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