CROSS Safety Report
Failure of supposedly grade 10.9 bolts
This report is over 2 years old
There was a high potential for collapse when supposedly grade 10.9 high strength holding bolts failed on a crane base.
Sixteen bolts, which had been pre-tensioned in the standard way, all failed within four days.
Key Learning Outcomes
For construction professionals:
Consider introducing a quality control procedure on site for the inspection of safety critical elements being delivered to ensure they are of the required quality
It is good practice to obtain certification to ensure that all components are sourced only from suppliers who can guarantee compliance with specification
The quality of supply is important to avoid variation in quality
There is always the possibility of counterfeit components and this was the subject of a Standing Committee on Structural Safety (SCOSS) alert: Anomalous documentation for proprietary products
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A report describes an incident where there was a high potential for collapse when supposedly grade 10.9 high strength holding bolts failed on a crane base. Sixteen bolts, which had been pre-tensioned in the standard way, all failed within four days.
This was found to be a brittle failure and because the bolts were pre-stressed, they failed in a manner which released energy, resulting in the bolt heads being ejected up to 4m above the lifting frame.
It is believed the failure was due to delayed hydrogen embrittlement which can be caused in the manufacturing process. This type of failure would be expected to occur within a few days of tensioning bolts as was seen in this case. The investigation is ongoing, and no-one was injured. The reporter says that in future, within their organisation:
Bolts of Grade 10.9 or higher to be approved by technically competent person(s) prior to use
Hardness and tensile tests to be used as a minimum to verify the metallurgy of all Grade 10.9 bolts
Permits to include the need for technical sign off
All using this type of bolt to be briefed on this incident
Expert Panel Comments
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This is another report about problems with fixings which is a frequent topic for CROSS. Presumably, the bolts were delivered with the correct certification/ markings, which is why they were installed without any questions? In general, but particularly for safety critical components, inspection and test plans should be developed prior to construction:
- Obtain certification to ensure that all components are sourced only from suppliers who can guarantee compliance with specification
- The quality of supply is very important to avoid variations in quality
- Ensure that the products are inspected by competent persons before installation
- Ensure products are installed in accordance with manufacturer’s and designer’s recommendations
There is always the possibility of counterfeit components and this was the subject of a Standing Committee on Structural Safety (SCOSS) alert: Anomalous documentation for proprietary products published in 2013.
In the UK organisations such as Highways England use only approved suppliers to avoid installing counterfeit components. For a company to be listed on the Schedule of Suppliers, it must be accredited by UKAS.
In the USA there are programs directed towards detecting counterfeit components including bolts and below is a list of General Indications:
- No manufacturer’s or grade mark (unless certified to a specification not requiring marking)
- Evidence of machining marks
- Poor thread form, evidence of wear, or dressing
- Headmarks shown on the Suspect Fastener Headmark List (USA Department of Energy list)
- Foreign manufacturer not meeting Public Law 101-592 (USA law)
- No markings for nuts or washers packaged with labels indicating that they were manufactured to a code or specification which requires marking
- Headmarkings are marred, missing, or appear to have been altered
- Headmarkings are inconsistent
- Double stamping
- Metric and SAE stamping
- Headmarks with raised marks and depressed marks on same bolt (not normal manufacturing process)
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