Three bolts of just under a meter in length have failed in as many months on London’s second tallest building, causing the building’s owner to replace dozens of bolts and raising fears more will fail.
In its latest announcement, London based developer British Land has acknowledged that following the earlier collapse of two bolts on its 224 meter Leadenhall Building in November, a third bolt had recently fractured and was captured by precautionary tethering put in place late last year following the collapse of the other bolts.
The company said site and laboratory testing of both the broken bolts and several other bolts by contractor Laing O’Rouke and structural engineering outfit Arup had confirmed fears the problem was caused by a material failure mechanism known as hydrogen embrittlement – a crack growth mechanism within the bolt material whereby according to the Gizmodo web site single atoms of hydrogen enter the steel, migrate through the crystal lattices and attract themselves to the area of highest stress where they cause small fractures to propagate and, in worst cases scenario, a cascading effect as the stress increases and attracts more free hydrogen, leading to a sudden catastrophic failure such as the shearing of a bolt.
Such a phenomenon creates substantial headaches regarding safety as it can cause bolts which have previously shown no sight of stress to snap suddenly as opposed to deforming over time and showing signs of stress during inspections.
In a statement, British Lands said the tests had indicated the building’s overall stability was not in jeopardy.
“These confirm that the problem is limited to certain bolts,” the company said, adding that a program to replace the bolts would take place as a ‘precautionary measure’.
“Arup has also confirmed that there is no adverse effect on the structural integrity of the building.”
Designed by Rogers Stirk Harbour + Partners and completed in September, the Leadenhall building – which is known as the Cheesgrater – is renowned for its sloping profile which was created to preserve the views of St Paul’s cathedral, and at 47 storeys in height is listed by the Council on Tall Buildings and Urban Habitat as the fifth tallest building in London.
The first two bolts broke in November when one came right off and landed in the hoarding area after a section of the bolt came loose whilst part of the second bolt came away from the node that connected it to the steel frame and glass structure – albeit with that bolt remaining in place.
Though no one was injured and the company insisted the building was safe, British Lands was forced to cordon off the building to pedestrians at that time.