Corrosion and mechanical issues in HSFG bolts

An investigation has been made of the environmental and structural effects that may have contributed to the loosening and loss of a small number of bolts from connections in the steelwork of a multi-span highway bridge on which vehicles (including road trains) frequently travel at speeds in excess of 100kph. The bridge, which was constructed about 40 years ago, crosses a substantial river in a rural area where estuarine crocodiles are likely to be found. The bridge itself is one of about four or five in the Northern Territory where the main deck structure is formed from large Universal Beams, connected to secondary steelwork by high strength friction grip (HSFG) bolts, and spanning between concrete cross-heads. Initial visual inspection of the underside of short sections of the bridge that were accessible from the river banks found that some of the bolts appeared to be missing, others were loose, and some appeared to have suffered from heavy corrosion. The underside of the bridge was repainted a number of years ago, and there is evidence to suggest that some bolts may have been loose or missing at that time, but no remedial action appears to have been taken. Remedial work has since been completed to replace all the bolts in the connections in the bridge and to ensure that these have been correctly tightened. Many of the old bolts that were removed from the bridge have been taken to Charles Darwin University for detailed examination and investigation, including load tests.