Synchronous lateral excitation is a dynamic phenomenon where pedestrians walking on a footbridge subconsciously synchronize their lateral footsteps with the bridge’s natural swaying motion, amplifying lateral vibrations. First widely recognized during the 2000 opening of the

London Millennium Bridge The Millennium Bridge, officially known as the London Millennium Footbridge, is a steel suspension bridge for pedestrians crossing the River Thames in London, England, linking Bankside with the City of London. It is owned and maintained by Brid ...

, synchronous lateral excitation has since become a critical consideration in the design of lightweight pedestrian structures.

Mechanism

Synchronous lateral excitation arises from two interrelated synchronization processes. The first is the pedestrian-structure synchronization, where slight lateral bridge movements (e.g., from wind or random pedestrian steps) prompt walkers to adjust their gait to match the bridge’s oscillation frequency, increasing lateral forces. The second is pedestrian-pedestrian synchronization, where individuals unconsciously align their stepping patterns, further reinforcing the resonant force.

Key cases

* The London Millennium Bridge experienced lateral vibrations up to 70 mm due to synchronous lateral excitation, requiring a £5M retrofit with dampers. * The

Auckland Harbour Bridge The Auckland Harbour Bridge is an eight-lane motorway bridge over Waitematā Harbour in Auckland, New Zealand. It joins Saint Marys Bay, New Zealand, St Marys Bay on the Auckland city side with Northcote, Auckland, Northcote on the North Shore, ...

experienced a lateral frequency of 0.67 Hz during a 1975 demonstration.Dallard, P. ''et al.'
"The London Millennium Footbridge,"
{{webarchive, url=https://web.archive.org/web/20110927001537/http://www.arup.com/MillenniumBridge/indepth/pdf/IStructE.pdf, date=27 September 2011 ''Structural Engineer'', 20 November 2001. 79:22, pp. 17–35. * The

Birmingham NEC The National Exhibition Centre (NEC) is an exhibition centre located in Marston Green, England, near to Birmingham and Solihull. It is near junction 6 of the M42 motorway, and is adjacent to Birmingham Airport and Birmingham International rai ...

Link bridge experienced a lateral frequency of 0.7 Hz. * The Toda Park Bridge in Japan is an early documented case (1990s) studied by Fujino et al., informing later synchronous lateral excitation models.

Mitigation strategies

Some ways to avoid synchronous lateral excitation are the implementation of

tuned mass dampers A tuned mass damper (TMD), also known as a harmonic absorber or seismic damper, is a device mounted in structures to reduce mechanical vibrations, consisting of a mass mounted on one or more damped springs. Its oscillation frequency is tune ...

, which were used in the Millennium Bridge to increase damping from 0.5% to 20% critical. Other strategies involve designing bridges with lateral frequencies outside the 0.5–1.1 Hz range as well as managing crows by limiting pedestrian density during events.

References

Engineering Physics Resonance