Miami Pedestrian Bridge, Part II 55

[Archie264]

A (hopefully somewhat gentle) reminder to all that:

1. This is a public forum. As such, it should be fair to quote anything posted here. Don't want to be quoted? Don't post.

2. Most participants aren't posting under their own name and those who do so have chosen to.

3. The title of this subforum is, "Engineering Failures and Disasters." If it shouldn't exist that subject should be discussed with the owners or moderators.

4. Would we all prefer that this discussion be left to the nonengineers?

5. There is far too much speech suppression in the world today as it is.

I suspect this post might earn me some ill-will but so be it.

 

[Phil1934]

If this were analyzed with FEA instead of as a truss and some fixity was assumed across the joint, would the diagonal pass a unity check?

 

[bridgebuster]

Roads and Bridges magazine has already sent out 2 surveys speculating as to the cause of the collapse.

Congratulations hokie66.

 

[dik]

If this were analyzed with FEA instead of as a truss and some fixity was assumed across the joint, would the diagonal pass a unity check?

With a 'brittle' concrete structure, the analysis would have to include for the fixity at the panel points and reinforced accordingly. If a joint rotates and reinforcing is not provided, it will likely crack... it may not have any issue with overall strength, but, may have an adverse impact on esthetics.

Dik

 

[dik]

Roads and Bridges magazine has already sent out 2 surveys speculating as to the cause of the collapse.

Interesting how FDOT is trying to distance themselves. It may not be that simple. Being a resource with money may make them a 'target'.

https://www.roadsbridges.com/fiu-broke-agreement-fdot-choice-firm-pedestrian-bridge

I guess one has to ask the questions:

Were they aware the construction was on going?

Were they aware of the agreement/requirement with FIU?

Did they have anyone on staff visit the site during construction?

Why did they not intervene?

Dik

 

[TehMightyEngineer]

...I suspect this post might earn me some ill-will but so be it.

On the contrary, nice summary. As I said before, I don't disparage anyone for commenting and discussing this failure. As you noted, I'm one of those people who chooses to attach identifiable information to my posts. This is part of the reason for my choice to not participate in the discussion yet. For those who post anonymously that's obviously not a concern.

In short, please keep discussing this; I just caution people keep in mind who may be reading it (public, media, etc.), I'm sure one or two FIGG engineers are watching this discussion and probably tearing their hair out that they can't join in and provide further details.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[racookpe1978]

I do not find cable-stayed brdiges (of any size) very attractive, but, my opinion about somebody's else's tastes don't matter. (That my tax dollars ARE taken to pay for somebody's (lack of) taste IS irritating, but I can do nothing about that right now.)

Regardless, I read more and more often that the large tower, the cable-stayed members themselves (hollow 8 inch pipes ?) and their connection fittings (8 inch pipe flanges and bolts ?) and the cable-stayed connections into the angled truss members are "decorative. "

If so, then these "decorations" are directly to blame, are they not? The "truss" cannot be symmetrical because every truss inclined member must align visually to its matching cable-unstay diagonal member coming up from the bridge lower walkway. Unsymmetrical members equal greater stress in some member connections than others, but all connections need to be "visually" identical = some are overbuilt = too expensive, take too long to build properly.

Regardless of intent, the 8 inch pipes are very long, and will sag under gravity: This creates an elegant appearance when they are installed in a traditional suspension bridge, but an ugly distraction when hung basically sideways in this case. Thus, whether intended or not, the cable un-stays MUST be under tension to be visually straight under their natural gravity load at the extreme angles of their design,(and must be heavier (stronger) to resist that sag if NOT under their proper function as true cable stays. (A true cable stay cable will be straight BECAUSE) it is in tension holding the bridge up.) These decorative cable un-stays would need to be even heavier and thicker-walled to resist the inevitable bending (sagging and droop) than needed to be in "real tension" as a true cable stay.

Further, a true cable-stay holding the original lighter weight of the bridge if it were a true cable-stayed bridge, would be a smaller diameter, lighter cable: Holding "up" the canal side of the bridge, thrusting down on the tower and foundation, and holding "up" the traffic side of the bridge. The smaller diameter of a true cable under simple tension creates less hurricane wind force under live load conditions. 100-150-200 feet of 8 inch pipe at 20 to 23 pounds per foot = added dead weight on each connection point in each truss, rather than a simple true cable stay that is lifting the walkway at the same point.

The pipe (fake) cable un-stays are themselves heavy and have mass and wind resistance. As long round objects in the unpredictable gusty winds of a hurricane, this round shape creates a near-maximum turbulence and whipping load in a hurricane, and this extra load must be added to their gravity load under wind conditions = heavier, more expensive bridge = greater profits for the brddige design team, right?

So, regardless of claim as decorative devices, there are gravity loads, extra stress, and extra un-symmetric stresses added to each connection point of every member. Forget the cost and time of the claimed "decorative" tower and its cable un-stays: The tower itself and its cable un-stays add dead weight to the walkway, add hurricane wind loadings to the walkway at each connectino point, and create even worse cases of the very conditions a true cable-stayed bridge is intended to reduce and support.

 

[racookpe1978]

There can be no question whether or not a title including the term "Unsafe" is appropriate.
The bridge failed catastrophically under conditions of essentially "no load" (no foot traffic, no winds, no snow (obviously!) or rain, and with innocents underneath.

 

[CWB1]

The stressing equipment used on this project at the time of collapse was very simple in design and use. Basically a 10,000 psi MAX pressure hydraulic pump (electrically operated, with a wired remote), with a 4-way valve (controls fluid directional flow), a gauge, a set of hyd hoses that connect to a center-hole, double-acting ram/jack, that sat upon a big-a$$ stressing stool/chair whereby the 'stressor' can turn the nut.

This hyd design/setup is about 40 years old - tried-and-true. Simple.

Interesting. Hydraulic fastener stretchers are very common in manufacturing and typically require redundant load cells for reasonable accuracy and repeatability, a hydraulic pressure gauge isnt even included on many.

 

[dik]

Engineers have vastly improved living conditions, chemicals, medical equipment, prescription drugs, transportation, petrochemicals, utilities, agriculture, food, etc., etc., etc., i.e. almost everything, to the point that our score card is nowhere close to being in the red.

Some of these items may not have improved living conditions... in addition, there may be other engineered products that have definitely harmed mankind. I tend not to be quite so smug about our accomplishments.

Dik

 

[WinelandV]

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

Any of us who has come into a project halfway through has had the thought, "This, this, and this look really strange and I hope that this, this and this were accounted for." Same situation here - we're all coming in after the fact and slowly learning what the design really is. I, for one, am willing to give Figg some slack here on all the design issues: the un-symmetry, the not-really cable stays, the non-redundancy, the kinda-a-truss but kinda-a-beam-with-holes issues. That firm isn't just 2 people in a garage somewhere. They have designed many, many bridges throughout the United States (including the I-35W replacement in Minneapolis).

Concerning the article title, to say that the design would have only "looked safe" is a tremendous extrapolation of the speculative comments here.

Maybe, when it's all said and done, we'll know one way of the other if the final design was safe (and maybe not if everything gets settled and everyone signs an NDA). Until then, all we're doing is throwing thoughts around. Yes, it failed, but there's only a couple hundred other factors that could have ultimately caused the collapse.

 

[ke27on]

With a 'brittle' concrete structure, the analysis would have to include for the fixity at the panel points and reinforced accordingly. If a joint rotates and reinforcing is not provided, it will likely crack... it may not have any issue with overall strength, but, may have an adverse impact on esthetics.

what program would this be used in? I'm familiar with using ETABS although I assume this would be in SAP or some other program. (I'm a buildings structures guy). I ask because when modeling in ETABS and to a large extent in RISA as well, most FEA objects just end up being line objects that connect at specific points rather than an object that you can see how stress varies across the thickness of the section etc.

 

[NITTANYRAY]

"Time will tell who failed whom, but, if it is discovered that an Engineer(s) did fail here, I am very comfortable that Engineers have vastly improved living conditions, chemicals, medical equipment, prescription drugs, transportation, petrochemicals, utilities, agriculture, food, etc., etc., etc., i.e. almost everything, to the point that our score card is nowhere close to being in the red. We just need to drill down to the facts in this case, so it never happens again."

In school if we got a 95/100 on an exam it was considered very good by most people (even the PE exam only requires a 70% to pass) but in the real world one miscalculation out of hundreds or thousands on a project can become a life and death issue. Very few professions require as much near perfection as ours. The fact that these types of tragedies are rare speaks well for our profession.

 

[Stephen Nuchia]

This is speculation of course but a theory of the failure I've not seen yet in this thread is sheer failure at the interface between 11/12 and the deck. In the dash cam footage there appears to be motion of 12 spanward as hinging starts to develop. But the base of 12 remains on the pylon so it might have been spalling? The last segment of the deck fell straight down and the upper flange initially moved downward rapidly before pivoting. One of the PT rods in 11, the inboard one, zippered to stay with the moving deck, the other stayed in the base of 12. The triangle consisting of 11,12 and the last segment of the upper flange retains substantially its original geometry post-failure. That sheer happened there is clear. Could it have been the point of initial failure and the joint failures at 9/10 and 10/11 secondary?

 

[dik]

what program would this be used in?

Any of those programs you mentioned would be able to do it... About 45 years back, I wrote a 2D Frame Analysis program that would run in 64K and swapped in and out of a 5-1/4 disk... By adding a couple of joints within a foot or so of the panel point I could increase the stiffness of the 'short' member to provide a better model of fixity. Almost any frame program can treat the joints as fixed and will generate a moment at the member-panel point junction.

Dik

 

[TehMightyEngineer]

This thread is closed. Please post in the new "continued" discussion.

Do not post any more in this thread.

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