CROSS Safety Report
Failure of anchor bolts holding suspended scaffold
This report is over 2 years old
During the removal of a suspended scaffold, it was found that a number of the screw bolt anchors had failed.
Key Learning Outcomes
For construction professionals:
Quality control and competent supervision on site can help to ensure that safety critical fixings are installed in accordance with the manufacturer’s requirements
For structural design engineers:
Connections can often be the weak link in structures and attention to detail is required
An attribute of ‘safety’ is to assure that the design is not disproportionately vulnerable to minor error
Selecting the correct fixings and corrosion protection for the given environment is important to ensure they perform as expected
For building owners/managers:
The specification of any inspection and maintenance requirements for fixings should be considered and recorded in the operation and maintenance manual
Fixings may require a combination of regular visual inspections with full inspections at appropriate intervals. They should be inspected by a person who is competent to do so.
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During the removal of a suspended scaffold, it was found that a number of the 12x100mm hexagon head screw bolt anchors had failed. The scaffold spanned a 10m wide, 20m deep pumping well which had been fully boarded as a crash deck and light use access platform.
Bolt failure rate
The overall failure rate was less than 8% (circa 14 bolts). At the main spine beam connection, two fixing brackets were used with six holes in each bracket, four at 15mm diameter and two at 13.5mm diameter. Four bolts were installed in each bracket fixing. At one of these fixings the bolt failure rate was found to be 87% with seven out of eight bolts having failed.
At one of these fixings the bolt failure rate was found to be 87% with seven out of eight bolts having failed
A full investigation was launched to establish the reasons for the failure. Due to the unusual site conditions the investigation proved to be quite complex in nature and was a collaborative exercise that involved the main contractor, the bolt manufacturer, and the scaffold installation team.
The investigation team could not identify a primary cause for the bolt failures but did identify significant contributory causes that could have led to the failure of the platform. These have been reflected in the key learning points and actions to be taken by all parties.
Key learning points:
Extensive laboratory tests were carried out on the recovered sheared bolts to help establish the cause of failure, but these were inconclusive
Quality control and inspection procedures have been revised by all parties
Ensure all fixings are installed as per the manufacturer’s instructions
Ensure that the contractor/installer is fully briefed on the approved and agreed method of installation
Ensure contractors are aware that any change of design specification must be checked and approved by the professional engineering services team before works can proceed
Action to be taken by site management:
Ensure all the required inspection regimes are in place and undertaken
Ensure all fixings are installed in accordance with the manufacturer’s requirements
Ensure all installers are briefed on the approved method prior to work commencing
Screw bolts can be used in less critical situations e.g. propping of wall forms where multiple fixings are present. The risk of potential failure should be assessed by a competent person.
If in doubt refer any questions to an in-house (or external) engineering specialist
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Expert Panel Comments
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The report shows a commendable degree of collaborative investigation and passing on lessons that may be learned. An additional aspect is ensuring the traceability of the fixings – are they actually what they say they are? Fixings problems amount to a considerable proportion of all CROSS reports and many are related to hanging structures.
In a redundant system it is not possible to predict the forces in hangers with accuracy. This means that fasteners must be capable of redistributing overload, which is not always possible. Either they pull out, as in many of the reported ceiling failures, or they can fail in tension setting off chain reactions.
In general, to guard against failure there should either be ample redundancy by means of multiple fixings, or the capacity for some yielding of the hangers so that loads can be re-distributed. Self-tapping concrete screws are a well-developed fixing technique carrying significant advantages over some traditional fixing types especially for temporary works applications.
In general, to guard against failure there should either be ample redundancy by means of multiple fixings, or the capacity for some yielding of the hangers so that loads can be re-distributed
Similar failures in one type of this sort of fixing have been recorded previously and it is thought that hydrogen embrittlement may have been a contributory factor. Hydrogen embrittlement is a phenomenon which afflicts very high strength steels when subjected to stresses and is a complex subject.
Failure is precipitated by stresses causing the migration of hydrogen atoms within the steel to the point of highest stress which, in a bolt type fastener, may be at the root of the first thread or a fillet radius under the head - particularly if either of these has been badly formed. Contributory factors leading to the incorporation of hydrogen in the steel include poorly controlled heat treatment during the hardening process and acid pickling as part of the plating process.
Self-tapping concrete screws are typically made from very high strength steels and are subject to a certain degree of stress when being installed into concrete so, if they happen to have been subject to poorly controlled manufacturing processes, failure due to hydrogen embrittlement is a possibility which may be exacerbated by over tightening. However, it is not known if these circumstances applied in this case.
Self-tapping concrete screw anchors that have been awarded an ETA (European Technical Approval/Assessment) will have been subjected to a comprehensive test regime which includes a test specifically designed to identify anchors with a high susceptibility to hydrogen induced brittle fracture.
To help avoid such failures choose an anchor with an ETA, ensure it is selected in accordance with the ETA, and is installed by a trained operator in accordance with the manufacturer's instructions. A practical issue is that bolts should be substantial enough to resist possible misuse by over-tightening.
Useful references include:
1. Guidance Note: ‘Self-tapping concrete screws' - Construction Fixings Association
2. ‘Fundamentals of hydrogen embrittlement in steel fasteners’ - Salim Brahimi Eng.
3. General guidance on the use of construction fixings in safety critical applications:
BS 8539:2012 - Code of practice for the selection and installation of post-installed anchors in concrete and masonry
Clarification on Report 430 Failure of anchor bolts holding suspended scaffold published in CROSS Newsletter 39 (October 2015)
‘In our report we showed images of a typical screw bolt and fractured bolts. The report indicated that it was thought that hydrogen embrittlement may have been a contributing factor to the failure of similar types of bolt. The report was accompanied by a picture of an Excalibur screwbolt. We have been asked by Excalibur to make clear that the incident concerning their bolt took place in 2012 and was subject to a full investigation and that there was no finding of any manufacturing fault or inherent defect in the product and, specifically, that there was no issue of hydrogen embrittlement.'