Sunday, August 2, 2009

Terror on a Bridge - A Truckers Nightmare

Port Townsend, today. I drove down to the bridge yesterday to get these "latest" views for you. I received many interesting questions about my last post, the most common was, why a floating bridge, instead of "bridge" bridge, such as a suspension bridge. As bridge technology has evolved, floating bridges are now correctly referred to as Very Large Floating Structures - VLFS.

Transition Span to VLFS. Hinged to rise
and fall with tide range up to 16 feet.

Floating bridges - VLFS - depending upon the construction site, run anywhere from 4 to 8 times less expensive to build than conventional alternatives. Hood Canal, a long narrow dead end gully carved out by ancient glacial activity, has very poor circulation and thus is slowly but surely silting in, from numerous creeks and streams flowing down into it from the Olympic Mountains.

Geologic surveys taken at the narrow throat of the Canal, where the bridge was to be constructed, revealed up to 300 feet of silt at the construction site. One estimate penciled out demonstrated that a structure like the Tacoma Narrows Bridge would have to extend 810 feet high from bedrock - 500+ feet beneath the surface.

That is taller than the Columbia Tower in Seattle. Total cost would be about $4 billion –eight times the cost of the Hood Canal Bridge.

· No expensive and dangerous caissons need to be built for tower foundations
· No expensive towers
· No expensive anchorage blocks for a
· very complex and expensive cable suspension system
· All construction takes place virtually at ground level

Ironically, the first recorded floating bridge in he US was a railroad bridge, constructed 1874 crossing the Mississippi River. It was a combination of pile structure and wooden floating boxes. I'd call it a four-shot bridge. I I were the locomotive engineer, I'd need four shots before crossing it!

Apparently there are 11 floating bridges in the world. I think the most unusual and graceful being the Nordhordland Bridge located near Bergen, Norway, a combination cable-stay and floating structure. What makes this VLFS unique is that this bridge is the longest free-floating unsupported span in the world. There are no underwater anchor cables!

And, the most unusual, a swinging floating bridge, the Yumenai Bridge in Japan. She opened in 2000, and requires tugboats to swing it open around the pivot pier for large vessels.

In my overly zealous attempt to get rid of stuff, I've been tossing tons of magazines, books, articles, photographs and other flotsam and jetsam. I came across an old copy of "Pacific Northwest Magazine" written on the occasion of the reopening in 1982 of the Hood Canal Floating Bridge, following its dramatic sinking in a storm in 1979.

The article recounted the terrifying experience of a truck driver who ventured eastbound onto the storm tossed bridge only to find the navigation opening at the far east end opened. The theory being to relieve pressure on the structure from wind and waves.

The magazine has changed over the years since 1982, and I finally gave up trying to figure out how to find the article. Mentioning this to the staff at the Washington State Department of Transportation, I was stunned to open my mailbox, to find that the Staff had located that magazine issue, and Joe Irwin at WDSOT sent me an Adobe PDF file of what I thought was lost forever!

Those who are interested in reading the full-length article about Red Taylor, a trucker caught on a disintegrating bridge over Hood Canal, will enjoy this read and the lessons learned from that sinking. Click on the PDF logo and read it just as it was published in 1982. And my sincere gratitude to the Staff at WSDOT!

I mentioned in my previous post that many lessons about floating bridges have been learned over the years - many lessons learned the hard way. Now, on all of Washington States four floating bridges, during a storm, once winds are clocked over 50 miles per hour, in a 15 minute period, the bridge is closed. No discussion. And remote controlled barricades prevent traffic from inadvertently venturing into harms way. And of course, traffic cameras monitor all activities on the structures.

I received another email today from Joe Irwin at the Washington State Department of Transportation. The "Content" line was simply "Aha!" I had queried him about video recordings showing how the new Hood Canal Bridge opens. Since the contractor had to demonstrate some 20 or so continuous open and close sequences to get their "write off" I figured some one must have recorded it.

Sure enough. My thanks to Joe Irwin at the Washington State Department of Transportation.

East Side Bridge Opening

video

West Side Bridge Opening.

video

Bridges, for the most part, are structures we cross without giving them much thought. I hope you've enjoyed learning a little more about this unique variety, the Very Large Floating Structure!

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