GLASGOW'S TROUBLED £8.5M Wing Tower could be shut for many months, it was revealed this week. Doubts are growing over the design of the suspect bearing which led to the 105m tall structure's closure last week.
Partial failure of the bearing due to accelerated corrosion was initially blamed for the structure sinking 15mm. This corrosion was thought to have been caused by a leak in waterproofing protecting the bearing's housing pit.
Tower owner Glasgow Science Centre (GSC), said that water found in the base pit was now thought to be rainwater that had leaked past seals between the tower and the podium roof.
These are thought to have been ruptured by the tower's gradual downward movement.
The massive, specially manufactured thrust bearing supports the entire structure and allows it to rotate 360¦ so that it always faces into the wind (NCE last week). It had been assumed that over the 12 months since the structure opened, water leaking into the 15m deep concrete pit housing the bearing had flooded the area and caused the problem.
The latest news, if confirmed by the current investigations, could imply that the thrust bearing initially failed due either to a manufacturing or design fault. This would cast doubt on plans by main contractor Carillion to replace the suspect bearing with an identical component.
However, operations to jack up the tower and remove the suspect bearing will not start until replacement bearings have been delivered from Swedish manufacturer SKF, the GSC spokeswoman confirmed.
'It could be eight weeks before the bearings arrive, ' she said.
Structural engineer Buro Happold senior partner Ian Liddell said that bearing replacement was designed to be a routine operation. 'Specially made beams have to be installed below the root cone to support the jacks, and these jacks act on a shoulder formed in the cone itself, ' Liddell explained.
'Before the tower sank 15mm it would only have been necessary to jack up the whole 470t structure by 5mm to take the weight off the steel stool which supports the suspect thrust bearing.'
Once the 300mm high stool is clear it will be slid out. The sump holding the oil that should surround the bearing will be drained and removed, and both the thrust bearing and the 'off the shelf' radial bearing above it dropped out of their housings and clear of the root cone.
Temporary chocks above the radial bearing will take transient wind loads during the replacement operation. At this point, the engineers will get their first look at the bearings, and decide if it is sensible to replace them with identical components.
A straightforward replacement operation should only take three or four days, unless there is evidence of serious underdesign, the tower may have to remain immobilised for many months.