Looks like there may be strain gauges on the beams -- but not on the failing (in 2003) gusset plates...
Posted on 01/15/2008 10:21:06 AM PST by jdm
Congratulations on your study of this collapse. I am a retired structural engineer and worked on a lot of those bridges in the forties and fifties. I’m too lazy to study your analysis of the failure, but it looks like you have nailed it. Thank you.
Thank you for your comments. Professional agreement, even ex-professional agreement, is always welcome.
I’m of two minds in one critical area of the failure.
On one hand, U10 east failed, U10 east looked different before the collapse, and U10 east was, along with U10 west, and 6 other corresponding similar points, one of the eight most important details on the bridge. Way too much coincidence to gloss over with a ‘design flaw’ final report, especially the similarity between the discoloration and the fracture lines in U10 east.
On the other hand, the discoloration doesn’t appear to be rust or corrosion, and repair paint doesn’t explain such a fracture.
The best I can come up with is that the odd paint covered graffiti, AND SLIGHTLY more corrosion effect than U10 west and the other reversal points. If all those points were close to failure, and the post collapse images show that all four mainspan reversal points did let go at or during failure, then perhaps U10 only let go milliseconds before the rest would have gone anyway.
But that still doesn’t explain the coincidence between the outlines of the discoloration and the fracture lines visible in the post collapse imagery.
So then next step then, looks to...human engineering. Suppose U10 east showed corrosion similar to the last image in my original post. One side or the other of the deck truss above U10 sheds more water, just as in that other image, and the south side of U10 is heavily corroded, perhaps even cracked along the bolt lines. The graffiti artist completes his work, and the repair guy, armed with a spray can or cans, covers up not only the graffiti, but also the ugly corrosion, after all, he’s right there with paint, tasked with improving the bridge’s appearance.
Thoughts?
Other possibilities that would explain the correlation between discoloration and fracture line placement?
In the original discussion, I had focused on the “frozen” roller bearings of pier 6. My idea was that the expansion of the main span in the heat pushed the top of the pier outwards until the rollers “jumped” causing the bottom bearing to fail.
The problem with this is that the rollers were flung towards the river and the top of the pier fell sideways and away from the river, inconsistent with a sudden movement of the bottom of the pier away from the river.
It now occurs to me that such a jump of the roller bearings could easily have been a trigger for the failure of the gusset. The inward force on the bottom of the pier in resistance to the expansion would lessen the tension along the bottom of the truss where the gusset failed. A jump of the bearings would cause a sudden increase in the tension, constituting a “yank” which could part the gusset.
I believe such bearing jumps were a known and accepted consequence of frozen bearings in general, and were mentioned in the circa 2000 analysis I read. I couldn’t and can’t understand how the dysfunction of these elaborate mechanisms could be so readily accepted, as they were a major design element, implemented at obvious trouble and expense, which signifies to me that someone at some time, at least, considered them to be quite important.
Yes, we agree on this. The report just released is a coverup. Its purpose is NOT to find the truth of the matter. Its purpose is to protect those who made recent bad decisions by blaming people from the distant past who are dead or retired.
I have read the engineering reports of a number of spectacular failures going back to the Kansas City hotel walkway collapse up through the Twin Towers collapse. In each case, the engineering was truthful and let the chips fall where they may. This is NOT the case here. I am disappointed with my profession. This is the kind of thing that politicians do, not engineers. I guess a PE in Minnesota does not mean much anymore.
Looks like there may be strain gauges on the beams -- but not on the failing (in 2003) gusset plates...
Where did the third image come from, the post collapse image?
The beam in the foreground of that image shows fresh paint, visible in the image, covering the discoloration visible before and immediately after the collapse.
I find the fresh paint there...very interesting.
The foreground beam matches the upper end of the L9-U10 compression diagonal at the U10 east gusset that I believe triggered the bridge’s collapse
I don’t know what the thing in the other two images is.
It doesn’t look familiar, it doesn’t look like it belongs there, it doesn’t appear to be serving any structural purpose.
Best guess is that it’s an add on, or something that was altered after construction. The H beam above it shows a stiffener/gusset in an unusual location. It is possible that cracks developed there, and the “bent” appendage was a temporary fix, while the unusual stiffeners were part of a permanent fix.
That third image, however, is at the core of this bridge’s failure, at least parts of it are. That sloppy job of fresh spray paint is highly disturbing.
Well, the third image certainly looks like a reconstruction using the recovered parts. The first image is looking northeast from the east side of the bridge at the south end, and the second picture is looking northwest from the west side, directly across from the first location.
The reconstruction looks like it could include the visible members of the second picture, where I think I can see the patchy paint from the foreground of the reconstruction. It looks like that paint was applied over the “wiring” of the sensors, or whatever. I surmise that the light patch in the foreground of the reconstruction is where that “loop” was attached, and ripped off.
Look at the edges of the paired hexagonal gusset plates midway between the diagonal and the upper box beam. Those edges (both plates in both photos) are "kinked" about halfway between the diagonal and the upper beam.
And they were bent at as least as far in the past as ca 2003...
Compare the foreground box beam in the third image above, to the beam at Arrow 2 in image six of my post 45 of this thread, shape only.
If it was black before the collapse, and black while still in the water right after the collapse, why is it green, and a sloppy job of green to boot, now?
I see what you refer to now. The deflection appears to be at least 1X gusset thickness. It shows a decreasing angle between U9-U10, and L9-U10. (Those probably are the strain sensors referred to in post 45, if so, yes, that could easily be the U10 gusset.) How the gusset deflection got that way, and what having it like that did for the bridge, structurally are problematic, at least.
But your third image appears to show that critical evidence has been tampered with, THE gusset that failed the bridge, altered, SINCE the collapse.
Where did that image come from?
Photos from the NTSB here:
http://www.ntsb.gov/dockets/Highway/HWY07MH024/387406.pdf
The first two photos above show a slight bend in the gusset plates on both sides of the joint. A similar bridge in St. Cloud was shut down a few days ago because the same kind of bending was found in the gusset plates.
The second in the series of three photos (with attempts at descriptions) is
You can use their arrows to page backward/forward to the other two photos.
I find it to be significant that prior to catastrophic failure, there had already been a condition that induced displacement/deflection/distortion of the critical gusset plates in the area of failure.
1) What stress/relative movement between components could have bent those plates?
2) Could continuation of that deflection in the same direction have led to structural failure of that assembly? More particularly, could increasing the angles of those "bends" have caused failure of the type that you see in the post-failure components?
I admit that I have long been surprised at the large number of rivet holes in those visibly thin plates...
Thank you.
Bridge in equilibrium, everything working as advertised.
If you then point load the midspan, or increase the distributed load across the mainspan, then the result is the tendancy for the U10 gusset to move towards midspan and down, basically along the line of the U10-L11 diagonal.
The gusset doesn’t, and wasn’t intended to resist that. The top chord shoreward from there is designed to resist that, and the diagonal shoreward from U10 is designed to resist that.
The deflection of the gusset there is an ancilliary side effect, not in and of itself weakening the bridge, but...
...the fact that the gusset did not return to original shape indicates that dimensions have changed since construction, and possibly that repetitve stresses were involved in those dimensional changes, which possibly exceeded elastictiy limits for the members that failed.
I consider these possible implications just as important as you.
However, even more important still, would be preserving evidence of an official cover-up, should something like that ever happen.
I need to go over those pics.
More later.
I think the visible surfaces in the 3rd image are exactly what we are looking at in the 2nd image, i.e. the inside of west U10. Note the little scar along the edge of the horizontal top piece.
Jeffers, those photos should keep you busy for a while!
I am struck by the absurd thinness of some of those gusset plates. They appear to be ~20% as thick as the width of the rivet heads. In places they look like torn paper...
Jeffers, look at the overhead, pre-collapse view of the bridge. Could live-load lateral thrust from traffic on that curve leading to the south end of the span have caused the lateral deflection/bending/failure that we see?
Too bad that NTSB report is a PDF. That prevents us from live-linking to specific photos for discussion. Even worse, they didn't even number the figures; that makes it extremely difficult for us to "stay on the same page"... :-(
False alarm on post collapse paint. Apparantly, both U10 east facing gussets failed identically, the west one and the east. Hard to believe, but the tears are identical, right down to the folded over triangular flaps, at the U10 end of L9-U10 East and West.
No coverup.
My mistake, apologies for confusion.
There is a wealth of data in that PDF.
It will take time to look through it all.
That type of steel will yield (bend) before ripping through: And the yielding will occur at point of highest stress.
Your observation (plus those above) sounds right.
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