Passers-by might have noticed King County crews giving the Novelty Bridge, located on NE 124th Street near Duvall, its first major cleaning this month. While the scrubbing can add sparkle and shine, there is a more practical benefit – a clean bridge is a more structurally sound one.
The reason is dirt can hold moisture that accelerates corrosion. A steel truss bridge’s horizontal gusset plates that connect the framework of beams together – such as the Novelty Bridge, rebuilt in 2000 – often accumulate dirt and debris that is trapped against the steel. That creates corrosion that can go unseen until someone cleans away the dirt, in some cases after it’s too late.
“I’ve heard horror stories from the bridge inspection world of steel truss sections that were completely rusted through,” King County Bridge Engineer Ben Roark said. “No one knew it because it was buried under a mound of dirt and debris.”
In addition to run-of-the mill dirt, bird guano and lichen (which is an algae or fungus) can attack the bridge’s protective layer of paint and reduce its lifespan if not removed. Once that layer fails, the underlying steel is exposed and begins to rust.
Bird guano usually accumulates along the top of the bridge, while lichen and moss tend to grow on the shadowy, north-facing surfaces. That’s why cleaning the nearly 300-foot-long, 65-foot-high Novelty Bridge is so important. County crews are using a high-pressure water spray – which helps with the hard-to-reach spots – along with a vacuum-like vactor truck to clean up loose debris.
They are expected to finish the cleaning this month. As far as the next time Novelty Bridge will get a scrubbing, bridge cleaning schedules are largely based on geography and can vary.