Following experimental and analytical studies of vibration serviceability of a 140m span steel footbridge which predicted excessive and uncomfortable vertical and lateral vibration levels due to crowd loading, a series of walking tests involving up to 150 pedestrians was aimed at assessing the prototype behaviour under ‘limiting typical’ pedestrian loads in two vibration modes judged to be critical.
In a walking test for possible instability resulting from so-called ‘synchronous lateral excitation’ (SLE), pedestrian volunteers were fed onto the bridge and told to walk casually. With all 150 available pedestrians circulating for several minutes, a steady increase in lateral vibrations was observed. This divergent response resembled the phenomenon observed during tests on the London Millennium Bridge (LMB), and while the maximum response reported here was an order of magnitude smaller than the largest levels reported for LMB on its opening day it was, apparently, uncomfortable for pedestrians. On the other hand, due to the apparent lack of synchronisation and random character of vertical loads together with enhanced damping due to the pedestrians themselves, vertical response levels were within acceptable comfort limits. From observations of pedestrian movement and the nature of response in vertical and lateral modes there appears to be an open question about the nature of any possible synchronisation of forces and the manner of generating and building up relatively large lateral amplitudes. To mitigate the potential for strong and unsafe lateral oscillation in the unlikely event of larger numbers of pedestrians, a tuned mass damping system has been installed. The damping in LS1 has been increased by a factor of approximately four, so that SLE is effectively prevented for any foreseeable reasonable pedestrian loading.
James M. W. Brownjohn, BSc, PhD, CEng, MIStructE, MIMechE
Professor, University of Plymouth, Faculty of Technology, Drake Circus, Plymouth, UK
Paul Fok, BSc(Hons), MSc, DIC, LLB (Hons), PE
Manager, Design Management, Land Transport Authority, Singapore
Mark Roche, BEng, DIC, MSc, CEng
Associate, Arup, New York
Piotr Omenzetter, MSc, PhD
Lecturer, Department of Civil Engineering, University of Auckland, New Zealand