A temporary bridge over a motorway was ready for commissioning when a fractured bolt came to light. As safety is always Locapal’s first priority, the site management team immediately returned the bolt to the manufacturer and asked for an explanation. The manufacturer’s verdict was that the problem most probably stemmed from incorrect tightening. For Locapal, whose teams of fitters work to strict assembly instructions, this explanation sounded highly implausible. The suspect bolt was then sent to Cetim for a second, independent assessment.
Extensive analyses
From the very first analyses carried out using the facilities available at the Centre laboratories, it was evident that the fracture surface was not the result of faulty tightening. As Locapal had suspected, the bolt manufacturer’s explanation simply didn’t hold water. Commissioning of the bridge was suspended with immediate effect while Cetim performed more extensive metallurgical analyses. These analyses revealed a problem of intergranular decohesion; a fault that could clearly not be put down to over-tightening, but was in all likelihood caused by a manufacturing issue, specifically during the heat treatment phase. This meant that all the bolts used in the assembly of the bridge presented a risk of fracture. On Cetim’s advice, Locapal took the decision to replace all the M30 bolts, numbering some 500, in the structure. However, to ensure the safety of the bridge, the company first commissioned the Centre to check the compliance of a number of bolts from the new batch. Both metallurgical characterisation and mechanical testing confirmed their compliance with the relevant requirements. The process of replacing each bolt in the structure one by one took a total of 16 hours.
In the words of David Thibaudeau, Works Director at Locapal, “The Cetim’s experts were highly responsive. Their advice, expertise and detailed responses to our questions were extremely valuable in enabling us to take the right decisions.”
