or Gustave Eiffel's 1878 scheme for the Pont d'Iena in Paris.
These ideas often seem megalomanical by today's standards, with examples of a tendency towards the gargantuan including Raymond Hood's "skyscraper" bridge in New York, and Cedric Price's proposal to bury much of the River Thames in a lengthy culvert. Although such ideas were more than a little crazy, they reflect a wider sense of the inhabited bridge as a utopian vision insensitive to any reasonable context. Many inhabited bridges seem visually attractive in their own right, but have the effect of blocking views both of riverbanks and along a river.
City rivers are now valued primarily as open space within a crowded urban context, and the idea of depositing a new building into such a context seems more than a little foolish. Nonetheless, Living Bridges offers numerous examples of modern proposals to do precisely this, with some kind of romantic attraction to visions of Old London Bridge trumping common sense again and again.
For such a niche subject, this is an ambitious and thought-provoking book, which ably explains the attractions of the habitable bridge while exposing its many flaws. Long since the Royal Academy exhibition, proposals for inhabited bridges continue to emerge, most of them absurd, but this excellent survey of the subject remains the definite resource for understanding what such bridges have been in the past, and might still be one day in the future.