An official inquiry was launched in mid-December into the closure of Scotland’s vital Forth road bridge, causing traffic chaos in the Edinburgh region. A crack discovered in a steelwork stiffening truss of the 1.8-kilometer-long suspension bridge forced closure on Dec. 4 for at least one month of repairs.

With a roughly 1-km-long main span, the 51-year-old bridge carries more than two million vehicles a month over the Firth of Forth near Edinburgh. Transport Scotland forecasts the cost of repairs to exceed $3 million. But with the nearest next crossing located over 30 km to the east, the economic impact of closure is said to be much greater.

Transport Scotland closed the crossing after its operations and maintenance contractor, Amey PLC., Oxford, discovered a crack in the 9-m-deep truss under the deck at the northeast pier during a routine inspection. The crack occurred at the bottom of a large vertical tie that hangs from a bracket at the pier to support the end of truss’s bottom chord.

Amey is reinforcing the cracked truss with a specially made steel “splint” welded to the vulnerable bottom end of the hanging tie. The contractor is being advised on the repairs by the design firms Arup Group, London, and W.A. Fairhurst & Partners, Glasgow.

Although only one of the eight similar connections along the bridge is cracked, the remaining seven are being reinforced and monitored as a precaution, says an Amey spokeswoman. Amey took over bridge operations under a five-year contract secured 12 months ago and valued at $60 million to $75 million.

The inquiry by the Scottish Assembly’s Infrastructure and Capital Investment Committee will focus “on the structural defects identified and whether these could have been avoided or dealt with differently.” One line of investigation will likely look at why a design contract to reinforce the trusses was cancelled four years ago.

In May 2010, the bridge owner advertised a contract to design “new truss end connections.” The contract notice explained that the connections had become overstressed. “A feasibility study has been undertaken and a preferred option identified to strengthen the existing truss end link connection,” it added. However, the invitation to bid was withdrawn the following March.