May 27, 2013

Collapsing Bridges: A Washington State Tradition!

There is probably no area of the country more famous for bridge collapses than Washington State, with many of them weather related.   The costs of local weather-related bridge disasters undoubtedly exceeds a quarter-billion 2013 dollars.   Our region is particularly vulnerable to such bridge losses, with long floating bridges and the powerful winds associated with our terrain and incoming Pacific cyclones.

The latest bridge collapse near Mt. Vernon is an exception to the rule, since weather did not play a role.  

Galloping Gertie (Tacoma Narrows Bridge):  November 7, 1940

Let's begin with perhaps the most famous of the bridge failures, the one shown in virtually every introductory physics class in the country:  the Tacoma Narrows Bridge, also known affectionately as Galloping Gertie.  ( Click on the picture below to see a video.)  Gertie, located in a channel between the Kitsap Peninsula and Tacoma (see map), started gyrating and then collapsed on November 7, 1940.

At 7:30 a.m. that morning a wind gust of 38 miles per hour was measured on the bridge. The cause was a strong low pressure center that was approaching the northwest coast of the State (see map at 4:30 AM below):

Two hours later, the winds reached 42 miles per hour near the bridge's east end, and fishermen near the west end reported "substantially" stronger gusts.  In the subsequent few hours the bridge started to oscillate or "gallop" and failed around 10:30 AM.   Poor design, coupled with strong winds, destroyed this suspension bridge.

The Hood Canal Bridge: February 13, 1979

The most expensive local bridge failure was undoubtedly the Hood Canal Bridge, which failed during a freak windstorm, one in which gusts at the bridge reached well over 100 mph.

At first the origin of the winds was a great mystery.   A strong Pacific cyclone was approaching the coast of central Vancouver Island (see figure).   Forecasters expected winds of 30-40 mph over Puget Sound and nearby waters.   But unexpectedly the winds accelerated to 100-130 mph over a very limited area around the Hood Canal Bridge.  Massive tree falls occurred in the vicinity.  After hours of pounding, a section of the bridge failed as the wind and wind driven waves struck the structure.

But why the huge winds?  Professor Richard Reed of my department was hired to figure it out and did.  By collecting every conventional and unconventional observation in the area he demonstrated that an intense mini-low pressure area formed near the bridge as a result of an unusual flow off the ocean interacting with the Olympic Mountains (sort of like an eddy in the lee of big rock in a stream--with the Olympics being a VERY big rock).   Here is a pressure analysis produced by Professor Reed.

Strong winds were coming in off the Pacific and then they accelerated wildly to the north as they approached the low center (winds accelerate as air moves from high to low pressure).

The bridge was replaced at a cost of approximately $ 143 millions dollars.

The Old I90 Bridge: November 25, 1990

November 24, 1990 was a very wet, windy day over western Washington.  There was widespread, major flooding on Western Washington rivers, two deaths,  and  $250 million in damage.  Records flows occurred on the Elwha, Cedar, Snoqualmie, Skykomish, Snohomish and Stillaguamish.  Another major Thankgiving holiday storm.  On the 24th Seattle received almost 3 inches of rain and wind gusted to 35-40 mph.

Heavy rain, rising lake levels, and open manhole covers contributed to the flooding of pontoons and the loss of the bridge, which was the world's first floating concrete pontoon bridge when it was dedicated in 1940.  Another issue was that the pontoons were used to store contaminated demolition water.   As the pontoons went down they also damaged cables for the new I90 bridge to its north.

Click on the picture below to watch it collapse!

The three weather-related bridge collapses may be the big ones, but weather related bridges incidents are legion around here.   Major windstorms have damaged the cables holding the 520 bridge across Lake Washington in  place.  A truck was blown through the guard rails of the Biggs Bridge across the Columbia Gorge.   The list is a long one.   One thing is sure:  Washington State bridges can be tested severely by local weather, flooding rivers, volcanic eruptions, and, of course, earthquakes.   The current 520 bridge would be very vulnerable to a truly major storm, such as the 1962 Columbus Day Storm...fortunately it is being replaced.  WSDOT did a simulation of what would happen with a storm with sustained 75 mph winds (about a once in 20-30 year event)--the current bridge would be history.  Click on the picture below to see the scary results!



Announcement:   I will be teaching Atmospheric Sciences 101 (WEATHER) at the UW this fall.  This class is accessible to folks 60 or older at very little cost (the UW Access Program) and, of course, to regular UW students.  This class will give you a good basic understanding of the atmosphere and Northwest weather.


  1. Cliff, I think you're wrong about one thing here... I think weather DID play a role in this latest bridge collapse! With three people in the river (water in the 40s, apparently), the fact that it was clear and sunny daylight, low wind, and reasonable air temps all played a role in those three people staying alive in the hour it took to scramble crews into the river and rescue them. If the weather had been bad (colder, windy, dark, raining) it would have massively complicated the rescue efforts and it probably also would have caused hypothermic deaths among those three, especially the woman that was unconscious (her husband had to hold her head out of the water, but the rest of her was in it). The hockey-player kid might have been OK, or might not; he was exposed sitting out there, wet, on top of his car. Not to mention that a windy day would have moved the debris around in the river, potentially making the whole thing so unstable that they would have survived the fall from the bridge deck in their vehicles only to be swept away by wind and water.
    I have to say "bridge collapse" ranks high on my "irrational things to fear in WA State" list. LOL It is truly the stuff of nightmares and I'm glad there was some good luck involved this time.
    All the folks who have voted for Eyman's initiatives to gut taxation for infrastructure spending have themselves to thank for this. It's depressing that more people don't make the connection between massive cuts to infrastructure maintenance and our willingness to tax ourselves reasonably if we use that infrastructure. Maybe the pendulum will start to swing back now that the state of our bridges is a bit more apparent to drivers.

  2. Res Tacoma Narrows, I was about 6 months old when parents drove across this bridge a few day before it went down. Mom always remembered cars in front disappearing from view. IIRC, insurance agent had not procured the insurance.

    I worked on the Hood Canal bridge in its first and second incarnations, the bolts fastening the pontoons did not work, the bridge had to be disassembled moved to a bay and reassembled with cables in tension and epoxy glue. Allegedly the second incarnation sank in part because hatches blew open. IIRC the state and insurance company did not want to open the can of worms as to actual blame, so settled without assigning blame.

  3. My 7 year old daughter and I were returning from the coast were perhaps the last car to cross the Hood Canal bridge before it was closed prior to the sinking...
    In 1963 I was the officer of the deck on the USS Skagit AKA105 as we approached the Hood Canal bridge from the south...the Captain and pilot were drinking coffee and did not notice that there was nobody in the control tower of the bridge to open it as scheduled for our passing....I noticed it and alerted the Captain and pilot...the pilot immediately ordered FULL BACK EMERGENCY (something that can really screw up an engine)and the force of the backing screw caused the ship to rotate 90 degrees as it slid toward the bridge at 15 knots...The ship came to a stop parallel to and about 50 feet from the bridge.
    I watched the I90 bridge sink from the other bridge as I was returning to Seattle from Easter WA.
    So if you want to see a bridge fail just ride along with me....

  4. I don't think it's really right to call a 42 mph breeze a weather-related bridge collapse. To me a weather-related collapse indicates a collapse due to extreme weather, not to weather that you'd expect routinely at any location on earth.

  5. Don't forget the Bogachiel River Bridge collapse on Highway 101 in 1978.

  6. Minute Physics has a nice, quick explanation of the Tacoma Narrows bridge collapse:

  7. Lousy weather in May. A Seattle tradition.

  8. I remember attempting to cross the existing 520 floating bridge in a horrific wind storm.

    Late 1980s, perhaps.

    They opened the bridge up in the middle to relieve pressure...after we unfortunate folks were already on the bridge... We all had to go to the middle of the span and do a U-turn.

    There was a WSP officer yelling through a bullhorn: "Get off the bridge. Get off the bridge".

    The bridge was rolling like a ship at sea. Land would appear, and disappear, appear, and disappear.

    I am telling you, my feet were was hard to press the clutch pedal of my Fiat Spider...

    I was scared...

  9. Patrick: "I don't think it's really right to call a 42 mph breeze a weather-related bridge collapse"

    True. But Cliff has to write to a general audience and it would take too much to explain resonant frequencies and aerodynamic forcing functions, which is exactly why it is famous in first year college physics. Though it is easy to find them on the internets: one complicated explanation.

    Coincidentally, the storm that collapsed the Hood Canal bridge also kept the aerodynamics professor from going over the 520 bridge that day, so our midterm was delayed by a day.

    Oh, and the Thanksgiving storm for the I-90 bridge was memorable to us. First it was another Thanksgiving that father-in-law was called by Puget Power to help restore power, and the one where kid #1 ended up in the hospital with croup. Fun times.

  10. I know that the UW's Kirsten Wind Tunnel has done buildings and bridges, but I could not find anything other than the postmortem tests on Galloping Gertie. I did find this wind test of a bridge.

    Mathematical modeling is lovely, but tests are needed.


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