A second and more in-depth inspection of critical parts of the Hawthorne Bridge drawbridge has confirmed that the bridge is not in need of urgent repairs, according to Multnomah County’s Transportation Division. The April inspection showed that an inspection last fall that found cracks in the counterweight trunnions was not accurate. The result means that Multnomah County will not need to secure about $15 million for an emergency repair that would have required an extended bridge closure.
The two towers of the Hawthorne Bridge lift span each have a cylindrical steel part called a trunnion at the top of the tower. Cables that connect to the million pound concrete counterweights on one end and the drawbridge on the other end pass over the trunnion. When the drawbridge raises and lowers, heavy loads are put on the trunnions, which were installed in the 1980’s.
In 2000, an inspection determined that the trunnions are slightly out of alignment, which increases their risk of cracking and failing. Replacing the trunnions would cost about $15 million. Since 2000, the County has paid for independent inspections of the trunnions every two years to check for cracks.
During an inspection in October 2020, an Oregon-based inspection team discovered what appeared to be the first signs of significant cracks in the trunnions. Due to the risk the cracks posed, and the cost for repair, the County hired a second inspector with additional testing tools to conduct the more in-depth April inspection and provide a second opinion. More time was allocated for the inspection to allow for a more thorough evaluation of the trunnions, which required a closure of the bridge for the April 10-11 weekend.
The April inspection used several methods to validate or disprove the findings of the October inspection. Both inspections used a wet magnetic particle test that is designed to locate cracks in steel. In April, the inspectors took more time (which was available due to the longer bridge closure that the County allowed) to evaluate every inspection test result. When the inspectors found indications that could indicate cracks or just variations in the surface of the trunnions, they used various techniques (buffing the trunnion surface, adjusting the angle of the applied magnetic field, and repeating the test) to assess the validity of those results.
The second inspection also used an ultrasound wand test focused on areas that were suspicious based on the wet magnetic particle tests to definitively confirm that cracks were not present. The ultrasonic wand test can only be performed by individuals with high level certifications that are based on extensive training and field experience. Since it is more expensive and few individuals are qualified to perform it, ultrasonic tests are typically only done to confirm positive results from less difficult inspection methods.
“This is good news for Hawthorne Bridge users, taxpayers and Multnomah County Transportation,” said Jon Henrichsen, director of Multnomah County’s Transportation Division. “The second inspection was more in-depth and we are confident that the trunnions are not in urgent need of replacement. Our capital improvement plan calls for the trunnions to be replaced in the 2030’s. We will continue to monitor these parts to ensure that the bridge is in good condition and safe for the public to use.”
The trunnions were installed in the 1980’s. Replacing them in the 2030’s would be a typical 50-year service life for these parts.Multnomah County maintains the Hawthorne Bridge, which opened in 1910 and is the oldest operating vertical lift bridge in the country. For information, visit www.multco.us/bridges or follow us on Twitter @MultCoBridges.