ENGINEERS LAST weekend completed an intricate operation to cure a 1km cable stayed bridge in Inverness of deafening clanging, caused by movement of its loose bearings.
Kessock Bridge, which crosses the Beauly Firth, is believed to be one of only two bridges in the UK fitted with pendel bearings, which behave like single links of a bicycle chain.
The bearings consist of a 5m long steel strut, the lower end of which is pinned to a plate mounted on one of the bridge's piers, and the top end to one of the deck's longitudinal girders.
On Kessock Bridge the four bearings are positioned at the first set of backspan piers. They take compression load and also restrain uplift forces from loading on the main span. Depending on traffic loading, these axially loaded connections act either as struts or tie bars. Under dead load they are load neutral.
Over the 20 years since Kessock Bridge was built, holes in the pendel bearings' pin joints became elongated by wear.
'Once this had begun grit could get inside and the degradation accelerated, ' said Andy Fraser, senior area manager for project manager Bear Scotland.
A significant amount of play developed so that, 'as the bearings switched from tension to compression you could see the steel surfaces crash down 15mm, ' explained Alan Radin, resident engineer with main contractor Mowlem.
The percussion was disturbing Kessock Bridge's neighbours, and was a threat to the structure's long term stability. A decision to replace the bearings was taken last year.
To complete the 26 week program, engineers required a pair of four day possessions, one for each side of the dual carriageway bridge. Traffic was cut down to a two lane contraflow, with the eastern side of the bridge closed first. Traffic was removed from the western side of the bridge and bearings replaced last weekend.
To carry out the work it was necessary to remove compression and tension reactions from the bearings.
Temporary hydraulic jacks were fitted to support the deck, so combating compression.
They were fixed to slider bearings to accept rotation but could not take tension.
To ensure the temporary jacks were subject only to compression forces, 16 trucks loaded with concrete blocks were parked on the deck directly above them. This was enough to resist maximum uplift created by contraflow traffic.
'The pendels were designed to take 600t of tension forces and 480t of compression, ' said consultant Babtie's principal engineer John Redpath.
'When we loaded the bearings with the 306t kentledge we calculated that once uplift forces had been considered we would have 200t of compression.
This proved to be spot on.'
Such unusual bearings were needed at Kessock because they offer flexibility. On top of vicious winds the bridge is designed to take seismic loading as the northern abutments sit on the Great Glen Fault. The 5m long pendel struts allow significant deck movement with relatively little rotation at the pin. They also allow some lateral flex.
Due to the deck's delicate nature - it is under strength for modern traffic loading and is to be stiffened later this year - engineers were wary of changing the type of support.
'After much deliberation we opted to replace like for like. The new bearings have the same articulation and geometry but we have opted for much higher tolerances. Tougher steel will be used for the pins, ' said Redpath.
The bridge was never designed for bearing replacement. To support the temporary jacks engineers had to build a new shoulder just below deck level around each of the backspan piers, increasing their area in plan from 2.6m2 to 3.8m2. A new inspection chamber was also provided.
Both were built to minimum dimensions to satisfy Royal Fine Art Commission demands that permanent works should not detract from the graceful structure.
'The new inspection chamber was also kept to a minimum.
We had only 5mm leeway to remove elements that weighed up to 5t, ' said Radin.
'We had to do a great deal of 'Egyptian engineering' with hoists, chain pulls and runner beams.'
Work also involved replacing the pendel bearings' lower connecting brackets, which were bolted onto the pier heads.
Local firm Isleburn fabricated the three-leafed couplings into which the twin-leafed pendel struts are pinned.
It was not possible to remove the pendels' top brackets, which connect to the delicate steel deck. Welding replacements would have introduced high temperature stresses to the deck.
For these brackets, which consist of a single leaf with a shoulder support on either side, it was decided to mill the deformed pin holes in situ.
Original connections were steel on steel but as part of the maintenance operation sacrificial bushes have been added. These can be easily replaced in the future.