Issue 16

The Changi Mezzanine Bridge is a 140m span flat arch footbridge constructed from welded tubular steel sections inside a tunnel that connects two passenger terminals at Changi Airport, Singapore. A series of vibration measurements were made on the bridge during construction, showing that non-structural cladding added mass and reduced the natural frequencies while also increasing the modal damping, from as little as 0.2% originally to around 0.4% for critical vibration modes. From these preliminary studies leading up to the opening of the bridge in early 2002, it was clear that the first symmetric lateral vibration mode (LS1) at approximately 0.9Hz and the first symmetric torsional vibration mode (TS1) at approximately 1.64Hz could be excited easily by pedestrian movement. The modal parameters for mode LS1 suggested that the bridge could suffer from synchronous lateral excitation for a walking pace of 1.8Hz while for TS1 the potential problem was the coincidence of the mode frequency with the lower range of predominant footfall frequencies together with a relatively low modal mass. These possibilities had been identified by the consultant who advised that an experimental study of the characteristics of low frequency vibration modes should be conducted to check vibration serviceability predictions based on analytical modeling. Forced vibration testing using a combination of shakers and humans was used to determine in a very short time scale, the properties for modes below 3Hz. The mode shapes and frequencies compared favourably with predictions from the consultant’s finite element model that had been used to show that with a large number of pedestrians, comfort levels would be exceeded and the bridge would be unserviceable. 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 Pilate Moyo, BSc, MSc, PhD Senior Lecturer, Dept of Civil Engineering, University of Cape Town, South Africa

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