CROSS Safety Report
Defects found in previously repaired bridge
This report is over 2 years old
While carrying out strengthening work on a bridge, there was concern that by removing the non-structural encasement, alternative load paths had been removed.
Key Learning Outcomes
For construction professionals and designers:
Be aware that even on well managed structures, there is the possibility that historic repairs can hide historic defects which may need to be investigated
Before carrying out any intrusive investigation on a structure, where the principle structural elements are hidden, it can be good practice to carry out a risk assessment based on the possible structural condition if the opening up could impact load paths
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A reporter was involved in the inspection and assessment of a concrete encased steel bridge structure constructed between the wars. The structure concerned was over a railway. Generally, the structure was in good condition although there was evidence of historic repairs. It was assumed that as a piece of managed infrastructure, these repairs were carried out by competent people.
The reporter had full historic details of the construction and so opening up was limited to accessible parts. It was mainly intended to validate the historic information, provide a limited number of confirmatory material samples, and give an indication of the condition of the encased steelwork.
This opening up confirmed the historic data and showed the steel to be in reasonable condition.
The subsequent structural assessment showed a limited number of areas where the structure was under strength to current loading standards and strengthening works were developed.
The work was to be carried out piecemeal in short night-time periods while the trains were not running. On breaking out of the non-structural concrete encasement to carry out the strengthening, a significant section loss was found at one location. This also occurred at a second location previously repaired, indicating the underlying cause of the previous damage had not been addressed.
On breaking out of the non-structural concrete encasement to carry out the strengthening, a significant section loss was found at one location
There was concern that by removing the non-structural encasement, alternative load paths had been removed and work was stopped. Load restrictions were put in place whilst temporary works were designed and installed.
Expert Panel Comments
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Even on an apparently well managed structures, the possibility that historic repairs hide historic defects should be investigated. Before carrying out any intrusive investigation on a structure, where the principle structural elements are hidden, a risk assessment based on the possible structural condition is required if the opening up could impact load paths.
In addition to addressing safety issues, the risk assessment should be seen as an opportunity to discuss the cost benefit to the client of investing in mitigation measures, even if they are never required, where commercial consequences are high.
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