Cross UK Report 1161

Collaborative reporting for safer structures. Report 1161 Corrosion concerns on a pedestrian bridge.

A pedestrian bridge may have reduced load capacity due to corrosion at the root of cantilever supports.

A reporter is concerned as hundreds of people use the bridge daily and the location of the corroded parts is 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. 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.

Comments

The corrosion appears significant, but it may be difficult to understand the structural implications without removing it 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.

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 that set down in the National Highways DMRB document CS 450 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:

• all structures to be included in a managed asset database
• undertaking reviews and audits of the quality of inspections
• an escalation process for safety issues discovered during an inspection; and
• using trained and competent inspectors – for example, the LANTRA-managed Bridge Inspector Certification Scheme (BICS).

Finally, but 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 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 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 may have an increased inspection and maintenance requirement.

CROSS report 772 Corrosion of bridge girder beams: bit.ly/CROSS_772

Image credit | Alamy