Isambard K Brunel’s Swivel Bridge of 1849: an overview

It is difficult now to appreciate the extraordinary pace of invention in civil engineering in the 1840s and 50’s: the years of the ‘Railway Mania’. The extraordinary achievements of those years were both led by and drove a relatively small group of engineers and contractors who worked at what now seems astonishing speed, risking both fortune and health. To picture invention as mothered solely by necessity surely misses what Rowland Mainstone termed ‘the springs of structural invention’. Its parentage is much richer, arising from the accumulation of and reflection on both tacit and formal knowledge, which in turn fuels imagination and intuition, before determination and ambition achieve something new. Those remarkable decades bear this out forcefully. In 1848, in the midst of work on much larger bridges, Brunel was commissioned to design and construct a bridge to span his new south entrance lock to Bristol’s Floating Harbour. It had to be mounted so that it could be moved out of the way of shipping using the lock and the double-cantilever Swivel Bridge was the outcome. It served that purpose until the mid-1960s, surviving being moved, shortened and damaged. It was then left neglected and rusting away at the side of the present entrance lock into the Harbour. Until the bicentenary of Brunel’s birth in 2006, it was among the least known of Brunel’s bridges and was certainly entirely un-researched. Now its history, construction and condition, mechanical and structural, have been investigated and significant mechanical repairs carried out as discussed in this issue. Members of the History Study Group of the Institution of Structural Engineers visited the bridge in 2015 and heard about the survey, conservation and repair work carried out since 2006. David Greenfield, the late Brian Murless and the late Graham Laucht showed some of their historical research including, in Laucht’s case, information about numerous later Brunel ‘balloon topped’ bridges for which he had found either archival or photographic evidence. I thought then that this work deserved a wider audience and in 2019 the Study Group and the Institution organised a conference in Bristol focussed on the bridge. The three papers published here were presented at this conference. David Greenfield has thoroughly researched the development of the design and construction of Brunel’s wrought iron tubular bridges which led to the 300’ span Chepstow railway bridge of 1849-52 and the 455’ spans of the Saltash railway bridge of 1854-9. With an overall length of 122’ and weighing around 70