Parapet and Barrier Testing
Last updated on 11 Jun 2010
In February 2003, the Board was advised of the need to carry out an assessment of the bridge parapets and barriers. These had been designed more than forty years previously and since then the design requirements for these elements had increased significantly in line with heavier modern vehicles.
Consulting Engineers, Flint & Neill Partnership carried out the evaluation which comprised a strength assessment and geometric checks to the current UK assessment standards. The assessment identified shortfalls in both strength and geometric requirements in a number of elements with a wide range in the degree of inadequacy.
Rankings were established for the parapets and barriers. These indicated the priority for replacement or upgrading and were found to be generally in the range medium to low. This level of ranking is not considered to be significant when compared with other highway bridges, although there is an argument for achieving a better standard on a crossing of such national importance as the Forth Road Bridge.
It must be emphasised that whilst the assessment of the barriers has shown that they fail to comply with current design standards, over 550 million vehicles have crossed the bridge since its opening and no cars have breached the barriers. Heavy goods vehicles overturning have damaged sections of barrier but this is not a load that bridge barriers are designed to withstand. The risk to bridge users is therefore minimal. It should also be noted that according to the British Standards Institution, across the UK accidents involving the penetration of parapets and barriers are fortunately rare and those resulting in secondary accidents due to penetrations are extremely rare.
Flint & Neill recommended testing, including off-site vehicle impact testing, to determine the containment level of the barriers. This was considered to be of particular benefit on the suspended structure where aerodynamic performance, and the strength of the underlying structure to carry the additional forces, would require careful consideration
The first of these tests were carried out in April 2007 at the MIRA Limited testing facility on a 54 metre long replica of the outer carriageway barriers to the suspended spans. The results were encouraging and showed that the barriers conform to the geometrical and strength requirements of the containment level appropriate for the bridge carriageways for speeds up to 80 km/h (50 mph).
With regard to the vehicular barriers on the approach viaducts, there is a particular risk due to the adjacent cycletrack/footway. Given this specific risk it was decided that the viaduct vehicular barriers should be replaced. However, given the encouraging results of the testing on the suspended span outer barriers, it was decided that a range of testing should also be carried out on the viaduct vehicular barriers to determine their containment level.
The testing programme on the viaduct vehicular barriers is now completed and the early indications are that the barriers are able to meet the required current containment standard and will not require to be replaced. Further desk studies are being carried out to determine whether or not some minor insitu modification need to be made to the barriers and to determine outstanding issues regarding the lamp standards. A full report will be published later this year.
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