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Welds at centre of B of the Bang row

Fatigue cracking caused by wind-induced vibration was revealed as the reason behind the B of the Bang failures in legal papers obtained by NCE this week.

Weld defects produced fatigue critical details in which cracks propagated leading to a 3.5m section of a spike falling off in January 2005, followed by further failures.

The failures are listed in papers lodged by Manchester City Council (MCC) at the Technology & Construction Court.

The mode of failure has been agreed as fatigue cracking by the council's checking engineer Arup and the design team being sued by the council.

The council is claiming £1.9M for damages and an indemnity arising out of breaches of contract and/or negligence by the defendants, Thomas Heatherwick Studio and its subcontractors Packman Lucas, Flint & Neill, and Westbury Structures.

The spikes should have been made from 7m lengths of weathered steel cold formed into semicones and welded in pairs. However, in 54 of the 175 spikes, the 7m lengths were replaced by two 3.5m long sections.

This introduced a fatigue sensitive joint close to the tip of the spike where the structure would experience the most movement.

Also at tack weld positions along each seam weld, the machine weld had failed to fuse the tack weld material in some places, meaning that the weld could unzip.

Manchester City Council claims that Heatherwick Studio acted in breach of contact and/or was negligent because it failed to ensure that the sculpture was designed in a substantial and workmanlike manner.

Heatherwick studio released a brief press statement this week which said: "Our priority is to open the sculpture for the public to enjoy as soon as possible and we hope this action leads to a positive resolution for the project and for Manchester."

The council claims that Packman Lucas failed to use the correct specification for a dynamic structure and did not correctly specify the welds or the weld inspection regime.

It also claims that vortex shedding was not taken into account in the design and that no action was taken when the fabrication subcontractors proposed to weld 3.5m wide plates together to form the plate for a 7m spike section.

Flint & Neil is alleged to have acted negligently because it also failed to take into account the effects of vortex shedding.

Westbury structures stands accused by Manchester City Council of poor quality welding and introducing the weld at 3.5m from the tip, creating the fatigue sensitive joint.

Inspection work carried out in July and August 2007 confirmed problems with about 25% of the repair welds, suggesting that
in-situ repair may not be the solution, claims the council.

The council also claims that further spike failures cannot be ruled out without knowing that full penetration and complete
fusion has been achieved in all of the welds.

Therefore regular inspections of the sculpture are required as well as fencing the structure off from the public.

Manchester City Council claims that the only way to minimise the risk of future failures with certainty is to remove all of the existing 7m long spike tips and replace them with new tips together with the introduction of a new damping and tethering system.

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