URGENT EXAMINATION of up to 300 bolts securing the Forth Road Bridges suspension hangers is likely to start next week after a bolt which failed in December was found to contain brittle steel.
Bridgemaster Alastair Andrew said that over the last 34 years only three bolts have failed in this way, but he added that it was possible more would be found.
We want to test over 90 bolts removed from the bridge some time ago, he said. If any fail to meet the standard, we will examine up to half the 768 out on the bridge.
It was already planned to renew all bolts over the next three years as part of a 9M hanger replacement contract due to be awarded in early April (NCE 4 September 1997). But Andrew considers the remote risk of a failure demands more immediate action. Client the Joint Bridge Board is tomorrow expected to approve a 100,000 testing programme.
The 1m long, 60mm diameter threaded rods secure each bridge hangers lower anchorage socket to steel deck plates. Four prestressed bolts fix each double hanger unit.
Metallurgical tests on the side span bolt which failed in December have revealed below-standard carbon, manganese and sulphur content resulting in the rod being too hard. We think the fault was caused during manufacture when rods were quenched in hot oil, said Andrew. If the timing was out by just seconds, the rods can vary in hardness over their length.