Could a bridge be built to float if it collapsed?
June 16, 2016 4:56 AM   Subscribe

I was told in San Diego that the bridge to Coronado Island had "air pockets in the concrete" so it would break up into buoyant pieces if it collapsed and not block the navy ships in the bay. I never quite understood this and Wikipedia says it's an urban legend, but would it even be possible from an engineering perspective? Has any large bridge been built this way? Is it even a realistic problem?
posted by neat graffitist to Grab Bag (12 answers total) 4 users marked this as a favorite
 
This discussion implies that while it is possible for a concrete/steel bridge to float (there are existing actual pontoon (floating) bridges made of concrete), no-one knows for sure if the Coronado bridge is built that way.
posted by EndsOfInvention at 5:04 AM on June 16, 2016


Massive pieces of concrete floating around a harbour and running into things either on the water or on the shore somewhere strike me as far more dangerous than a pile of debris that takes a bit of work to blast a channel through.

Which is to say, I don't know if the bridge will float when you blow it apart, but it sure sounds like a bad idea.
posted by cardboard at 5:14 AM on June 16, 2016 [1 favorite]


Seattle has two floating bridges, so there's that technological example. (still two right? One sunk but they rebuilt/refloated right?)
posted by sammyo at 5:32 AM on June 16, 2016 [1 favorite]


Seattle has two floating bridges, so there's that technological example. (still two right? One sunk but they rebuilt/refloated right?)

Technically four, soon to be three. Two bridges carry I-90, so they're often referred to as one bridge despite being separate. Then, the SR-520 bridge was just replaced a month or two ago but the old bridge, while in the process of being removed, is still there. So, four right now, soon to be three.
posted by Special Agent Dale Cooper at 5:58 AM on June 16, 2016 [2 favorites]




Concrete boats are a real thing.
posted by valkane at 7:16 AM on June 16, 2016


I never quite understood this and Wikipedia says it's an urban legend, but would it even be possible from an engineering perspective?

Yup! Everything's possible - what's the budget?

Has any large bridge been built this way?

Not to my knowledge. There's really not a purpose to doing so. If, for example, the bridge were to fall into the harbor, it would sit at the bottom, about 37' or 38'. The drafts of the ships would let them clear the debris on the bottom.

Is it even a realistic problem?
Not really. Bridges are built to go over bodies of water that are navigable to traffic, and so have enough depth. If its deep enough for boats, it's deep enough for debris.
posted by the man of twists and turns at 8:15 AM on June 16, 2016 [1 favorite]


Hampton Roads Virginia (think Norfolk / Portsmouth Naval Stations) has various bridge/tunnels to avoid the possibility of a bridge collapse blocking the harbor.
posted by DanSachs at 8:26 AM on June 16, 2016


From KPBS' "Ken Kramer's About San Diego": Can the Coronado Bridge Float?.

Short answer: no.

What I was told in my Navy family was the design was optimized for not floating, but to allow any ships to easily pass underneath it, and to be lightweight enough that in the instance of a "Pearl Harbor" type attack on the Port of San Diego (major Navy town back in 1970, and still) that whatever remained of the bridge could be easily cleared due to the more lightweight construction.

When my father served on an (an LPH) in the 1980s it docked to the south of the bridge. But of note, the most powerful/valuable (and also too tall to clear the span!) ships the Navy has docked in San Diego, the aircraft carriers, dock along North Island to the South of the bridge and in the event of an attack on the bridge would have no debris to clear.

And for those of you wondering, the whole Wikipedia article on the bridge is worthwhile, here's the floaty bit: A decades-old local urban legend claims the center span of the bridge was engineered to float in the event of collapse, allowing Naval ships to push the debris and clear the bay. The myth may have developed as a result of the hollow box design of the 1,880-foot center span, combined with the low-profile barges that made it appear to float on its own during construction. However, Caltrans and the bridge's principal architect, Robert Mosher, maintain that the legend is false.
posted by artlung at 8:46 AM on June 16, 2016


The Puget Sound also has the Hood Canal floating bridge, made of concrete.
posted by humboldt32 at 8:57 AM on June 16, 2016


For those of you who have not seen it, it it truly a beautiful thing. Pictures do not do it justice.
posted by SLC Mom at 5:41 PM on June 16, 2016


As a late addition to this thread, it seems that Norway is developing an even stranger twist on the floating bridge concept: submarine floating bridges, spanning the fjords. I like a floating bridge more than the next guy, but that makes me uncomfortable.
posted by Sunburnt at 12:27 PM on August 3, 2016


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