Over 2 years areas on the cable bundles were tested

In 2004 the Forth Estuary Transport Authority (FETA) conducted a major assessment of the expected lifespan of the bridge, engaging the consultant engineering companies Faber Maunsell and Weidlinger Associates for the engineering assessments and Exova’s Edinburgh laboratory to undertake all laboratory material testing and examinations.

Scope

Cliff McGuire, Metallurgical Manager at Exova Edinburgh explains: “FETA operates a rigorous maintenance regime on the Forth Road Bridge that ensures regular external inspections of the main load bearing cables. However, this type of inspection cannot directly assess the condition of the individual wires within a cable bundle”.

Current practice in the USA, where there are a large number of these suspension bridges, is that aerially spun cables on suspension bridges should be internally inspected after 30 years. The American guidelines for such inspections, NCHRP report 534, were only published in 2005 although FETA adopted the draft guidelines in 2004 for this assessment of the 40 year old bridge.

Over a two year period 10 areas on the cable bundles were exposed for inspection and sampling. Conditions of the wires in the first panel opened were found to be far more deteriorated than expected. Moisture had permeated through the protective layers of paint and red lead paste has corroded the zinc coated steel wires. In addition to corrosion, cracked wires were also discovered that had failed by a hydrogen embrittlement process.

Results

From the information generated by the inspection and testing programme, it was established that although the calculated deterioration of cable strength was between 8 to 10%, the factor of safety still well exceeded the minimum of 2.0 as specified in the guidelines.