CROSS Safety Report
Fixings related to 395 partial roof collapse at shopping centre
This report is over 2 years old
A reporter shares their own experience and concerns after reading report 395 which discussed the partial roof collapse at a shopping centre.
Key Learning Outcomes
For civil and structural design engineers:
- Connections and fixings 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
- Manufacturers' instructions provide helpful guidance on fixings. The Construction Fixings Association (CFA) website and CIRIA publication C778 Management of safety-critical fixings are also useful references.
For building owners/managers:
- The specification of any inspection and maintenance requirements for façade fixings should be recorded in the operation and maintenance manual
- The fixings may require a combination of regular visual inspections with full inspections at appropriate intervals
- The fixings should be inspected by a person who is competent to inspect them
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A correspondent refers to the report on the roof failure (report 395) that appeared to be the result of the failure of the securing bolts. It is not reported which type of bolt was used, and this can have a significant effect of the behaviour of the fixing. The detail is such that prying tension will occur in the fixing. It could be significant depending on the factors described in the expert panel comments.
Any type of fixing that cannot take significant tension reliably could contribute to this failure. The fixing bolt in the photo looks suspiciously like a loose wedge type of fixing, where the impact driver expands the shield over the wedge. This is considered OK for fixing light loads (they are quick to set and suit services and suchlike) but the shield can pull off the wedge, deforming back to near its original shape, and the bolt come loose under high tensile loads.
If this is the case, then what we are looking at in the photo is the complete assembly, less the wedge, which will still be in the concrete. This type of anchor can be obtained up to M16, which looks the size of bolt shown. It may well be that the fixing has not been working as intended for a number of years and a particularly high load triggered the failure. It is quite possible that the loads that occurred were within the bolt's published capacity.
Tensile capacity of fixings
The typical sleeve anchor, with a pair of captive wedges expanding a loose sleeve can also be suspect, and its tensile capacity is dependent on the bolt staying tight. If the bolt tension relaxes, then the sleeve expansion relaxes and can slide out. The commentator does not think this applies here. I have known contractors select the loose wedge option as it is cheap to set and the test capacity is seen to fit the requirements.
However, vibration and long term use are not included for in the published data. They take the view (not only theirs) that such fixings are properly done using stud anchors or resin anchors. The former is where the wedge is formed in the bolt shaft and the shield is captive; these anchors cannot be removed once set (it requires failure of concrete or bolt).
The latter give a good anchorage, subject to workmanship, though there may be problems where there are large number of repeated loading cycles. The HA have specified these for fixing acoustic barriers and suchlike to bridge decks and the reporter has not heard of a failure.
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