The latest cracks on the US-77/SH-39 Bridge were in areas where none had been anticipated, said Oklahoma Department of Transportation (ODOT). However, ODOT remains confident that the bridge will reopen as planned on or before 14 June.
The bridge was initially closed on 31 January following inspections that revealed several cracks in lower members made of carbon manganese steel, which is not conducive to welding.
The contractor was fitting repair brackets when two additional cracks were found near the gusset plates in the manganese alloy lower chord. Ninety-six areas of the bridge are now slated for repair.
The recently formed cracks are associated with welded manganese steel but were unexpected because the welds were relatively small compared to theose in the initial cracked locations. Engineers had believed that not enough heat was generated to cause the material to become weakened.
When the repair contract was awarded, ultrasonic impact treatments were specified for use on all welds made to manganese members, regardless of cracking. The device uses vibrating needles to relax tension on the base of the welds. The treatment appeared to be working on the gusset plate weld areas until cracks began to appear. The ultrasonic impact treatments are an ongoing repair and have no bearing on the reopening as they could be completed under traffic.
"Initially it was thought that these locations could be sufficiently different from the welded manganese areas that they might not be susceptible to cracking, especially since no cracking had been observed during the time when about 180 cracks were developing at the original cracked locations," said Modjeski & Masters chariman emeritus and senior technical advisor John Kulicki.
The new repairs will involve adding reinforcement plates connected to the existing plates using bolts.
