CANTILEVER STONE stair cases in 60 Grade 1 listed buildings at London's Bedford Square could be in danger of collapse, a structures consultant warned this week.
The warning follows last week's sudden collapse of a section of stone cantilever stair in one building as aluminium nosings were being removed from the treads during a refurbishment.
No one was hurt, but building leaseholder Gardiner & Theobald called in consultant Cameron Taylor Bedford to investigate.
As part of the investigation the consultant also checked seven other Gardiner & Theobald leasehold properties in the Georgian square and found problems in five of them.
'In view of the high proportion of stairs with significant defects we felt we had no alternative but to warn the leaseholders of the other buildings in the square, ' said Cameron Taylor Bedford technical director Clive Richardson.
'This type of stone stair, mistakenly dubbed cantilever, can collapse catastrophically and without warning if it isn't properly maintained.'
Richardson said this type of stair could rarely act in a pure cantilever mode as the ends of the treads were usually only built a short distance into flanking walls, making the connection too weak to support cantilever stresses.
In practice the tread was supported along one edge by the tread below and prevented from rotating by its embedment in the wall.
Problems usually start when the 3mm or so of lime putty between the treads begins to crumble.
'Normally the balustrade acts as a safety belt if anything goes wrong, ' Richardson explained.
'Loads from a failed tread pass up the baluster into the handrail and are shared between adjacent treads.
'But we found that the lead caulking that's supposed to connect the bottom ends of the balusters to the stone treads had failed in many cases.'
Richardson said he would be reporting his findings to the Standing Committee on Structural Safety.