introduction
oscillation
test programme
results
solution
modifications

video

There are two fundamental ways to limit dynamic excitation:

• Stiffen the structure, so the frequency of the bridge and our footsteps no longer match
  
  
• Add damping to absorb the energy.

It was concluded that stiffening the bridge to change its frequency was not a feasible option. The bridge would need to be at least tenfold stiffer laterally to move its frequency out of the excitation range. The additional structure required to do this would dramatically change the appearance of the bridge.

It was decided to adopt a damping solution, either active damping or passive damping. Active damping uses powered devices to apply forces to the structure to counteract vibrations. Passive damping relies on harnessing the movements of the structure to absorb energy.

Active damping
Active dampers are commonly used in other engineering fields such as aeronautics. However, although active damping systems have been used in buildings, no previously designed systems were sufficiently developed for a more complex multimodal system such as the bridge. Maintenance requirements were also a cause for concern. Following discussions with manufacturers, Arup reached the conclusion that active damping was too complex, expensive and production times were too long for this to be a viable solution in this instance.

Passive damping
The bridge deploys two forms of passive damping to reduce bridge movement: Viscous dampers and Tuned Mass Dampers.

Viscous dampers are located under the deck, around the piers and the south landing to control the lateral motions. They function in a similar way to shock absorbers. Each damper dissipates energy by the movement of a piston passing back and forth through a fluid. Distinctive new chevron steelwork transfers the bridge movements to the under deck viscous dampers.

The tuned mass dampers are also located beneath the deck and reduce vertical movements. Tuned to a specific frequency these inertial devices, simplistically weights on springs, are attached to discrete points on the structure. Although no excessive vertical movement occurred on the Millennium Bridge, these were added to the solution as a precaution since some researchers suggested that synchronous pedestrian vertical loading is also possible and has been observed elsewhere. Synchronous pedestrian vertical loading would not be the same as a group of soldiers deliberately marching in step; instead it would be a subconscious response of ordinary pedestrians reacting to a moving surface.