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

 

[bimr]

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.

To be clear, the Collapse of I-35W report also indicated other factors:

"The National Transportation Safety Board determines that the probable cause of the collapse of the I-35W bridge in Minneapolis, Minnesota, was the inadequate load capacity, due to a design error by Sverdrup & Parcel and Associates, Inc., of the gusset plates at the U10 nodes, which failed under a combination of (1) substantial increases in the weight of the bridge, which resulted from previous bridge modifications, and (2) the traffic and concentrated construction loads on the bridge on the day of the collapse. Contributing to the design error was the failure of Sverdrup & Parcel's quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the I-35W bridge and the inadequate design review by Federal and State transportation officials. Contributing to the accident was the generally accepted practice among Federal and State transportation officials of giving inadequate attention to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analyses."

 

[RetiredSE]

Thanks Meerkat. PT ducts are tougher than I thought and the widely spaced ties are not as ductile as I imagined. I envisioned member 11 looking like a centipede where the PT ripped out.

 

[Ingenuity]

Over tensioning a PT rod is not dangerous.

Yes it is. How many have you personally stressed to state this? I have personally stressed (and sometime intentionally failed) many PT tendons - some longer than 1,000 ft (yes, not a typo - one-thousand feet long). Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.

I assume that this was an experienced crew. They were sent to work alone without direct supervision.

For more than 15 years the Post-Tensioning Insitute (PTI) has conducted training and certification for field personnel pertaining to PT. I would expect that at least one of the field crew on the canopy undertaking the stressing works that day would be PTI certified. Additionally, the parent company of the subcontractor who did the PT work has possibly one of most comprehensive and industry-leading training and safety program in the US.

Normally a structure is supported by some type of false work until post tensioning is completed.

Often, but certainly not always, highly depends on the type of works. For temporary PT works, often PT is undertaken under full-traffic. For more complex (higher risk?) PT, there may be a traffic shutdown, but falsework, generally speaking, is a big time and cost item. I am not saying it was 'not justified' for this structure, it may have been. A traffic shutdown would have been PRUDENT, on a Saturday evening!

We may never know truthfully what the instructions and warnings were given to the crew before the work began.
It is doubtful that the crew was aware of the reduced safety margin.

I think it will be revealed precisely what instructions were given by FIGG after the 2-hour trailer meeting after cracking was noticed and necessitated the stressing works. There are also two surviving members so the stressing crew who I am sure will do the honorable thing and report the facts of what they did on that canopy - it is the very least they will do for the sake of their now deceased work colleague and 5 others.

It appears that one worker falls from his safety harness. If true, that says something about this workers profesionalism.

Total speculation.

The man who will take shortcuts with important personal protective equipment may also be the man who will overtension a PT rod without a second thought.

You really have it in for the field guys, don't you! I have spent half of my 30 year career in the office and half in the field (with dirt on my hands and under my fingernails) - and for all the $hit field guys get from office 'pimps' it is not justified in the majority of cases.

I believe that the root cause of the wye may be a tiny grain of sand or other foreign matter jamming the threads of the PT nut and the following cause was over tensioning the rod in an effort to free the jammed nut.

Highly unlikely. Consider than member #11 (and it PT bars) is more than 30 feet in length. So the elongation of the PT bar stressed to 70% of yieldEDIT ultimate is (70%*390 k = 273 kips) about 1-1/4". To achieve 'lift off' of the nut to the bearing plate all you need to 'daylight' a gap - let assume 1/8" (very conservative). So the bar has now elongated 1/8" above its original 1-1/4" so it elongates a total of 1-3/8" - an increment of 10% - and therefore a 10% increase in corresponding force (stress) too. 110% of 273 kips is 300 kips - way below ultimate tensile of 390 kips.

Additionally, the very design of the threads to PT bar is to handle the field conditions. It is a very robust thread - a grain of sand or cement paste splatter has little effect. Additionally, it is common for the manufacturer to supply a 'thread cleaner' nuts that you 'run' over the bar projection to clean away any deleterious materials BEFORE you install a stressing setup.

 

[SwinnyGG]

Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

 

[dik]

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

Because they 'have big pockets', they may not be able to distance themselves far enough... we'll have to wait and find out... hopefully, their presence on site, during any part of this work, is nil or next to... If they were frequently on site, or attending meetings, they may have a problem.

Dik

 

[Ingenuity]

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

It was closer to 1,100 feet long, 31 x 0.5" dia 7-wire multistrand tendons (all 31 pulled one-time), 8 tendons total. Approx 8 feet of elongation. I may have a photo I can dig up...

 

[SAIL3]

thanks Ingenuity...finally someone with actual experience in PT design and construction....

 

[3DDave]

bimr - I have read the entire I-35W report and still have in mind the photos of the warped gussets. I didn't feel that pasting the full contents was required to make the point that a bridge that had the same redundancies as suggested as a fix for this one still failed because a fundamental design element was never addressed.

 

[Ingenuity]

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Yes, the engineer would (should?) have considered all aspects of component capacity - all the PT components and the structural member with its load histories/cases and including reductions for area occupied by the tendon ducts (not yet grouted etc).

For member #11 with 2 each x 1-3/4" PT bars on a 24" x 21" concrete cross section, with PT bars stressed to about 70% of UTS - that is about 540 kips over about 500 in[sup]2[/sup] of concrete - so about 1,000 psi of axial compressive stress. Self weight support reaction of 950 kips and a member #11 angle of 36[sup]o[/sup] equates to a compressive force of about 1600 kips (3,100 psi axial compressive stress) - add the self weight force to the stress induced by the PT to member #11 and you get a net of about 2,140 kips (or 4,100 psi axial compressive stress). IF (big IF) there was a 10% overstress to PT bars that would only increase the force (and stress) by 54 kips (100 psi) to the cross section, for a total of about 2,200 kips (4,400 psi). For 8,000+ psi concrete, adequately reinforced with mild steel reinforcing, (including significant confinement reinforcement) and the level of stress to the member should be okay...now the joint of #11 is another matter!

 

[wiktor]

[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.]

Yes, we do not know what the bridge was designed for - but we know for sure that it collapsed, and based on this fact, we know that there was some major error somewhere leading to the catastrophe. I have seen already another design driven by the appearance, without any understanding of the V-pier design.
And I have seen indiscriminate application of the code, without understanding of the meaning of these factors, and resulting safety of the design.
No code, or design guide are perfect - and there are many variables which should be taken into consideration, like the percentage of the particular loads in the overall design forces. And perhaps the codes should specify the limits, to which particular formulas applies. ("when DL/PL ratio is >2, use DLx1.6" - just example)
Almost anybody, with some basic computer skills, could operate Midas, or Bridge Designer, or Robot program, to name just few. One could model a bridge, and size it without even knowing how the structure works.
And I really hope, that that's not the case, and the cracking moment and shear for the span was checked, and compared with the design loads and minimum sensible safety factor achieved.
And I also hope, that somebody designing another heavy pedestrian bridge is reading this, and will alter a design to achieve minimum 1.7 SF.

 

[Archie264]

>>>One could model a bridge, and size it without even knowing how the structure works.<<<

Given our current overly-complex codes and our reliance on software to navigate those codes I'm afraid I agree with you. I've also come to believe we may have built better in the days of the slide rule. Just my opinion.

 

[Lomarandil]

wiktor, regarding this discussion of load combinations, the ratio of DL/PL concerns are exactly why bridges also have to satisfy the Strength IV combination, using DLx1.5

Maybe not the 1.7 you're shooting for, but a higher SF than has been discussed to this point.

----
The name is a long story -- just call me Lo.

 

[dik]

Received this eMail at home; I've been discussing this with a couple of engineers on DesignToEurocodes...

I’m not a member of the ENG-TTPS [sic] forum, but I thought you might be interested in this:

https://facilities.fiu.edu/projects/BT_904/FIU-Pedestrian-Bridge-Design-Criteria-2015-05-06_REV.pdf

Other internet finds:

RFP:

https://www.flrules.org/gateway/notice_Files.asp?ID=14762059

FDOT Pedestrian Bridge Info

http://www.fdot.gov/structures/structuresmanual/2007january/Components/BridgesPedestrian.htm

The information may already be posted, but, I didn't notice it.

Dik

 

[dik]

we may have built better in the days of the slide rule. Just my opinion.

Still have mine... just in case...

Dik

 

[3DDave]

For those who don't have one:

http://www.mathsinstruments.me.uk/page3.html

 

[hokie66]

The discussion will continue in Part IV. This thread is closed.

Post here:

thread815-436924