Miami Pedestrian Bridge, Part III 99

[JStephen]

"Site management has requested that we limit the length of the other thread by forming a new one. This subject may require III, IV...."
So here's Part III. Please don't post any more in Parts 1 and 2.

Part I thread815-436595
Part II thread815-436699

 

[samwise753]

Not sure why you're arguing against redundancy. Non-redundant structures are avoided a majority of the time, and, when they can't be avoided, the safety factors are much higher. As for the elements of a redundant structure, they are always designed with some reserve capacity with load redistribution in mind should one element fail. Inherently, Load Resistance and Factor Design makes provision for that by increasing raw loads with load factors and decreasing capacity with phi factors. The more critical the failure mode, the lower the phi factor. If i recall correctly, the issue with the I-35W bridge in MN, was that dead load had been added over time and the inspection frequency was very low. Had the bridge been inspected more often, they likely would have noted which joints were becoming more and more overloaded.

As for redundancy fixing the issue with the FIU bridge, it would have prevented catastrophic failure. The structural concept in general could not be non redundant because they chose to go with a single, giant, post-tensioned, simply supported I-beam. The critical feature of their design was going with the open-web, truss-like structure. I could only imagine focusing moment, shear, and axial forces as well as the post-tensioned forces at nodal points would be very hard to design and detail for and equally difficult to construct properly. I would think that a solid web would have been easier to design though you would lose the aesthetics of the open web.

 

[Meerkat 007]

Any have a clue what role had PT anchor of member #11 into #12 from structural design point of view ?

 

[bimr]

Looks like someone is clamming up, From Fox News:

“The city attorney is advising us not to speak about anything to do with the bridge,” said Sandra Antonio, a spokeswoman for the city of Sweetwater,

...National Transportation Safety Board had forbidden engineers or contractors from talking about the project pending its investigation.

A FIGG spokeswoman, Cheryl Stopnick, said it would be “inappropriate to discuss specific detail about bridge design” during the investigation.

A spokeswoman for FIU did not immediately respond Tuesday to questions about the pylon relocation in the bridge design.

Happy as a clam...

Dik

This issue was noted in other accident discussions. All parties take a vow of silence until the NTSB delivers its final report on an accident. So secret is the process that for some portions of the inquest the partners gather in a secure section of the NTSB building that is equipped with a unique computer system that allows no communication outside the room. Partners at those sessions take notes on color-coded paper that is collected before they leave the room.

NTSB Form

This is not a 'conspiracy', but a means to determine the cause of the accident as well as to prevent future accidents. If all parties to an accident stand in the public square, behind their attorneys, point fingers, and fail to disclose what they know, the true cause of an accident may never be determined.

This accident has developed intense interest, but it will take some time before the accident investigation is finished.

 

[Jerehmy]

jrs87 - Condit, C.W. 1983. American Building: Materials and Techniques from the First Colonial Settlements to the Present. University of Chicago Press. ISBN 978-0-226-11450-7

 

[3DDave]

The I-35W gussets had been known to be buckling for some time and were just being observed. I think the accidents reports indicated they weren't specifically analyzed by the original team who considered the frame members to be the load path which is why no one noticed they were undersized.

Regardless of later developments in use, the I-35W trusses were seemingly redundant and still allowed a catastrophic failure.

 

[dik]

Winelandy:

I gave you a star for your post, but, it comes with a caveat. Maybe half a star, and, someone can provide the missing half. This posting goes to others in the thread, just to bring our feet back to earth.

We have a reasonable likelihood of having identified the collapse mechanism, or, something similar to it. We could be wrong. We are basing our conjecture on some poor photos, some poor videos, and a preliminary construction 'document' (Photos are generally consistent with the preliminary drawings).

The final as constructed documents may differ substantially from the preliminary ones. The preliminary document could have provided design simulation material, simply for marketing. To do anything, but speculate, we need final construction documents and shop drawings. I think our current endeavour is close, but, there could be 'big holes' in it. Myself, and, others likely don't have enough information of offer a real cause and may have some reservations/concerns about the Eng-tips proposed failure.

Regarding codifying everything, there are a few thoughts and maybe more. Overall, this is a *BAD* idea.

Codifying goes a long way to indemnifying an engineer... all is done to prescribed standards, and, it is more difficult to fault an engineer. The Nuremberg defense comes to mind.

Codifying also curtails the profession by tending to restrict innovation. With things not codified, you can reach beyond a bit, using your engineering skills.

Another issue with codifying is that there are so many problems in engineering that don't fall into a 'cubby hole' and there is no prescribed manner of addressing them other than engineering skill. All you have to remember is that E=mc^2 and you cannot push on a rope (you can, but, you reduce the effective prestress)... and go from there.

There is talk about insisting that construction be ductile. As engineers, we know how to build redundancy into our Work so there are multiple load paths to failure, or enhanced safety factors, if ductility is limited. A ductile structure is a little easier to accommodate. It started, I think, with Ronan Point.

We don't know, that the original structure may have had some ductility built in, but, this failed.

The failure appears/was a brittle failure from some mechanism that may have eluded the original designer. There was no redistribution. Just so much is unknown. We have to sit calmly back, and, let things unfold;, else, we put ourselves in the same position as 'the Professor'. Additional information should be forthcoming and we will be able to provide more enlightened responses.

Dik

 

[dik]

This is not a 'conspiracy',

Never thought it was... just reminds me of the old saying, "When everyone is thinking alike, no one's thinking."

Dik

 

[dik]

I wish NTSB all the best with this one.

It's not luck they need... hopefully, they can come up with an explanation of the cause of the failure. If not, they can speculate on the possible cause (and identify it as a reasonable, possible cause), but, the report should not have to stipulate on an exact cause. They should strive for that, but, not be 'driven' by it.

Dik

 

[Meerkat 007]

I don't get your point. Redundancy role isn't to cover design errors.

 

[wiktor]

Dik,
The known facts are:
1.Bridge design as per AASHTO Guide Design Specification for Pedestrian Bridges, and AASHTO 2015 (Drawing B-2)
2.Overall SF factor (DLx1.25+PLx1.75)/(DL +PL)=1.35
So, applying blindly the code it's not the best idea. I do not think that the authors of the "design guide" envision a pedestrian bridge with such ratio of DL to PL.
Any bridge suppose to have a minimum SF of 1.7 or above for bending, and 2 for shear.
And under-reinforced transition from blister to the deck was not ductile - it was relying on the shear strength of uncracked concrete

 

[3DDave]

Meerkat 007,

This bridge seems to have failed due to design deficiency and many are claiming that redundancy would have prevented the failure.

There is no difference to the people who are killed between a design failure to account for expected loads and a design which fails because it is loaded more than the expected loads. The only thing that is important is if the failure mode is such that people have some warning that significant failure is imminent. Redundancy isn't guaranteed to do that.

OTOH overcapacity and designing a specific failure sequence would be helpful, but I have rarely seen it done. For example, I don't see many cases where there is a levee that has a separate, higher levee structure placed behind it. Instead there are designs such as the space shuttle booster O-ring that allows the secondary o-ring to cover for failure of the primary o-ring, resulting in catastrophic loss of the entire space shuttle.

 

[Alfie.Tx]

Meerkat007, your comparison between bridge collapse movie and theoretical simulation of truss without member #11 looks similar. It tells that the diagonal #11 crumbles at the base first, otherwise, it would have pushed out #12 a great distance based on the geometry as shown below, before the base slab tore away from #11 and #12.

 

[RetiredSE]

Please forgive me if I missed this in skimming the previous threads but:
-I noticed the PT sheath that I assume is from member 11 is nearly intact. I would think it would be destroyed as it ripped out.
-Maybe the old eyes missed it but if the PT strand was within the ties of member 11, I would expect to see the broken tie ends projecting where the PT rod ripped them apart. Was the PT rod a late addition outside the ties when it was discovered that the transporter needed moved inward and member 11 became a tension member (temporarily)?

 

[bimr]

Relevant webinar today:

The Role of the Design Professional During Construction

 

[bimr]

It's not luck they need... hopefully, they can come up with an explanation of the cause of the failure. If not, they can speculate on the possible cause (and identify it as a reasonable, possible cause), but, the report should not have to stipulate on an exact cause. They should strive for that, but, not be 'driven' by it.

Dik

Probable cause will be stated.

Example: Board Meeting : Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007

 

[Meerkat 007]

RetiredSE - see below...

You can see more details here.

 

[dik]

The known facts are:
1.Bridge design as per AASHTO Guide Design Specification for Pedestrian Bridges, and AASHTO 2015 (Drawing B-2)
2.Overall SF factor (DLx1.25+PLx1.75)/(DL +PL)=1.35
So, applying blindly the code it's not the best idea. I do not think that the authors of the "design guide" envision a pedestrian bridge with such ratio of DL to PL.
Any bridge suppose to have a minimum SF of 1.7 or above for bending, and 2 for shear.
And under-reinforced transition from blister to the deck was not ductile - it was relying on the shear strength of uncracked concrete

That's where I have a problem...

minimum design standards as you have noted... could have been more than AASHTO required, or, may not have been designed in conformance with AASHTO... It should have been, but, just don't know this fundamental information...

Safety factor predicated on AASHTO design... see note above.

DL to PL ratio *should* make a bridge more safe because the variable quantity is diminished... not, that the standard includes for this loading differential... this weight disparity may be reflected in any geotech report, but, not likely in a bridge standard.

...appears to be under-reinforced, else, it shouldn't have collapsed catastrophically. I cannot yet get to the 'real' cause of the collapse although I think the thread is going in the right direction.

Dik

 

[dik]

Probable cause will be stated.

That's great...

Dik

 

[Ingenuity]

who in their right mind allows a crew to work on 150ksi PT rods on a non-redundant structure without closing traffic????

More common than you think.

Ever since the 1950's when Finsterwalder pioneered free-cantilever (non-redundant) post-tensioned bridge construction, which are constructed over traffic-occupied freeways and highways all over the world to this day.

For match-cast segments consider that the only thing passing through the precast joints are PT (and UNbonded until grouted) and some epoxy, without a lick of mild steel reinforcing.

 

[epoxybot]

Correct me if I'm wrong but with the pylon moved 11 feet further to the north, this part of the bridge was 186 feet (57.2m) long instead of 175 feet.

FDOT reasonably distances itself from this project because Florida International University received Local Authority Program (LAP) Certification to oversee the project. Link

FIU installed Smart Bridge Technology on a new flyover from eastbound State Road 836 to northbound State Road 826 in Miami back in 2013, why they didn't do so here is frustrating. Link

The City of Sweetwater really has no roll in this project. They transferred title of the land to FIU at an inflated price of $1.3 or $1.6 million and agreed to participate in a shuttle transportation program, requiring they provide a "refurbished" 10+ passenger bus. When the bridge was moved from one side of 107th St. to the other, they agreed to assume the costs of completing the remaining improvements to the existing "historic" pedestrian bridge access. They did it on the cheap. Their approval of the ADA work is laughable.

According to the FDOT LAP guidelines FIU had the responsibility to have the FDOT sign off on the Right-of-Way BEFORE requesting Bids for the project. The delay & redesign fall squarely on their shoulders. TY Lin seems to have been providing Planning Services since 2014 for the global "University City" plan. Should they have been prompting FIU to the ROW requirement? Or did FIU just bite off more than it could chew.

Watching the videos in the DropBox, it seems they paused at one point to adjust the chains securing the bridge on the north end. I think this was needed in order to raise the bridge enough to clear the pylon. Finally, the bridge just sat in one place until a daylight hour convenient to ~dignitaries~.