To the top of the Snohomish River Bridge...

Friday, March 20, 2009

I’ve never been known as someone with a fear of heights, but when I was recently offered the opportunity to tag along with our bridge maintenance crew to check out the SR 529 Snohomish River Bridge; well let’s say, the hidden fear quickly came to light.

Upon arriving at the bridge, we began climbing the first of what turned out to be several flights of stairs leading to a never ending spiral staircase. From the bridge deck, we climbed roughly 60 feet to the near top of the bridge (80 feet from the water). My lightheadedness at the top made me wonder if it was a combination of holding on for dear life and/or the bridge vibration as cars passed at 55 m.p.h. below…Who knows?

My fear of heights gradually subsided and was replaced by my interest in learning more about the bridge. During annual inspections, structural engineers found small “fatigue” cracks in the machinery designed to help lift the bridge for large boats passing underneath. The cracks were hidden in the machinery itself, which makes diagnosing and repairing a challenge for crews.
You’re probably asking…”If the cracks are hidden, how did we find them?” Our crews use the most advanced technology available to inspect and track how our bridges are holding up to years of wear and tear. One method is an ultra-sonic test, similar to pregnancy ultrasounds. A liquid chemical is applied onto the machinery and then a scanner searches for sub-surface cracks.


Another method is called wet fluorescent magnetic particle inspection. Crews apply a liquid spray and then use a black light to detect any visible surface cracks. The particle alignment highlights the cracking.

While small and rarely visible to the naked eye, these cracks don’t pose any serious threat now, but swift action is required to prevent a future catastrophic event, such as a bridge collapse. Using advanced technology helps us find small cracks before they become big, costly problems. It also extends the life of the lifting mechanisms significantly.


There are roughly 7,000 bridges on the state, city and county road systems and most are inspected every two years. WSDOT crews inspected 1,909 state and local agency-owned bridges in 2008. This year, crews are scheduled to inspect 2,018 bridges. For more information about bridge inspections, visit www.wsdot.wa.gov/Bridge/Reporting/Inspection.htm.



Submitted by Patrick Conrad.

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