Roads and Bridges 2016: Ultrasonic Examination and Monitoring of Original Sheave Trunnions for the Lift-Span of the I-5 Interstate Bridge Crossing the Columbia River

Sheave trunnions that facilitate opening of the I-5 Bridge to allow passage of marine vessels on the Columbia River were installed in counterweight towers at each end of a moveable, vertical lift span. The Columbia River Bridge, shown in Figure 1, which later became part of the I-5 Interstate Highway System, was completed in 1917. Retrofit of the bridge followed the 1958 opening of a companion bridge, constructed parallel to the original structure. The bridge is maintained by the Oregon Department of Transportation (ODOT). Suspected cracks prompted the 1997 removal and replacement of two sheave trunnions in the north counterweight tower of the bridge. Initial concern originated from indications produced in ultrasonic examinations. The evaluation procedures used to interpret the ultrasonic indications were incorrect, but the existence of cracks was verified in post removal fractographic studies. The two identical original trunnions in the south tower of the bridge have continued in service, with periodic ultrasonic examinations for condition monitoring. In the original configuration, the trunnions were seated in bronze bearings. A retrofit in 1960 reconfigured the trunnions, to fit newly-installed roller bearings. The reconfiguration shortened the trunnions, but increased the geometric complexity of the journal end for ultrasonic examination. Ultrasonic examinations of south tower trunnions, using enhanced ultrasonic procedures and a calibration standard machined from the north tower removed trunnions, detected indications of apparent crack initiations. Subsequent periodic examinations of the trunnions monitored the cracks for possible extension, while scanning the remainder of the journal for further initiations. Ultrasonic procedures, utilizing a conventional pulse echo method, have remained similar over the years; however, transducers and apparatus have evolved to improve test equipment calibrations and the effectiveness of bore-hole scans. The examination procedures, with enhancements, have resulted in a protocol that will detect discontinuities in critical regions of the trunnions and contribute to characterizations of the ultrasonic indications.

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