After 54 years of service — and at least 5,000 bridge lifts — one of the trunnions on the northbound span of the Interstate 5 Bridge is showing some age.
The 20-inch-diameter trunnion, or axle, is one of eight on the bridge that are responsible for lifting and lowering the 170-ton bridge and its 170-ton counterweights. A routine ultrasonic test has found that one, high on the southeast tower of the northbound span, has a half-inch crack, Oregon Department of Transportation spokesman Dave Thompson said Thursday.
But no need to fear — or even change lanes. ODOT officials say they believe the trunnion will last at least until 2026.
“It poses no safety issue at all at this point,” Thompson said. “We’re just being transparent and letting people know that we found the crack.”
The northbound span was built in 1917; its trunnions were modified when the southbound bridge was constructed in 1958.
The inspector, an independent engineering expert from Lehigh University in Pennsylvania, said the trunnion could last until 2045 and grow to at least 1.5 inches before it is a safety concern, Thompson said.
Together with the pulley wheels, the trunnion weighs 21 tons, so ODOT is choosing to be more conservative and is putting a three-fourths-inch crack as its maximum. At
the crack’s current rate of growth, that puts it in need of repair or replacement by 2026. The timeline for the Columbia River Crossing, a project that includes plans to replace both I-5 spans, shows construction ending early in the 2020s.
“It’s 54 years old and had how many bridge lifts in 54 years?” Thompson said. “At even 100 a year would be 5,400. … That’s just phenomenal. A little wear and tear’s not a big surprise.”
Two trunnions on the northbound span were replaced in 1997.
The inspection of the trunnion has nothing to do with the work being done to realign the counterweights on the southbound span of the I-5 Bridge. Crews will close the bridge at 11 tonight and have it reopened by 7 a.m. Saturday. More closures may be in the works.