CROSS Safety Report
Corrosion concerns on a pedestrian bridge
A reporter is concerned about a pedestrian bridge that may have reduced load capacity due to corrosion at the root of cantilever supports.
Key Learning Outcomes
For asset owners and managers:
- Regular inspections and maintenance can help keep a structure safe before it deteriorates and requires costly repairs
- Inspections should be carried out by suitably qualified and experienced personnel
For civil and structural design engineers:
- The detailing of structures, particularly to ensure the effective shedding of water, is fundamental to their durability
- The corrosion protection specification for structures is important
- Cantilevers give cause for concern as they have no redundancy
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A reporter is concerned about a pedestrian bridge that they believe has reduced load capability due to corrosion in cantilever deck supports as shown in Figure 1 below. With hundreds of people using the bridge daily, says the reporter, the danger should not be neglected. The location of the corroded parts is very unfavourable being at the root of cantilevers where there is high bending moment and shear.
The reporter believes the situation has come about due to the structure not being adequately checked as a matter of routine. The reporter goes on to say that all existing structures used by the public need to be checked periodically to avoid unacceptable structural deteriorations. In this case, the capacity of the cantilevers should be assessed and verified that they can withstand the loadings according to appropriate regulations.
The reporter has notified the local authority as to the condition of the structure.
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Expert Panel Comments
An Expert Panel 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-US Expert Panel page.
The corrosion appears significant, but in cases such as this, it may be difficult to understand the structural implications without removing the corrosion and assessing the steel section loss. Assessing the extent of deterioration and measuring the thickness of the steel that remains appears to be necessary. This investigation and the follow-on assessment of capacity should be done as a priority. Cantilevers always give cause for concern as they have no redundancy and corrosion at the root can be a serious matter. Corrosion, as shown in this example, will continue and the structure potentially fail, unless steps are taken to remediate it.
Asset management regimes
Regular inspections of assets such as bridges are essential. All bridge structures should have an appropriate inspection and maintenance regime regardless of their ownership by a public or private body. Good practice, such as set down in National Highways DMRB document CS450 Inspection of Highway Structures, is followed by many UK highway authorities. This standard sets down inspection types and frequencies that are designed to ensure the safety of the structure and facilitate effective long-term management including maintenance. Design sign-off would normally include confirmation of the operational plan to manage the structure during its life. Other key related aspects of asset management include:
Finally, but very importantly, too often ‘design’ is thought of as a process to keep stresses under limits, however, that is just part of the process. Design includes achieving sensible durability and maintenance and certainly, on exposed structures, ensuring that water can be shed effectively. The value of a least weight design, which just a few years later when some of the steel section is lost and repairs are required, should be questioned. Detailing the fabric of the structure to achieve the required durability and avoid expensive repairs and breakdowns is essential. Alongside detailing, the corrosion protection specification for structures is also fundamental. It should also be noted that once a structure has been allowed to corrode significantly, after repairs, it may be impossible to achieve good surface preparation before repainting. Whatever paint system is subsequently applied is unlikely to last as long as the original surface protection system. So, structures with significant deterioration may have an increased inspection and maintenance requirement.
CROSS has reported previously on bridge corrosion, including report 772 Corrosion of bridge girder beams published in 2022 where hidden critical elements were of concern.