Interstate Bridge Falls Into Mississippi

[glenn kangiser]

Seems nobody wanted to be the bad guy and say "Stop the traffic, Captain Kirk, aye, shes gonna blow." :-/

[John Raabe]

This is a very interesting review of the structural concerns (if you haven't already seen it)

http://www.nytimes.com/ref/us/20070803_BRIDGE_GRAPHIC.html?#step1

[glenn kangiser]

Interesting concerns John and it seems to put the rest of the picture together for me.

In talking to an engineer about Nelson studs on top a beam in a composite (concrete and beam work together to provide extra strength) second floor design, he explained that the Nelson studs anchored the concrete to the top flange of the beam  (think top chord of the truss on the bridge) so that the beam could be much smaller and still support the same load.  In a composite design the steel provides the tensile strength and the concrete provides compression strength.

This bridge was a composite design in that the concrete deck was in compression and the truss was in tension.  The truss couldn't support the massive load if the concrete was not sound.

The reports I posted above refer to the hinges no longer sliding -blocks falling over 90 degrees - being at full extension etc.. There were thousands of feet of cracks noted in the underside and top of the concrete deck as the concrete released it's grip on the rebar that was the backbone of this structure.   All of this indicates that because of failure of the concrete deck and probably being made worse as workmen were currently repairing the deck, there was no or little compression member left at the time of failure.

This is like cutting off the limb that you are standing on, but the picture was so big that the workmen could not have known.

The cracking of the steel - twisting of girders, cracking of decking have all been noted by the inspectors as a slow motion recording of the ultimate failure of the bridge.

I think the center span concrete deck failed as they were working on it, under heavy loading, causing it to shorten and over stressing the already damaged end sections to where they could no longer stand the strain.   I haven't reviewed the photos at the various sites yet but the above seems like a reasonable explanation to me.

[PEG688]

So in essence thier attempt at repairing the concrete exacerbated the stress load on the steel .   So in trying to  fixing they destroyed it :-/

 Did you read anything about the harmonic vibrations theroy?  Maybe some of both  :-/

So now every bridge thats being worked on will need to be closed to all traffic  :-/

[glenn kangiser]

I didn't read about the harmonic vibrations theory but funny you should mention it.  I mentioned that to Sassy a couple days ago - the day someone mentioned the noise of a jackhammer then the bridge fell.

Tesla made a very small machine in the early 1900s  he could tune to the harmonic frequency of the building and with a tap - tap - tap nearly brought a building down.  Luckily he shut it down before it got out of control.  Harmonic vibrations brought down the Tacoma narrows bridge also.,,, but I think this one was brought down by neglected major repairs and not wanting to shut the bridge down to do the fix.

Whether a bridge would have to be shut down for repairs to do fixes or not would depend on the bridge and the repair - analyzed one by one then someone to carry through if it needed the repairs.

It is common for us to have to provide temporary bracing when making major welds to building beams that are under load.

[glenn kangiser]

So in essence thier attempt at repairing the concrete exacerbated the stress load on the steel .   So in trying to  fixing they destroyed it :-/

 Did you read anything about the harmonic vibrations theroy?  Maybe some of both  :-/

So now every bridge thats being worked on will need to be closed to all traffic  :-/

That last piece of concrete they busted out could have been the final one holding the bridge up from collapsing -- not the workers fault though -- this bridge was already gone.

From the reports I read it looks like this thing has been sinking in the center as the concrete has been breaking away from the rebar.  Hinges were streched out to full extension if I recall correctly and they readjusted members under the bridge - and a comment by one engineer - inspector seemed to indicate that what they did was the wrong thing to do.

I think that the size of this problem was so big and repairs were so major that nobody wanted to be responsible for shutting it down and saying -- it's gone - let's get rid of it.  It also seems that nobody put the big picture together.  Always easier after it's done and looking back. :-/

But -- government handles things like this.  We pay taxes so they will take care of things like this and we won't have to worry about it.

[Sassy]

http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=57017

PREMEDITATED MERGER
NAFTA Superhighway traffic tied to bridge collapse
WND uncovers federal study warning of high risk in 1998
Posted: August 5, 2007
5:45 p.m. Eastern

By Jerome R. Corsi
© 2007 WorldNetDaily.com

Evidence of increasing international trade truck traffic on Interstate 35 through Minnesota raises concerns that NAFTA Superhighway traffic contributed to last week's collapse of the freeway bridge in Minneapolis.
(con't at link above)