CROSS Safety Report
Liquid metal assisted cracking
This report is over 2 years old
Large cracks appeared down the sides of gusset welds after a truss was erected on site.
Key Learning Outcomes
For civil and structural design engineers:
Be aware that liquid metal assisted cracking can occur if due attention is not paid to the design, fabrication, galvanising and inspection processes of susceptible assemblies
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Another reporter had to advise on a large timber grillage system that had been connected at the nodes with steel connectors formed of a central bar (about 100mm diameter) fitted with radial gussets welded on to it. The whole assembly was then galvanised. The trusses were erected on site, but during erection it was noted that there were large cracks down the sides of the gusset welds into the parent body of the bar. Fortunately, this was spotted before it was too late, nevertheless, the whole truss system had to be dismantled for safety reasons.
The reporter’s firm took expert advice to establish the cause of cracking and the consensus was that it had occurred in the galvanising bath and that it was Liquid Metal Assisted Cracking (LMAC). It looked likely that the cracks had been masked by zinc so were not visibly detected until the structure had been erected and tension applied across the crack surface. The nodes were remade with tougher steel and the cracking did not re-occur.
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BCSA document 40/05 emphasises that LMAC is a rare phenomenon. However, it can occur if due attention is not paid to the design, fabrication, galvanising and inspection processes of susceptible assemblies. It is apparent that not all designers are yet familiar with the guidance.