Interstate 5 Bridge Maintenance Project

Trunnion Repair

Planned Work

Replace the Counterweight cables, drums and shafts on the North tower of the Northbound Interstate 5 Drawbridge.

Reason

One of the shafts has developed a small crack. This crack is slowly growing and will ultimately result in failure of the shaft. The predicted time for the crack to grow sufficiently to cause failure is between the years 1999 and 2019, with the most probable year being 2009. Crack growth occurs during the raising and lowering of the lift span. Although the shaft is currently safe, it is unacceptable to ODOT to delay repairs more than two years.

Background

In 1987 ODOT began an inspection of the Interstate Drawbridges, specifically to examine the trunnion shafts. It is not possible to visually examine the surface of the shaft, which is enclosed by the drum and the bearings. An ultrasonic examination was performed, and an indication of a possible crack was found. In 1994, newer technology called acoustic emission monitoring, made it possible to locate an actively growing crack. The crack was identified as a future problem, but did not need immediate replacement.

In early 1996, the shaft was examined again, and confirmed the crack was continuing to grow. Work began to plan how to replace both the cracking shaft and its companion on the North tower as well as cost-efficient replacement of the drums, bearings and cables.

Construction Impacts

The North drawbridge counterweight must be supported by an extremely large crane mounted on a barge or platform in the water, in order to replace any of the lift system components. This renders the lift span inoperable for the duration of the project. The shortest practical, continuous time to accomplish this is 21 straight days.

The impact on motorists could be reduced by requiring the contractor to work only at night and on weekends. This spreads the work out, requiring the contractor to pay crane costs and labor costs for nonproductive time. Working at night increases the risk of accidents and significantly slows down crane work. Maritime traffic would be significantly impacted by only weekend and night work since this alternative would greatly increase the project time frame. The draw bridge will be closed for the duration of the project.

Hayden Island Access

The north bound structure will be closed during work. Hayden Island will retain its access to the on and off ramps to I-5 south bound and the off ramp (Janzten Beach exit) north bound. The on ramp to I-5 north bound will be closed for the duration of the project. The bridge closure will occur prior to where the north ramp meets the bridge.

Timing

Unlike most construction or maintenance projects, this project requires specialized forging, machining and welding of very large components. The time to fabricate these components is somewhere between 3 and 9 months, depending on the workload of the shops capable of doing the work. Work cannot begin on the project until these parts are completed and inspected.

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