CROSS Safety Report
Wrong length blind bolts lead to unsafe bridge structure
Bridge strengthening being carried out using blind bolts left a structure in an unsafe condition with traffic loading on it.
Key Learning Outcomes
For civil and structural design engineers:
- Connections can often be the weak link in structures and attention to detail is required
- Specifications supported by an appropriate inspection and test plan are required for critical connections
- Critical connections must be checked after or during installation using the inspection and test plan
- If you are uncertain about technical information provided by the fixing manufacturer, seek clarification from their technical support team
- Construction drawings can be used to draw attention to critical connections
- The Steel Construction Institute has published a number of articles on the use of blind bolts (sometimes referred to as toggle bolts)
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The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
Bridge strengthening being carried out using blind bolts left a structure in an unsafe condition with live traffic loading on it.
A reporter described how a metal bridge was strengthened by overplating the web of a girder by attaching new plates, using blind bolts, because one side of the girder was mostly inaccessible. Existing rivets were knocked out and replaced with blind bolts, and lap plates were also connected to the new plates with blind bolts. On a snagging visit after the works, on a part of the girder that was visible from the other side, it was noticed that the blind bolts had not penetrated far enough. It was found that the wrong bolt lengths had been specified. It had been assumed that proper engagement of the blind bolts would be confirmed when tightened, however, the bolts had merely tightened in the hole without protruding on the far side sufficiently. The reporter goes on to say that with normal bolts this error would be obvious as the nuts could not be tightened.
The reporter says that the issue was caused by human error compounded by the limited range of blind bolt sizes available. In the inspection and test plan, there was no requirement to verify the correct bolt lengths. Accordingly, the reporter highlights the need for inspection and test plans to include checking of blind bolt lengths and for warnings to be considered for inclusion on construction drawings.
Expert Panel Comments
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Fixings are critical and often the weak point in any system. It is essential that robust inspection and test plans (ITP) are in place to ensure that structural components, particularly fixings, are suitably and sufficiently designed and installed. In this case, the reporter has acknowledged as a point of learning, that the ITP was not sufficient to identify the faulty installations.
fixings are critical and often the weak point in any system
A number of different assurance routes may be taken when checking installations such as that presented in this case. One model requires the provision of a specification by the designer, the contractor to provide an ITP, and a check by the designer that the ITP meets the requirements of the specification. Thereafter the contractor needs to construct in accordance with the specification with validation being undertaken according to the ITP. Any proposed changes (such as alternative materials) being agreed upon by the designer before execution.
Another model, preferred by some designers as prudent when there are multiple unknowns, sees test installations undertaken to prove the methodology before the ITP is developed. The designers need to be involved in the test installations to ensure that any variances and anomalies (such as material thicknesses) are catered for in the design by, for example, appropriate sizing and spacing of the bolts. The ITP then needs to provide a route map such that all installations, regardless of variances, are confirmed as adequate.
Clearly, regardless of how the ITP is developed and used, designers must be sure that any item specified is indeed available. The reporter draws attention to the provision of ‘warnings’ on construction drawings; this method of highlighting critical parts of any construction can be effective.
‘warnings’ on construction drawings can be an effective method to highlight critical parts of a construction
When dealing with installations where visual inspections are difficult, inspections using a borescope may be useful. The borescope can be inserted through small holes made in low stress areas that are subsequently made good.
CROSS has published a number of reports concerning blind bolts including Blind bolt query which although published in 2008 is still a useful reference. The Steel Construction Institute has published several articles on the use of blind bolts (sometimes referred to as toggle bolts).
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