Malcolm Reading, project director for the Millennium Bridge Trust, confirmed that engineer Arup’s report into ways to reduce the swaying is now under scrutiny and one of the options listed will be chosen by the end of this month.
After exhaustive investigation, Arup has proposed two types of damping system to add greater stability to the 370 m footbridge, which started swaying in June when 100 000 pedestrians crossed it. The third option is to make the structure more rigid using extra cables and steel.
In a passive system, dampers are fitted as an integral part of the bridge, enabling the structure to absorb more energy. Active dampers are computer operated and react to vibrations by triggering an external device such as a hydraulic piston to dissipate them.
There are two types of passive damper: viscous and tuned mass. Viscous dampers are metal pistons that pump into pots of silicone, absorbing vibrations like a shock absorber in a car. These could be fitted underneath the bridge deck every 8 or 16 m.
A tuned mass damper could be a lump of steel suspended under the deck in a cradle at the points of maximum movement. It creates counteractive frequency that offsets the s-shaped movement triggered by pedestrians and stops vibrations from amplifying.
However, the bridge experienced at least three problematic types of vibration in June. For a tuned mass damping system to work, several smaller lumps of steel would have to be slung at different points under the bridge to counter each of these frequencies.
Active damping is a computer-operated system in which sensors attached to a computer server detect movement in the bridge and trigger an equal and opposite force. The sensors could be positioned where cables are anchored to the supporting piers, where the cross beams connect to clamps or onto cables.
Professor Yozo Fujino, the Tokyo-based bridge expert advising Arup on the repair of the bridge, is understood to favour this system, which is manufactured by a Japanese company.
Arup, however, is understood to have reservations about the active system, which is used in aircraft engineering and to stabilise tall buildings during an earthquake, but is untested on a bridge of this kind.
The engineer’s reservations hinge on the system’s reliance on new software, and on regular testing and maintenance, all of which add to cost. Arup is understood to favour a less expensive mix of viscous and tuned mass dampers.
None of the options is expected to stop the bridge moving completely.
The Millennium Bridge clients – the Millennium Bridge Trust, Southwark Council and the Corporation of London – have been in continuous talks with Arup during the last month over the issue.
Multidisciplinary firm WS Atkins is advising Southwark Council and the Corporation of London, and Professor David Newland, head of engineering at the University of Cambridge, is advising the Millennium Bridge Trust.