As a result of a series of upgrades and resurfacing works carried out on Kessock Bridge in recent years the Tuned Mass Dampers originally installed during the bridge construction in 1982 were seizing-up and failing to function properly and therefore unable to effectively reduce the vibrations and resonance of the bridge.
The weight of the bridge had increased due to improvements to the crash barriers, pedestrian barriers, access platforms and resurfacing works, resulting in a necessity to replace the original dampers for heavier dampers to compensate for the changing dynamics of the bridge.
Millar Callaghan carried out a full and detailed survey of the existing dampers and supporting steelwork and eight new customised dampers were ordered from a specialist supplier in Germany. The additional replacement steelwork was then fabricated and galvanised, before being transported to site.
Specially designed lifting frames were constructed to remove the old dampers and install the new dampers in position on the underside of the bridge deck.
The dampers were located, not only in the centre of the bridge but also in the middle of the shipping lane, which caused some difficulty as any access put in place would need to be completely removed at a day’s notice to keep the shipping lane open. The solution was to raise a 24-meter aluminium platform from a barge anchored in the shipping lane and secure it to the underside of the bridge. This could be winched along the underside of the bridge to clear of the shipping lane when required.
Lifting equipment was positioned around the existing damper and the lifting frame was raised into position to cradle the old damper. With the damper secured in the frame it was disconnected from the bridge steelwork and lowered onto to the barge. The old dampers were unloaded from the cradle, using a crane positioned on the barge, and the new dampers then loaded and lifted into position, secured to the steelwork and fully torqued into position. On completion of this process for all eight dampers a specialist engineer tuned each damper to match the exact frequency of the bridge. A full quality package was presented to the client on completion of the contract.
