Dynamic behaviour and collapses of early suspension bridges

In the mid-1800s a number of the early suspension bridges collapsed for various reasons, some with loss of life. In this paper, four collapses caused by pedestrians are re-visited; Broughton and Angers failed when excessive vibrations were imposed by marching troops, Montrose and Yarmouth failed when crowds rushed from one side to the other to watch events passing beneath. It was found that both the failures caused by troops were at the suspension anchorages. Those caused by asymmetric crowd loading were in the main chains at badly forge-welded connections. Poor-quality iron, and shortfalls in design and constructions were identified as contributory causes. In the aftermath of the collapses it was recommended that troops should break step when crossing bridges, structures should be periodically inspected and chains should be load-tested. Unfortunately these actions were not mandatory and an order for troops to break step was the only one to be generally adopted at the time. Various design schemes were adopted to limit vibrations but the single most important was to stiffen decks by the addition of truss-parapets, as adopted by Rendel after Montrose Bridge was damaged by wind in 1838. Dynamically lively bridges continue to be designed to this day, sometimes unwittingly, sometimes knowingly.