CROSS Safety Report
Failure of metal cladding panels on bridges
Overview
This report discusses the findings of an investigation into the causes of the failure of metal cladding panels on a bridge parapet and provides recommendations concerning the use of pop rivets to fix such panels.
Key Learning Outcomes
For civil and structural design engineers:
- Connections can often be the weak link in structures and attention to detail is required particularly at interfaces between different materials. The role of tolerances should not be overlooked
- Be aware that the capacities of fixings such as pop rivets and self tapping screws in thin gauge steel can be acutely sensitive to the material thickness. Fractions of a millimetre can make a difference
- It can be good practice to consult with the technical team of the fixing manufacturer to ensure the fixing nominated is suitable for the specific application
- The anticipated life span of the fixings should be noted in the operation and maintenance manual
- Manufacturer 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 the construction team:
- Quality control and competent supervision onsite can ensure the correct fixings are installed in accordance with the design
- Fixings should be installed in accordance with the manufacturer’s specifications and requirements
For asset owners and managers:
- Be aware that all elements of cladding present a significant risk to the public and require proper design and installation by suitably qualified and experienced practitioners
- Consider having a regular inspection and maintenance programme by a competent engineer who has experience with the form of construction
Full Report
Find out more about the Full Report
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.
This report discusses the findings of an investigation into the causes of the failure of metal cladding panels on a bridge and provides recommendations concerning the use of pop rivets to fix such panels.
Missing fixings
Following a site visit and inspection of the fallen panels it was established that they had each been fixed in place using ten pop rivets (five fixings to the upper rail and five fixings the lower rail). During the inspection it was also noted that fixings were missing on other panels on the same bridge.
The immediate cause of the failure is considered to have been the pulling out and shearing of the fixings due to high winds. The underlying cause is less evident.
Pop rivets if properly specified and correctly installed can be designed to have a high factor of safety against failure. As quite a number of fixings were missing on the bridge panels, it is apparent that a failure has occurred in either the specification or installation of the pop rivets.
Concerns over specification on fixings
When specifying the pop rivets, it is likely that inadequate consideration was given to the length of rivet needed to produce a secure fixing to the rails that the panels were fixed to.
It is also likely that no onsite checks were carried out to confirm that the correct pop rivets had been specified. This can be done by measuring the length of the pop rivets and the thickness of the materials to be fixed together.
The following actions were recommended following the failure:
1. It was recommended for the bridge concerned that the existing panel fixings were replaced with more robust fixings designed with an adequate factor of safety against tension and shear failure.
2. That future metal panels at bridge locations are designed and detailed to use vibration resistant High Strength Friction Grip (HSFG) bolts as the primary fixings and pop rivets as secondary fixings (e.g., to seal plates together).
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Expert Panel Comments
Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.
The weak link in any structure is often the joints or fixings. Yet in many ways, these are the parts that receive least attention in design. Often, their design is delegated down the supply chain.
Design capabilities may also be masked by fundamental uncertainties in the loading. In this case, what might be the tension and shear on any pop rivet that is anchoring a relatively large flexible sheet, and perhaps responding dynamically in the wind?
Designers may need to consider how installation sequencing may affect the loads acting on fixings
The report says there were five fixings on the top rail and five on the bottom. It is easy to imagine that any suction load varied widely amongst the fixings and, if an end pop rivet were to fail, a progressive unzipping could take place.
Likewise, there were ten rivets in total. But is the vertical load of the sheet (shear / rivet) load divided by ten or, given the size of the sheet and rivet clearances, more likely to be carried by just a few rivets? And how is that number to be determined? Designers may need to consider how installation sequencing may affect the loads acting on fixings.
Fixings and vibration
The report also hints at something we have seen in other recent reports - a mode of failure generated by vibration. Loss of fixings from vibration is a real issue, but how often is it considered? It looks possible that these panels were subject to wind buffeting and vibration. Putting those factors together you would imagine simple calculations might be misleading.
The pull-out capacity of fixings in thin gauge steel is also acutely sensitive to the material thickness. Fractions of a millimetre can make a difference on fixing capacities. It can be good practice to consult with the technical team of the manufacturer to ensure the fixing nominated is suitable for the specific application.
CROSS has received a large number of reports concerning fixings over the years
If the consequences of failure are intolerable (for example, falling on overhead lines) then bear in mind that the sizing of fixings has great uncertainty within it and be guided by what has proven adequate previously or be extra cautious. A robust design also needs to be supported with a verified installation procedure.
CROSS has received a large number of reports concerning fixings over the years and you can search for further safety information on fixings on the CROSS website. The capacity of a pop rivet and its safe design might not seem a glamorous topic but, as we know; it is frequently the details that let structures down and modes of failure not anticipated.