Structure: Millennium Bridge
Location: Bankside to the City of London, England
Completed: February 2002
Architect: Foster and Partners
Artist: Sir Anthony Caro
Contractors: Monberg Thorsen and Sir Robert McAlpine
The design for the Millennium Bridge began in 1996. The height restrictions due to the bridge’s location required a unique design for a shallow suspension foot bridge spanning three hundred and thirty-three feet. The cable sag is a mere 2.3 meters.
The unusual profile for the cables created concern about the structure’s stiffness and response to torsion. Extensive analysis was conducted to make sure the tension in the cables would stabilize the bridge enough to meet lateral stiffness standards. The cables were placed wide from the bridge to resist torsional forces.
Despite the extensive analysis, on opening day the bridge experienced strong lateral vibrations. While no vertical vibrations were experienced, the lateral vibrations were strong enough to make many pedestrians grab onto the rails. Two days later, the bridge was closed for investigation.
Many tests were conducted and a theory of synchronous lateral excitation was proposed. This phenomenon results when a large number of people cross the bridge, all contributing a minuscule lateral force that is typically neglected in design analysis. The sensation of lateral vibration becomes noticeable because the pedestrians often find it more comfortable to step in synchronization with the bridges slight lateral movements, therefore creating a large force in time with the bridge’s natural frequency, eventually exaggerating the lateral movement. The investigation included studying bridges that also demonstrate this phenomenon, proving that the sensation is not unique to the Millennium Bridge’s unusual shallow cable design.
Thirty-seven viscous dampers were installed to control the horizontal motion, and twenty-nine pairs of tuned mass dampers to control vertical motion.
While synchronous lateral excitation had previously been witnessed, the extensive research into it was a break through in bridge design and stands as the Millennium Bridge’s main contribution to the engineering world.
Pat Dallard, Tony Fitzpatrick, Anthony Flint, Angus Low, Roger Ridsdill Smith, Michael Willford and Mark Roche, “London Millennium Bridge: Pedestrian-Induced Lateral Vibration”, J. of Bridge Engineering, Trans. ASCE, 6, 412-417 (2001).
P. Dallard et al. “The London Millennium Footbridge”, The Structural Engineer, 79, No. 22, 17-33 (2001).