Mineapolis bridge - yes again

[csd72]

I noticed the following news thread on the aol site:

http://news.aol.com/story/_a/2003-p...e/20080323153909990001?ncid=NWS00010000000001

I just love how they quote lawyers as if they are expert witnesses.

 

[jmw]

Interesting.
I know nothing about bridge design but it seems to me that the wording of this report is slanted toward the assumption that the original design was flawed.

However, it also states that a concrete deck was added and a central concrete divider.

Thus:

NTSB Chairman Mark Rosenker didn't comment on the photos, but has said the original design for the bridge specified steel for those and other gusset plates that was too thin.

This appears to judge that the original design was wrong and that the concrete deck was not a factor.

So how was the concrete decking and divider not a factor?
AM I wrong in thinking that adding the concrete should have required a design review that should have either exposed the original design flaws or shown that the design was not such as to support the added loads. In other words, whether the original design was right or wrong, wouldn't there have been a design review necessary to permit adding the concrete and wasn't this flawed whatever else comes to light?

Or, is it possible that the concrete addition design review simply looked to see if the added load was within the original loading allowances? In which case it would suggest that the review did not go back to first principles but assmed the original calculations or design were sound.



JMW

http://www.ViscoAnalyser.com

 

[Qshake]

JMW, you are quite correct in your questions and comments. I'm sure this will haunt someone (somebodies) for quite some time.

1. Original design appears to be flawed but bridge stood for 40 yrs.
2. At some point plates bend, bridge stands for years.
3. Addtional weight was added but bridge stood for years
4. Contractor makes modifications and adds construction materials to bridge, bridge stands for weeks, months.
5. When contractor redecked the bridge the first time what did they do differently?

I don't mean to imply it was the contractor but just suggesting that the bridge has seen it's share of loads and renovation in the past. Something must have been different.





Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[csd72]

It is very easy to look at the calculations and say hey these were 2% under therefore that must be the cause, but it is much harder to actually find the cause from other sources.

Lawyers are not interested in the truth, only potential liability. They will take the easiest path to compensation for their clients.

 

[Dinosaur]

I attended a conference in West Virginia not long ago where a federal government engineer with a substantial reputation in the structural engineering community explained what he learned in the investigation. Gusset plates connecting the two ends of a diagonal member were undersized. As memory serves, he computed the needed thickness as 1 and 3/8ths inch. The plan detail showed 3/8ths inch thick. The original plan detail was off by a factor between 3 and 4, which is consistent with bridge design factors of safety typically used. The construction loading did contribute to the failure he believed, but was on the order of 115-130% of the original design, not 300-400%. After the fact, folks are looking at inspection photos where a bit of a bend may be observed in some gussets. Unfortunately, I wouldn't expect an inspector to proclaim that as an indication of imminent failure. How is he to know it wasn't bent slightly in the shop or during erection? It just goes to show all the effort we put into quality control, whether it is during the plan development process or during construction or somewhere in-between, is all for a good purpose.

 

[Qshake]

Dinorsaur -

I'm afraid if those were the numbers for the actual plate size, then you've been mis-informed. Similarly with the correct size of the plates.

It's amazing how much mis-information there is and how willing engineers are to spread it.

While I don't disagree that an inspector should observe a bent plate as a sign of imminent diaster, it should have been dealt with more promptly.

That this is true is to remember the I-90 Cuyahoga River Bridge in Cleveland which underwent an extensive retrofit for gusset plate stiffening.

I think this all shows that we, as engineers, need to do more to get our lessons learned out to the community of engineers and owners as well as our achievements.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[msquared48]

I'm not a bridge engineer, but I did take basic drafting and know a "true view" when I see one.

Such is not the case of the two diagonal photos on the site. Taken two days apart, neither photo confirms in my mind that the gussets were bent. That could only be confirmed in an end view photo of the gusset. Unless they have more definitive photos in their hip pocket here, I would contend they are not bent. I believe what you are seeing is just a corner of the gusset at the intersection of two cut faces.

And the lawyer's comments, typical BS...

Mike McCann
MMC Engineering

 

[Dinosaur]

Qshake,

I'll speak to him and write the values down when next we speak. Could you check your notes and see if you can find the actual values also?

 

[Qshake]

Dinosaur - First, please keep in mind that my statement was not a personal attack...not that you took it that way but I wanted to be clear.

From the material on the website at MnDOT for the original plans, they list the gusset plate thickness as 1/2". The interim report by FHWA (available on the internet) which focused on the U10 and L11 plates determined that the plates were about 1/2 their proper size. In that report I reference you to Table 4. Earlier in that report you will see a table of D/C ratios wherein, a 2.07 for shear is indicated for the U10 plate. This is where the conclusion that the plate is half the necessary size.

I hope this helps to clear up a few things. And I hope, for the sake of you presenter's reputation, that he was misunderstood. Otherwise, he was misinformed and is spreading mis-information.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[SomptingGuy]

"Gusset Stiffening"

Yak, snort, etc.

- Steve

 

[Qshake]

Mike - The plans on are on the MnDOT website as are the shop drawings. The only cut line for the plates that are not vertical are the lines normal to the diagonals and those are basically the width of the diagonal. Hence there really is no other oblique planes for the plates.

I'm not really trying to convince you of the likelihood of a bent plate, but rather trying to lead you to information that may help support or refute your claim.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[jmw]

so, if they decided the gusset plates were too thin, when were they planning to change them or did they only find out after the bridge failed?

JMW

http://www.ViscoAnalyser.com

 

[msquared48]

Thanks Qshake - I'll check it out.

Mike McCann
MMC Engineering

 

[Qshake]

jmw - since I am a bridge engineer, I have followed this matter. It's my understanding that the plates were not found to be thin until after the collapse.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[msquared48]

Qshake:

Could not find the drawings on the website, so I'll have to take your word for it. Too many drawings to go through!

I'm curious about the structural design theory here though with the utility of the gusset plates. As I see it, the only way they could buckle is if they were resisting a moment at the joint and buckled due to overloading, whether it was gravity induced or cylical.

Sounds to me like these joints were not designed as pinned, but moment resisting - possibly partial moment resisting. What is the design spec on the gusset plates that drives the thickness requirement? Minimum moment capacity, bolt/rivet bearing, brittle fracture... what?

Thoughts?

Mike McCann
MMC Engineering

 

[Qshake]

Mike, All those items need to be checked including shear for different planes and for block shear failure.

On the matter of designed as moment resistant, I doubt that in 1965, all truss bridges were designed with member stiffnesses accounted for. I think the joint was designed as pinned and through the typical details of the connection, it winds up being a moment resistant connection. In other words, I think that the joint itself is very stiff, in plane, versus the members. Out of plane is a different story and when edges aren't stiffened may be prone to buckling. Either laboratory testing or lessons learned were part of the development for the AASHTO plate edge stiffening.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[jmw]

Thanks Qshake,
But I see the photos purporting to show distortion are from and intact bridge.
The question is if these photos induced concern at the time they were taken, in which case that should have been the time they started to check the calculations and plan remedial work, or these photos were only examined after the event.
If they were only examined after the event why did they not create concern at the time?
What else was photographed and why?

JMW

http://www.ViscoAnalyser.com

 

[csd72]

So many questions, so few answers. Best to wait until the investigations and court cases have run their course.

 

[msquared48]

Qshake:

OK, I can accept that. My only other concern though is that if the gussets provided joint rigidity that was not designed for then the following happens:

1. The members are seeing forces less than what was designed for and have KL/r values less than the original design due to joint fixity.

2. The joints are seeing moments for which they were not designed.

3. If the joints were designed as pins, the lack of stiffness of a failed gusset should not contribute to the bridge failure as it would be merely another level of energy absorption that was not included in the original design adding to the overall safety factor. In effect, the gussets would be springs that were overloaded. A further redundancy for the system.

Item 2, to me, is the critical one here, but there are undoubtedly others.

Just thinking out loud here. And, yes csd, we will have to wait, but it's still fun to guess.

Mike McCann
MMC Engineering

 

[Qshake]

Mike - I don't disagree with your points but will note that:

1. Not necessarily a bad thing, eh?
2. Since the gusset plates are being designed for (by AASHTO specs), the average of the actual stress and member strength but no less than 75% of the member strength, and since both criteria are independent of the actual load caeses since same load case may not elicit maximum stress in all members framing into gusset, it is likely that the gusset was designed to resist quite a force above what it should.
3. Failing the gusset is not good due to it's relatively weak out of plane behavior. Addiitonally any large displacements in the gusset from failure will negatively impact the truss by changing the geometry and load paths. This loss of stability can collapse the bridge.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.