An expert says the cracked axle can safely last until 2045; ODOT would replace it in about 2026
For more information: Dave Thompson, 503-731-8263, David.H.Thompson@odot.state.or.us
One of the trunnions (axles of the lift span’s pulley) on the Interstate Bridge northbound lift span has developed a half-inch crack, after more than a half-century of use.
The northbound side of the Interstate Bridge was built in 1917; the trunnions were modified in 1958, as part of the project that built the southbound bridge. The welding process in those modifications introduced the eventual possibility of fatigue cracking. ODOT replaced two trunnions on the north tower of the northbound side in 1997, one of which exhibited fatigue cracks. An independent engineering expert from Lehigh University studied the cracked trunnion over the intervening years and produced a report on the expected life of the remaining trunnions.
Using ultrasonic tests, ODOT discovered the new crack in a trunnion on the southeast tower of the northbound bridge lift span in the fall of 2011. The same independent expert studied this new information and issued a report last month concluding the cracked trunnion could safely remain in use until about the year 2045, when the crack will have grown to about 1.5 inches based on analytical models. An OSU engineering expert reviewed the report and concurred.
ODOT engineers have adopted an extra margin of safety, recommending the trunnion be replaced when the crack grows to ¾-inch, about the year 2026 at the present rate of growth.
ODOT will perform detailed ultrasonic inspections in the summer of 2012, 2013 and 2014. This inspection schedule is twice as often as the usual two-year inspection regimen.
The 20-inch diameter trunnion and pulley wheel together weigh about 21 tons.
Editors: Attached PDF file contains timeline and graphic
May 24, 2012 HQ 12-140