Thursday, August 02, 2007

Google Earth image of Minneapolis bridge before it collapsed


(Point and click to enlarge)


Current affairs


According to press reports, it was the Interstate highway 35W over the Mississippi that collapsed, with a current death toll at least 7, and many dozens injured. One is reminded of that iconic footage of the Tacoma Narrows bridge undulating for seconds, if not minutes, before collapsing, but that was a suspension bridge, as I recall, prone to build up of destructive resonance, not as here, a rigid steel framework bridge.

Using just the I35 identification, and the fact that there's an adjacent bridge in the photograph, I've located the above image of what I hope is the correct bridge, as it might have looked yesterday before the rush hour. Existence can be a terrifyingly ephemeral commodity in our developed societies.

Update Thur 2 Aug, 10:41 Paris time: a quick look at the New York Times confirms I have the correct river crossing, the adjacent one being called "Cedar Avenue". In fact the NYT also has oblique image of the bridge before it collapsed, presumably from a plane or helicopter. It's not an doctored "oblique angle" satellite image, since the side structure of the bridge is visible. (Or can satellites take oblique-angle as well as plan-view pix these days?)

Their small graphic enlarges on demand. Here's the link to the enlarged picture. One can just about make out the lacy supporting structure. Maybe one's being wise after the event, but that framework looks distinctly insubstantial, not so say flimsy, wouldn't you say ?

Update Thur 2 Aug, 11:15 Paris time: The Telegraph describes it as a "concrete" bridge in one of its picture captions.

Whilst there was undoubtedly concrete in the superstructure, supporting the road surface, the main support would appear to have been a network of steel girders, essentially the same principle as the Forth railway bridge. One should avoid playing the blame game at this early stage, but one cannot but help wonder if the bridge might still be standing had it really been made entirely of reinforced concrete.

Update Thur 2 Aug, 23:06 Paris time: One of the pleasures of being an independent blogger in mid 2007 has been seeeing the speed with which one's new posts becoming accessible on Google (assuming, needless to say, that the right search words are entered). During the Annie Leibovitz/Buckingham Palace controversy, for example, folk were visiting my site the same day, following Google links. I only mention this because, unusually, this post is so far totally invisible on Google. Now why is that, one may ask ?

Here's my theory for what it's worth. I have touched on the question of blame, using words like "flimsy". It's my guess that in that litigious society called the USA, Google is screening out, electronically or manually, anything that might influence the mind of jurors in any future legal action.


Update:Friday 3 Aug 08:35

A Google search under " minneapolis bridge flimsy" turned up the following photograph on flickr of the underside of the bridge before its collapse, provided by http://www.bridgepix.com/ from its archives.

Looking at the sheer mass of concrete which that steel lattice work had to support (and that's without including the traffic) I feel , admittedly with the wisdom of hindsight, that corners were cut in the design and commissioning of the bridge.

To be candid, I cannot for the life of me see how a bridge whose span depends on two slim arcing strips of steel ever got off the drawing board. No amount of ties or cross members can alter the fact that if one of those slender strips should fail, the entire bridge collapses. It's not engineering, we're talking about here, but simple physics, to do with weight and gravity !

The article gives some interesting insight into technical and cost factors that resulted in choosing that particular design of bridge. The key fact to consider is that the bridge was unsupported in the middle, so as not to obstruct navigation on the Mississippi. Yet the adjacent concrete Cedar Ave bridge, built much earlier, back in the 1920s, did have obstructing piers in the centre of the river, so it's an enigma wrapped in a mystery, one might say.

This contribution of mine is now finally "searchable" by Google, so the world's major search engine is still trawling, but perhaps a bit slower than usual.

I find it fascinating to track the way one's internet profile can rise in the days and weeks following publication of a post. This one at the moment still needs "dreams" or "daemons" in the profile, which means it's at present essentially invisible. That was the situation when I first posted on Annie Leibovitz (see above) but it's now risen in the Google rankings such that it can be picked up on Page 2 simply with " leibovitz queen" Or was it the other way round (queen leibovitz)? The word order is curiously (and illogically) important. Reversing the order yesterday moved that earlier D&D post from Page 2 to Page 8. The latter is the boondocks, needless to say, in internet search terms. Few people, we're told, go much beyond the second or third page of returns.

Interestingly Shane Richmond was saying yesterday that Google is a major factor in bringing his telegraph.co.uk and its blogs etc to the attention of overseas readers, especially in the US, but was strangely coy on what proportion of readers are now non-UK.

If statistics on personal blogs, such as this one, are anything to go by, the US, Canada, Australia, the entire world, in fact, are discovering and logging on to our scribblings, but it's nothing to get excited about. They rarely linger, once they realize that Google has taken them to something that was not quite what they were looking for!

2 comments:

rbobjim said...

You wrote:
Looking at the sheer mass of concrete which that steel lattice work had to support (and that's without including the traffic) I feel , admittedly with the wisdom of hindsight, that corners were cut in the design and commissioning of the bridge.

To be candid, I cannot for the life of me see how a bridge whose span depends on two slim arcing strips of steel ever got off the drawing board. No amount of ties or cross members can alter the fact that if one of those slender strips should fail, the entire bridge collapses. It's not engineering, we're talking about here, but simple physics, to do with weight and gravity!
==============================
My comments:
Actually, it IS engineering that we're talking about. I've lived in Minneapolis for 70 years and traveled this IS35W bridge thousands of times. But not on August 1, 2007. I am an engineer (chemical, not structural) but I do agree that the design and the "pre-fall" photos present an unlikely scenario to laypeople like you and me.

However, I'll also note that this bridge had been in place (with no known movement or issues) for 40 years - despite the flimsy appearance that you and I might see. Forty years is not a trivial fact, and neither is the huge volume of traffic this bridge has carried for decades. The cause of this failure will be determined and published, but the highest probability is that something changed over these 40 years. Perhaps subtle corrosion; perhaps loading; perhaps an so-far unknown engineering issue.

I must note that the Eiffel Tower has not impressed me as a visually confident engineering structure either. Yes, it's been there a long time, and yes - - the engineering has been good so far. But every time I go to the top I want to stop and call for my mommy!

Bob in Minneapolis

ColinB said...

Hi rbobjim

Apologies for the delay in replying - just got back from a trip.

I suspect that the layman's take on a structure is one not to be taken lightly - we're all quite good at detecting whether a structure is under- or over-engineered.


The roof of the new De Gaulle air terminal in Paris collapsed recently, with loss of life. I frankly can't see the same thing happening at Stansted Airport, where the supports look comfortingly solid.
Can't agree with you, thoughn, about the Eiffel tower - that'a a belt and braces job, surely ? If it were built today, I'm sure that modern day computers would have proposed a much lighter structure, with only a fraction of the cross braces.

The term is "redundancy" I believe: cost pressures, to say nothing of bidding competition, means we end up with structures that have little or no redunndancy. They are fine for resisting all the recognized forces, but become progressively dangerous as they age, especially if when one bit snaps, the whole lot goes.

I still maintain that the Minneapolis bridge looked under-engineered, and had little or no redundancy. Exceptional (?)circumstances - whether corrosion or maintenance work - caused it to collapse catastrophically. If properly engineered, it would have given warning signs - buckling, sections out of joint etc.

Corners were cut, risks taken with its flimsy-looking design.

We in the UK learned our lessons in the 19th century (Tay Bridge disaster), and tend to over-engineer our bridges. But then, we don't need quite so many as you do !