Watson was forced to replace all 14 hangers on the £20.3M bowstring arch structure, and now blames poor manufacturing of connection holes and faulty steel for the failures. In legal papers obtained by the NCE this week, Watson claims Macalloy’s connections did not meet specifications and failed to match supplied test certificates.
The Clyde Arc had to close on 14 January last year after a connection on the two year old road bridge failed, causing a 35m long Macalloy bar to fall onto the carriageway below (NCE 17 January 2008). The bar was one of 14 tension bars which suspend the deck from the bridge’s bowstring arch.
A second crack in another connection was found 10 days later, prompting a decision to replace all the existing connections (NCE 31 January 2008).
The connection had two flattened lugs sitting either side of
a fin welded onto the main arch structure. A pin through the two
lugs and the fin connected the two. The connection failed in a
brittle fracture in the lugs across the holes for the pin.
Watson claims that investigations show that the steel used
to create the lugs did not match any grades of steel recognised by
BS 3100:1991 Specification for steel castings for general engineering
Tests found that the steel had an elongation value over 10%
below the 13% minimum specified and a Charpy impact value which
was significantly below that specified. A Charpy impact value is a measure of brittleness.
Certificates supplied by Macalloy, which was responsible for the testing of components before delivery to ensure that they met the specification, failed to match results of tests on the steel in the bridge forks taken after the event, Watson claims.
The carbon content of the forks was 0.33% rather than between 0.18% and 0.28% as indicated in the certificate. Traces of other metals were found in the fork including 0.096% chromium, but these had not been mentioned in earlier certificates.
Poor manufacture of the forks was also thought to have contributed
to the failure. Watson Steel claims that the pin bores were neither concentric to each other nor perpendicular to the forks with the result that forces were distributed unevenly across the holes allowing stress concentrations to build up.
Macalloy has yet to respond to Watson’s claim and was unavailable
for comment this week.
The Clyde Arc was designed by consultant Halcrow and built by
contractor Nuttall for Glasgow City Council. It opened to traffic
in September 2006 but was closed in January 2008 due to the connection failures.
Repair works, undertaken by Nuttall and supported by Halcrow
and fabricators Watson Steel, resulted in the cast steel connection
components replaced by milled steel (NCE 24 April 2008).
The bridge reopened in June 2008.