First thing that sticks out to me are the guy-wires at mid-span. It seems like a pretty inefficient arrangement, the slope being about 1:5 or thereabouts. I would have thought that this kind of structure would have been proportioned so that the end supports would be enough to make it stable under wind.

The guy-wires kind of ruin the aesthetic of the arches being otherwise free-standing, and I wonder if this came late in the design? Certainly the pier crosshead was designed to be wide enough for these.



Also, interesting excerpt from the article linked:

The first arch was lifted in March with no problem, but crews stopped work after they tried lifting the second arch. They said they heard popping noises, so the arch was reinforced and re-inspected before it was raised in July 2021.

Officials in Hickory confirmed that the splintering issue last year was “repaired with adhesive, clamps and self-tapping screws.”

The steel girders under the bridge, plus the fact that the actual walkway is still standing, hints strongly that this wood thing was only decoration.

so, who's the structural engineer who affirmed that it was not only clever and attractive, but could also stand up for a long time?


spsalso

So the timber arch is a decoration, not a structural member. It is top heavy and has hinge points in its lower portion and at its summit.

This seems like an improbable structure but then, once again, hindsight.

is the arch structural or decorative. Is it me or does that arch become a beam in bending if you break / loss tension in one of those guy wires (assuming them are not just to stabilize the arch against wind side loads.

If that wood structure was structural, it would have to support a load. The only connections between any part of the arch and the roadway are the two steel cables dropping down from the middle. And it is VERY curious that they land right on the top of a pier. Could the wood arch be secretly holding the pier up, keeping it from sinking into the ground? I think not.

It is ONLY decorative.

It was interesting about how the general contractor passed the blame off to a subcontractor who seems to be passing it off to a place FAR out of town (in OREGON!!!), and thus obviously at fault.



spsalso

It's a shame... it looked like a beautiful structure. It would have looked better with concealed connections...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (gusmurr (Structural) (OP) 20 Feb 22 23:23)

First thing that sticks out to me are the guy-wires at mid-span. It seems like a pretty inefficient arrangement ...

Even worse, with the hinge points approximately 1/3 the height, the guy wires become even less effective, maybe just enough resistance to tease the installation.



As we can see in these images, the top portion buckled from the hinge points while the ends fell inward.

Note that what you see in the picture with all the colored lines and circles and arrows is NOT the completed structure that fell down.


spsalso

Note that the complete structure includes two arches, one within the other. The failure axis extends through the hinge points on both arches. This is a very bad, no good, horrible design.

Winds at the time S 30 km/hr (Feb 17/18 midnight)


(With help from Google Earth and other)

No respectable structural engineer designed that unstable thing. Go find the architect. His monument fell down.

"No respectable structural engineer designed that unstable thing."

It's sure as hell going to be cheaper to deal with than Millennium Towers. But, as you say, no respectable structural engineer designed......


spsalso

Think those wires will actually make matters worse for the top joint.

Tapered wood onto a metal plate with the grain the way it is wasn't sensible either.

I think the gods of engineering are smiling whenever non structural pieces fall over. Just a good thing they are ignoring Melbourne. We have so many such things here. (Bolte bridge)

There are some gems in the reporting

Quote (city offical)

Officials in Hickory confirmed that the splintering issue last year was “repaired with adhesive, clamps and self-tapping screws.”

Think they must have used my wife as a consultant. That's her solution to any wood work broken. And its never yet worked. But she keeps trying....

The project had the installation repair solution reviewed and approved sometime in May 2021

https://www.fox46.com/news/hickory-iconic-bridge-p...

Western Wood Structures appears to be well qualified for designing and fabricating wooden structures. This is not their first or most ambitious project.
https://www.westernwoodstructures.com/our-story#h
https://youtu.be/MDl21tNpjXs

The shape was interesting when it was standing - almost like a DNA strand.

unfortunately Europe blocked to the fox46.

It was more the wording of the reporting that amused me.

And agree they have done some lovely structures.

These white papers are interesting

https://www.westernwoodstructures.com/index.php/wh...

Quote (Alistair_Heaton)

Yes - WWS has some very interesting white papers. Their bowstring truss analysis PDFs will have me looking overhead whenever I am in a structure with glulam construction.

Link

Big pic of the top

There seems to be an awful lot of concentration of bending moment at those mechanical flanges alright. Were the two of them tied together at the top? It looks like it to me from a bit of googling.

Maybe those wires are like the ones on the FIU bridge and there to prevent the bridge from bouncing too much?

Google street view hasn't updated with it built unfortunately but there are plenty of pictures out there.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

When function follows form, instead of the other way around, you end up with a bridge like this;
Link
It's not apparent from most of the pictures, but this is a single inclined arch

Opening was delayed for some time until everyone was sure it wouldn't collapse.

Brad Waybright

The more you know, the more you know you don't know.

Ridiculous connections! No transfer of outside fiber stresses. They don't really transfer moment at all.

A black swan to a turkey is a white swan to the butcher.

The hinge connections looked pretty flimsy to me; structure with a certain level of stiffness based on its footprint narrows down to less than 1/10th its base width, supplemented by at most 1" of steel and screws that appear to have pulled out pretty easily

The base connections in the image above also appear to have been flimsy; whatever screws they used to secure the wood to the metal plates simply pulled straight out, while the hinge connection screws simply splintered their way through the wood.

This is a pretty good picture


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I suspect gusmurr identified the real cause of the failure.

I agree that it looks frighteningly unstable at first glance, but most of that is resolved by internal bracing. I think the interplay of the two arches confuses the shape - some of the 'straight-through' shots make it look symmetrical. The global shape appears to be roughly parabolic, and 3 pinned arches are relatively common. So no moment is transferred from one half to the other at the middle connector by design.

Regarding fasteners in the connector plates, I wouldn't go so far as to say they "pulled through easily" - perhaps they did, but perhaps that was because the joint experienced 10x its design stress due to a failure elsewhere in the structure.

Check this from the picture IRstuff posted. This looks suspiciously like a terminal connection of a cable to the concrete pier. I would not expect this to be a secondary failure - if the arch itself had failed a wind of only 30mph our so would not be sufficient to blow it out and maintain significant tension as it went down. It likely would have crumpled and come down, leaving those connections in tact.

My theory: the incredibly inefficient cable brace angle led to excessive tension in the cable, and it precipitated a failure in the cable connection. (I wonder if there was a request to use adhesive anchors for the cable terminals...) When that came out, the arch lost all lateral stability, which then caused the joints to experience out of plane moments for which they were not designed.

Quote:

When that came out, the arch lost all lateral stability

That would seem to argue that the cable brace was woefully inadequate, given that it was a single-point failure mechanism.

Quote:

but perhaps that was because the joint experienced 10x its design stress due to a failure elsewhere in the structure.

Given that the cable was a single-point failure, the joints should have been designed to handle the loads that would arise from that failure, one would think.

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I think the cables were there to pretend to hold up the bridge.

Not to pretend to stabilize the arch.

Quote (IRstuff)

That would seem to argue that the cable brace was woefully inadequate, given that it was a single-point failure mechanism.

That it would. I never said it was a good design.

Quote (IRstuff)

Given that the cable was a single-point failure, the joints should have been designed to handle the loads that would arise from that failure, one would think.

If that were the case, it wouldn't be a single point failure. And if this were an integral part of the bridge and not decorative, you'd probably be right. But as the failure of the arch had nothing whatsoever to do with the stability of the bridge itself, I'm guessing it didn't undergo the same kind of scrutiny with respect to design adequacy and redundancy.

I'm not sure what the cables are for... if to provide support of something below, they might have been OK. If to provide lateral support at the top, not so good. Any horizontal load at the top would greatly increase the force in the cable because of the 'steep slope'. We'll have to wait and see why it 'broke'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (IRstuff (Aerospace) 21 Feb 22 17:44)

Quote (phamENG (Structural) 21 Feb 22 18:11)

The connectors require more info:
1. Is the connector from the top of the arch or from the pier crosshead?
2. The cable appears cut close to the loose connector. Is that from tension prior to collapse or from being restrained/sheared from the momentum of the collapse? The loose ends do not appear to be frayed, at least not significantly.

Could the cables be for keeping the whole thing in compression all the time?

The two cables had to be in tension, if nothing else, so they didn't look all loose and flappy and stupid.

How much tension?

If one side failed, then the horizontal vector up top for the other cable would surely add lateral force up there.

Would that be a bad thing?


spsalso

Those two legged 'Y' shaped supports at the end of the arches looks hauntingly like another failure which I remember. But can't recall any more about that one at the moment.

Some buff body dynamic action and the wires are to stop vibrating?

As of Mar 23, 2021, prior to the installation of the second arch:


(hickoryrecord.com)

The top connector resembles the loose connector found on the bridge deck after collapse:



In December 2021, after the installation of the second arch, it appears that a second cable is connected to the upper bracket, to the outside of the initial cables.


(newsadvance.com)


(newsadvance.com)

The lower connector is distinctly different from the loose connector on the bridge deck.


(newsadvance.com)

Sym P. le - looks like that debunks my theory. Oh well. I'll be interested to see what comes out of the official investigation.

Sorry 'bout that. I was just trying to get a better understanding of the connectors. It's seems odd that the cable snapped near the upper connector while at the same time the connector broke free of the larger weldment. There's more to learn.

Upper connector, enlarged from previous image:


(hickoryrecord.com)

Quote (MintJulep)

I think the cables were there to pretend to hold up the bridge.

Not to pretend to stabilize the arch.

I think you got it. The cables might have helped stabilise it, but they're essentially decorative. Decorative cables on a decorative arch.

In any case, those cables could be tensioned up fairly tight, like guitar strings. You're not tensioning a gravity load, you're tensioning between hard points (the headstock, and the arch).

Just for fun, 3D wreckage!


(wset.com and newsadvance.com)

That particular picture is really bothersome. My impression of the connections is that end plates were screwed onto the ends of the beams and the plates were bolted together at the connections, such as the hinge point, but that picture is showing plates but no protruding screws/bolts. Either the bolted-together plates are still intact, then there should be lag screws, or whatever, protruding from the exposed side of plates, or, the plates separated, which would seem to require some plastic deformation as each bolt broke, but the plates look kind of pristine

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Probably all treated, so not good for burning in your fireplace.

Altogether a poor advertisement for exposed wood.

This is the arch bridge I was trying to remember, in which the arch splits into 'wishbones' at the ends. Perhaps not a proximate reason for that collapse, but a weak point nevertheless.

https://www.eng-tips.com/viewthread.cfm?qid=458618

Lateral stability aside, these appears as 5 hinged arches? Isn't that inherently unstable?

There was also the Chirajara bridge in Colombia
https://www.youtube.com/watch?v=q-Vbe4ZqRak
https://www.bridgeweb.com/Report-published-on-fata...

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There appears to be a significant gap at the apex of the arches with a simple, single bolt connection. I can't yet imagine how the guy wire mounts were included but the mount for the lower arch came away clean (as seen in previous images laying on the deck) and the mount for the upper arch appears still fixed to the west weldment but there is no damage to the east weldment. I guess the guy wire for the upper arch can pass through the gap in the lower arch connection.

That pin connection looks awfully flimsy ^. The pin holes in the clevis plates have almost no edge distance. The plates appear very thin. Was this really holding the whole arch together??

The area circled in green appears to have been torn out. Could this tear-out failure of the pin have caused the arch to collapse? Alternatively, the other side of the clevis appears to have collapsed at its base, which is not surprising considering how thin it is. Could this have initiated the pin to become dislodged and tear out of the other side?

I would image that under wind loading, one side of the clevis would have been in tension and the other in compression. Perhaps this bending through the connection was overlooked, because it might have been thought that the guy wires would eliminate such an effect. But the guy wires appear to be awfully flimsy as well and not in a particularly effective arrangement as I mentioned in my first post.

Connection of lower arch prior to work stoppage to address structural concerns (May 20, 2021):


(Video source)

Compare to the piece on the bridge deck ???:



EDIT: It appears that this clevis mount is indeed modified before the completion of the project. (see further posts below)

Quote (gusmurr (Structural) (OP) 22 Feb 22 01:48)

... under wind loading, ...

The structure likely experienced buffeting during its entire life. It is a large sail of significant mass so any movement would likely work over the joints pretty good.

Comedy Break - Clarke and Dawe - The Front Fell Off except in this case, The Top Fell Off! ... not typical, not normal, unusual

Wow! If this "connection" isn't the cause of the collapse, I will eat my hat.

Thanks, Sym P. le for finding that picture.

Quote (The cables might have helped stabilise it, but they're essentially decorative.)

That was the purpose of those huge cable stays on the Florida pedestrian bridge that collapsed, too. No one seems to want to take really clear photos of the connections, it would appear. That top 'pinned' connection is really strange looking and really flimsy by the look of it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Those fake cable stays on the Miami bridge never had a chance to get installed.

gusmurr:

"Wow! If this "connection" isn't the cause of the collapse, I will eat my hat."



When I look at the pictures of the completed decoration, I don't see a connection as shown in that photo.

The connection you are referring to appears to have been temporary and long gone. If so, it's kind of hard to see how it could cause a collapse, since it didn't exist anymore.


spsalso

I was able to see the news article... had to set my VPN to the US to see it. Hokie's correct in the the material is incised, so it's PT and not likely good firewood. The manner of repair seems flaky. I don't know what failed, but adhesives and clamps with self tapping screws seems a little odd.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (Those fake cable stays on the Miami bridge never had a chance to get installed.)

agree Hokie... the similarity is that they, too, were decorative only, like the cables in this one. I don't know their function (even if only decorative) and what affect they had, if any, on the collapse. The Florida stays had the capability of supporting a load of several hundred kips... they weren't detailed as decorative... and they had nothing to do with the Florida failure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Are the knuckle connections supposed to be rigid, ie designed as rigid? Would that be the design intent? The dont look very rigid. Yet I can’t see how it could ever be stable without them being rigid.

dik,
The Florida bridge stays may not have been detailed as decoration, but that was the intent, according to the project description. But the whole wooden monstrosity in NC was a sculpture, not a load bearing span. The cables may have been intended for lateral stability...maybe as an afterthought.

Tomfh, agreed. I would say that a traditional 3-hinge arch turned itself into a 5-hinge arch with the failure of those 'knuckles' at the top of the wishbones, and that is not a stable structure.

That may have been the intent, but had the structure 'stood up', whether it wanted to or not, the cable stays would have had a real impact on the strength and behavour of the structure. Just because you label them as decorative, unless you detail them as such, they may not be.

As I noted earlier, "I'm not sure what the cables are for... if to provide support of something below, they might have been OK. If to provide lateral support at the top, not so good. Any horizontal load at the top would greatly increase the force in the cable because of the 'steep slope'. We'll have to wait and see why it 'broke'."

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

I have a feeling the cables are a part of the design mainly for dynamic response.

The thing does have an aerofoil look to it from certain angles. Including the one that the wind was blowing when it collapsed.

Quote (I have a feeling the cables are a part of the design mainly for dynamic response.)

I don't know... if they were for dynamic response, it would likely be lateral since that's the weak direction. The steep slope would increase cable loads substantially, and the vertical loading on the arches. The arches had been subjected to greater wind loads earlier this year. We'll have to wait to find out the cause from some other source. I'm still curious about what the earlier failure was and the manner of repair. That, too, might have bearing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

dik,
I never saw any shop drawings of those pipes. Granted, the design drawings did show them with end plate connections to the top deck, but I always assumed there would be slip connections of some sort when they got around to installing them.

The wishbone connections on the lower arch appear to have suffered worse damage than those on the upper arch. The south east leg of the lower arch even ended up beneath the bridge. Also notable is that the center of the arch pushed slightly east, in keeping with the wind direction relative to the span.







It's hard to say exactly how it all went down (so to speak) but I don't think any of the engineering imagined the degree of freedom represented in the following gif, which is what I tried to explain earlier in this thread.

dik the lack any real beef to them is the reason why I think it might be dynamic response.

As for the wind loading being ok initially with one span and then fail with two.

I am thinking biplane.



Lift off the spans would put even more load on the wires.

This picture was posted earlier. These perforated metal strips must have been the repair as they're not visible in other images.

The arches seem to be flanged together in th lower 1/3. I question the strength of the flanges that connect the arches at 1/3rd height. They are so very thin. It's so bad it's as of someone forgot to carry a zero.

Actually it turns out the north bound carriageway of the road underneath the bridge has street view photos from Sep 21.

Make of these what you will. I don't understand them but that centre connection looks like a pin to me. Also the turnbuckles on the wires are nearly out of room on tension.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Obviously designed by an artist and not an engineer. That pin joint at the top would have been rejected in 15 seconds by any half competent stress analyst. The other “connections” look horrible also.

The gif is neat, but did anyone notice that degree of motion? The cable would hang and not flex and would 'snap back' when movement went the other direction. If the movement were an inch or two rather than what appears to be a couple of feet, hopefully someone would have run up a red flag. The wind speed was low (like 35mph), but the kicker would be the gustiness of it. Does anyone know? Maybe set up harmonics, or overload... like 35mph gusting to 70mph?

Alistair: you notice that the cable bracings are at a 'shallow angle' If they were at a steep angle, the tension load induced in the strand would be huge with any lateral loading on the and they would likely tear free from whatever they were attached to.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (Obviously designed by an artist and not an engineer. That pin joint at the top would have been rejected in 15 seconds by any half competent stress analyst. The other “connections” look horrible also.)

Does anyone know if an engineer was involved? Western Wood Structures is a fairly reputable supplier and I suspect the connections would have been designed by them or at least checked...

It appears the wind speeds were in km/hour, and not mph on my last post.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (That pin joint at the top would have been rejected in 15 seconds by any half competent stress analyst.)

Dunno... the top hinge is fairly tall for the span and the reaction at the crown pin in horizontal shear, if half arch is 20T, would be approx 40x0.5x0.5 about 10K plus whatever wind loads are applied (they could be big)... these are not a real big loads, so the connection may be adequate. Looking at the framing, 80K seems like a pretty big weight for the timbers, but I haven't tried to see what the real load is.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (dik)

Alistair: you notice that the cable bracings are at a 'shallow angle' If they were at a steep angle, the tension load induced in the strand would be huge with any lateral loading on the and they would likely tear free from whatever they were attached to.

Yes I did and there was no load fuse.

Thats why I think they were only envisioned to alter the natural frequency but only in one plane.

'load fuse'? Something that 'breaks' with overload?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

I also wonder if the top arch was really designed to sit or rest on the bottom one? Maybe it's all shrunk a little or bent a bit more and the two were supposed to be separate? Or the tension in the wires was not as it should have been and the top set were forced more onto the bottom set of beams?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

yes, the one I saw with a similar purpose, shear pin but then it had a secondary load path came into action which had a sprung damping element to it.

When I say saw, it was a FEA model. The attachment to the support was many times stronger than the tensioning wire. It was a controlled failure analysis if that makes sense. The dynamic engineer took it off me once I had created the input deck for nonlinear geom, it was as well a piece of "Art" and I don't have a clue if it was ever made.

It was a welder that apparently triggered a more in depth look at the dynamics of it.

Where are the engineering drawings with the PE stamp on the corner ?

No drawings ???.... Then where are the ARCHETECT Drawings with his stamp on the corner ???

Why is no one discussing responsibility here ?????

MJCronin
Sr. Process Engineer

Quote (MJCronin)

Why is no one discussing responsibility here ?????

1) We're more interested in why it happened - we're engineers, not lawyers.
2) Until you know why it happened, it's difficult to determine who is responsible. Was it an inherently flawed design? Was there an error in the shop and the 1-1/2" plate callout on the drawings was recopied onto the shops as 1/2"? Was the epoxy adhesive applied correctly? Each of those has a different person who is "responsible" for the error. The EOR is involved at each of those stages, but we're not going to burn him/her at the stake until we know that it was his/her mistake that was the root cause.

Glad there were no fatalities...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

pham... you forgot to include, "so we don't make the same mistake, ourselves!"

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Well the first thing everyone did was promptly pass on responsibility to the people making it....

I strongly suspect Western Wood Services provided the design, manufacture and installation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

One way that it would help to figure out WHY it happened is to have the drawings for the project. If there were any changes between start and finish, those could be informative, also.

I expect those should come out in the near future. Unless, of course, there's a reason not to "share".



spsalso

Quote:

I strongly suspect Western Wood Services provided the design, manufacture and installation.

I think the articles say that the sub did the installation, with some support from Western Wood

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Can someone provide details of the fix last year?

The link is blocked to the eu

My VPN allows me access to many countries sometimes...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Look the least bit familiar?:


https://res.cloudinary.com/engineering-com/image/upload/v1645563680/tips/Screen_Shot_2022-02-22_at_12.54.55_PM_v3xa2k.tiff


Skyline Ranch Road, Bend OR, by Western Wood Products


spsalso

If you look at Western Wood Structures portfolio ( https://www.westernwoodstructures.com ) as well as some of their YouTube videos, they use this type of pin connection quite a bit, through this arch uses the 'lightest' example of it that I have seen. I suspect it is used in dealing with the expansion/contraction of their structures.

None the less, Western Wood Structures is pretty clearly not a run-of-the-mill 'design' firm as they have some simply phenomenal structures across the world (some of which I've seen in person) - which makes this arch creation (and failure) all the more baffling.

Quote:

Look the least bit familiar?:

That bridge's arches are structural and required to keep the walkway in place

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https://hickoryrecord.com/news/local/watch-now-see...

Good find. At 7:03 in the video, it certainly looks like the structure folded over on the lower hinge points.

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Hickory Record story concerning the work stoppage prior to raising the second arch.

https://hickoryrecord.com/news/local/govt-and-poli...

The lower UNINTENDED hinge points, that is. You can't have a stable arch with more than 3 hinges.

It is hard for me to tell with the grainy security camera footage (seems to be a staple here at eng-tips...), but it sure looks to me like the tip started to fail first and then overloaded the lower fixed "hinge" points causing them to fail and twist as it rolled over.

I cannot imagine it would take much horizontal load on that single pin joint at the top to cause it to tear out as it appears in gusmurr's earlier post.

Maybe the lower part is intended to be fixed?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Those cables definitely look decorative, with the way they're attached and the angle. I guess they were just tensioned enough to look good. So what was supposed to be stopping it from blowing over?

As soon as that failed and it started going, it would have put the windward side cable under massive tension and it would have pinged, which looks like what happened in the pics where you can see the broken cable. But that's a symptom not a cause imo. Looks like those crossover joints half way up just weren't up to the job.

EDIT: posted this before seeing the vid. It looks like this halfway up hinge failed on one side, that side then dragged the other half down and that would have pulled the cable as well.

Quote (dik (Structural) 22 Feb 22 16:20)

The gif is neat, but did anyone notice that degree of motion?

Thanks for the neat comment but I think you missed the point. The gifs only purpose was to convey the idea of the freedom of movement, full stop. What might be obvious (and may not) is that once any movement starts, other factors quickly come into play and complicate the collapse sequence. I for one am not ready to meld half a dozen or more factors into one animation, let alone fine tune it to inches or less. The gif was neat but it's also wrong (I fear) and not because it moves south instead of north or moves to far. Much to my dismay, I finally took a closer look at the math of the guy wire and realized I was out to lunch, or should have just had lunch instead. In order for that gif motion to occur, something else had to break first (the guy wire or another connection).

Quote (TugboatEng (Marine/Ocean) 22 Feb 22 07:27)

... perforated metal strips must have been the repair as they're not visible in other images ...

Thanks for posting, I was looking for images relating to the repair or where the crew may have been working at the time but had not found any. That flimsy repair may be the key. A closer analysis of the videos just posted may reveal that leg buckling first.

I believe it's the connector of the lower arch on the east end of the structure. In the closeup, the timber to the right of the connector is of the upper arch. Only a fragment of the lower wishbone remains. That southeast leg fell off and under the bridge.

Quote (Red Corona)

Looks like those crossover joints half way up just weren't up to the job.

Seems like it. They needed to be rigid to prevent 5 hinged arch formation, and also to prevent it falling over laterally like it ended up doing.

Quote (The gif is neat, but did anyone notice that degree of motion?)

The gif may have obscurred or detracted from the failure. Movement would be nowhere near that magnitude, and people seeing the clip might be expecting that type of failure.

Failure did not appear to be from a cyclical loading. From the clip posted by Hokie, it appears that there was no movement due to wind and that the one cable may have failed, allowing the top of the arch to fall to the right, bringing down the 'fixed' base parts. All is speculation; it's a matter of 'those in the know' posting the real cause of the collapse.

It would be nice to see the failure parts to see how the components fared... I suspect strongly that the one cable 'snapped' for whatever reason. Cable failure may have precipitated the collapse, or some component/connection failed causing the overload to the cable. I suspect that the one cable is 'broken', but I don't know for sure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (dik)

Movement would be nowhere near that magnitude, and people seeing the clip might be expecting that type of failure.

The failure looked pretty much like that from the first video. Isn't that all the gif was saying likely happened? That the top fell over? I didn't think he was saying it looked like that in service. Merely that it looked like that when falling over.


The wind was blowing in the video. Flags near horizontal. So likely it blew over in the end.

Direct Links to videos:

https://whky.com/wp-content/uploads/2022/02/2022-02-18-13-06-39-2.mov
https://whky.com/wp-content/uploads/2022/02/FPS-arch-video.mov
https://whky.com/video-footage-of-collapse-of-arches-shared-by-city-of-hickory/ (click on the video icon near the bottom of the story for a third video)

I just discovered that the location of the elevated hinges may have contributed to the collapse. Take a gander at the sketch. By elevating the hinges, the extension to the cable can be minimised so it offers little lateral resistance. That may be the cause.



The blue is the trajectory of the cable and the red is the trajectory of the crown of the arch from the elevated 'base'. The white is the trajectory of the crown of the arch if the rotation were about a non-elevated base. The difference in the two schemes is the location of the elevated base.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

The top fell off.

yes, the red thing and the red trajectory... nearly matches the blue cable trajectory. The white shows the extension to the cable required if the base of the arch were at the bottom, and not elevated to the point at the intersection of the green and red vertical line. The cable may not even be broken. I'm just trying to show that the cables may have been totally ineffective, or seriously compromised. the white lines starting from the base of the cable are 'trim' lines only for cutting the circle circumference.

I just realised that in the Right Sketch, the cable is in tension. In the Left Sketch the cable is mostly partially in compression and the first rule of engineering is that you can't push on a rope... maybe we've solved the manner of collapse.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (dik (Structural) 23 Feb 22 01:38)

... nearly matches the blue cable trajectory.

Yes, that is what I was trying to say earlier, however from a pure technical standpoint, no matter how less effective the guy wire is, something still has to bend or break before the motion can proceed. But the idea that the guy wire might become irrelevant is of concern. As I've since looked at the videos closer, I do see the structure start to lean over at the top before anything else starts to fail, at least anything that can be reconciled in the videos.

That's correct, but the cable if it's in compression will not restrain the structure to prevent it from breaking somewhere else. Depending where the elevated hinge is, the cables may be totally useless and people may have been relying on them for lateral support.

Quote (I do see the structure start to lean over at the top before anything else starts to fail)

Cable in compression will allow that to happen. I got the idea from your clips, because it appeared they started to rotate with little restraint.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Dik, the elevated pivot point certainly doesn't help, however the red curve and white curve agree until about 10 degrees rotation, which is well past the point of collapse. I.e. it was unstable anyway.

Agreed, Tom, but the cables may be nearly ineffective, and would be less effective if there was any horizontal movement of the base to the left.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Improved sketch... moving the elevated base higher... more compression zone for the cable.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Dik, please review my first and second posts in this thread (Sym P. le (Mechanical) 21 Feb 22 00:06, and 21 Feb 22 02:07). I've been saying this since this thread started. I've just been trying to suss out the reasonableness of such an assertion in light of the many other factors that could be at play. The mount blocks for the clevis connections at the top of the arch, for instance, are either bolted in place or field welded. They appear not to have suffered any damage as they detached from the structure (see previously posted images). That is odd.

What's all this about cables in compression? Cables only work in tension. The cables did nothing to restrain the unintended hinges at the top of the wishbones.

Interestingly enough, this artist render from 2017-2018 had a lot more stays...

The stays appear to hold up the walkway in that sketch. The concept must have changed over time.

Quote (What's all this about cables in compression? Cables only work in tension.)

Oh sh*t... I should have realised that.

If you look at the sketches, the white curve shows the approx cable location if the arches were pinned at the base... the red curve shows the cable location with the arch rotating about an elevated point... the effect is that the bracing is severely compromised. Earlier in my comments, I noted:

...and the first rule of engineering is that you can't push on a rope.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

As I think about this more, if the wishbone connection swivels, the structure is indeterminate. The wishbone pivots on its base, the guy wire pivots off the crosshead, and the upper arch just takes up the space in between (the top of the wishbone and the top of the guy wire). It's a miracle this thing didn't fall over sooner. Q.E.D.

To simplify the analysis, did they possibly introduce a pin at the upper intersection so they could be handled with simple statics?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (the structure is indeterminate.)

Yup, but because of symmetry you should have a fairly close static analysis. They may have chosen pins to do this. Moment connections with wood are a real b*tch... so maybe to eliminate these, too. Just WAGs until there is more information.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

With lateral load, I believe one cable is in tension and the other slack. For the tension cable to resist the lateral load it would have to impart 5 times that lateral load in vertical load on the structure. Maybe the cables overloaded the structure vertically trying to resist the wind. Please correct me if I am wrong.

You're correct about the vertical component being 5x approx the horizontal component, to a point. You can see from the three sketches that because of the elevated location of the base that the tensile force may be a bit unstable. As the base is elevated the amount of leaway for tension diminishes to the point that the cables no longer provide any bracing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

A 3 hinged arch is statically determinate. Thus the hinge at the top. But if you introduce additional hinges, that does not make it indeterminate, it makes it unstable.

There is loads of wood outside the hinge. Wouldn't take much movement to put the CoG outside the hinge point. Then you have rapidly all the weight on one cable.

I've increased the height of the pinned connection in Sketch D. As this point move upwards the amount of tension required is diminished due to the reduced difference in strain and the amount of rotation is also diminished before the cable ceases to be a tension member. This type of instability may have precipitated the failure.

We still don't have enough information to be certain of the cause; this is just another scenario. Elevating the base of the arch diminishes the effect of the cable bracing. Any movement of the base locaton to the left further diminishes the effect of the cable bracing.



Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

I'm not sure what happens, but it is likely that if one cable is in tension, the other is taking no load. I think it will reach a point where both cables are unloaded... I use the term compression for humour... See the first rule... I used to joke that you could; you just diminished the effective pre-stress...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

from what I can see its two fish symbols in an arch.

And you could get the ruff measurements off google maps.

Quote (A 3 hinged arch is statically determinate.)

In 2D space for sure, but maybe not in 3D space unless you can use symmetry, I think (I'm not 100% sure); I think it's reasonably close to use for analysis. BART knows these things. The bottom connections have to be 'rigid' else you have a mechanism (without plastic design).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

In following the artists impression I think they overreached themselves in trying to create a wooden structure in which two curved pieces of wood are shown crossing each other in the same plane without any apparent fixings. Easy to draw - Hell to try and make.

Hence we ended up with a visually pretty pure representation of the sketch, but now with seriously lower bending strength and essentially another hinge.

Doubt anyone will know why it just decided to fall over at that particular moment - maybe a gust of wind, maybe a wire did snap, maybe the hidden hinge between the two sections halfway along decided to bend.

Don't think those videos are going to give us those answers, but it sure looks to me like it bent at that connection from the wishbone to the strut.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Connections would be a nightmare... Catch the following, Precious Blood Church in Winnipeg





I understand the shop drawings were a b*tch and they ended up just getting a bunch of glulams and cut them to suit on site.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

video 1 of 3 attached: 2022-02-18-13-06-39-2.mov

• https://files.engineering.com/getfile.aspx?folder=c8b19cfe-f29e-4249-897b-f

video 2 of 3 attached: FPS-arch-video.mov

• https://files.engineering.com/getfile.aspx?folder=ffd964b9-cee0-4216-bc4f-d

video 3 of 3 attached: 2022-02-189508-39-13-806951-2.mp4

• https://files.engineering.com/getfile.aspx?folder=a1f82530-8f59-4eb7-8b01-c

Aren't they the same ones you linked above?

Western Wood Structures doesn't say anything about this collapse on their website. I guess they're still "thinking". And I guess their thinking is kind of slooowwww.

However:

"A dedicated engineering department sets Western Wood Structures apart from the rest."

Does that mean apart from the others that DIDN'T fall down?


And the mayor says it's extremely unlikely they'll rebuild the arches. The mayor SHOULD have said "We're going to rebuild those glorious arches bigger and better and STRONGER than before. And our good friends at Western Wood Structures, and their insurance company, will be covering most of the costs!"

As is, he looks like a loser mayor of a loser town.


I'm going to mention that the more I look at the "before" photos, the more I like this non-structure. I would have liked to walk around/under it. It MIGHT have been a pretty neat piece of work. My biggest aesthetic problem is that there are three bridges here, all together. And the fact that the other two bridges are FLAT takes away from the visual interest. Can't exactly move the other bridges, but it would have looked better with this thing all by itself.


I think what they should have done with all this extra money that they had to spend is to put some nice shady sitting spots on the perfectly flat bridge, so as to allow folks to have a picnic while waiting for a train to watch pass by. THAT is a pleasant way to pass the time!!


spsalso

We don't know the cause, yet, and Western Wood Structures is very likely looking at this now. Their site is for advertising and this collapse is not good advertising. They may not be the cause; it's a matter of waiting to find out more particulars. It's not good to speculate on who's at fault... who's on first! It's also not the mayor's fault... We have to wait a bit.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Nevertheless, a company in that position should at least post some sort of throwaway release ala,
"Western Wood is dedicated to identifying the root cause of the failure of our structure installed by SOMEONE ELSE, and will work diligently with the city to show that it was not our fault, etc. "

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Western Wood Structure's website is certainly for advertising. But it is also the face of the company, to anyone who goes there. It's what you SEE when you choose to visit.

I suspect we all agree that the design of the structure was by Western Wood, with perhaps some artistic sketches contributed by a Hickory NC local. But they were the folks who ran the numbers.

Yes. It would be interesting to see the installation instructions. If any.

spsalso

That picture of the turnbuckle several posts back keeps sticking. WHY was so much takeup used? My guess is that it was because the top of the structure was brought downwards by the tensioning, which likely would have brought some bending to any joints in the structure.


spsalso

I agree... my oversight. They should acknowledge the problem, be greatful that no one was injured and stipulate that they are currently investigating what happened and provide information when it becomes available and vet it through their legal department before publishing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (Spsalso)

That picture of the turnbuckle several posts back keeps sticking. WHY was so much takeup used?

To tighten it up. But with a 5 hinged arch it would only tighten so much. And like you say, the motion would have stressed the joints. This is likely related to the joint damage that was repaired earlier on.

The moment of collapse live, from a rather distant camera.

https://www.youtube.com/watch?v=c_D6RfCLZA0

That video is one of Sym's links. It's the cleanest of all the videos I've seen, and it certainly looks like the arch bent over at the lower hinges

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Quote (it certainly looks like the arch bent over at the lower hinges)

that's what prompted me to do the sketches...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Quote (spsalso)

WHY was so much takeup used?

Quote (Tomfh)

To tighten it up. But with a 5 hinged arch it would only tighten so much.

Well, it wasn't actually "loose", so there was nothing to tighten up.

I wasn't actually asking why some person chose to do this. I was asking how the structure allowed so much movement.

The cables pulled downwards with a pretty significant force. This then put a compressive load onto the structure. And rather than being decently rigid, and refusing to compress much, it most likely did a whole lot of bending/bowing. And, yes, twisting of any joints in a way that was likely not contemplated by the designer.

I wonder if the thing would have been still standing if the cables were not used.


spsalso

Quote (Spsalso)

I was asking how the structure allowed so much movement.

It’s a 5 hinged arch.

So there are 5 pinned connections, all allowing rotation about the pin?

spsalso

The 5 “pins” would have provided some rotational resistance, but not enough to keep it standing.

It's all dependent on how secure the bottom supports are. If they can provide a rigid base, then it's a three pinner... if not, it's a 'turkeyshoot'. the movement appeared to be limited to the top three pins, but my failing eyes cannot tell for sure.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

It was only a 5 pin arch for forces parallel to the axis. For lateral force (wind), it was unstable above the wishbones once the rotational resistance of the connections was overcome. The cables may have been provided in recognition of that instability, but did not do the job.

Concur... even the cables may have had a 'snap through' point where they became useless... and see how quickly it becomes unstable with movement:



Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

The turnbuckles used a huge amount of takeup because the structure provided too little resistance. There were no loose joints that needed slop removed. The damn thing bent. This doesn't HAVE to be bad--lots of things bend by design.

I don't think this one bent by design. If it DID, I surely look forward to the explanation.

spsalso

No doubt those knuckles were imagined as continuous parts of the arch, and yet ended up bending a lot. Bending down due to the tensioning of the rods, and then bending the other way when it fell over.

Quote (I don't think this one bent by design.)

Bending makes it less stable, and once it reaches the 'snap through' point, the cables become useless, no matter how well they are anchored.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

The latest news from Hickory, NC regarding arches:

https://hickoryrecord.com/news/local/govt-and-poli...

The mayor says it was a "catastrophic event".

As I watch various news sources and video feeds today, I do think the mayor just might be overstating his case.

Maybe they need a new mayor, who has a sense of proportion. Or just sense.

Meanwhile, Western Wood Products still has nothing to say about this on their website. Perhaps they feel there's no reason to rush.


spsalso

small town mayor,

WWP is sensible say nothing until the facts are known. Even if they know already its not their fault.

I’m sure they’re busy working out all the ways it’s not.

It was a potentially catastrophic event for sure - if anyone was actually crossing the bridges or if a train had been running and it fell the other way.

Difficult to see how a structure which is less than a year old and fell down in nothing more than a stiff breeze isn't a design issue. They were supervising during construction so can't back away from that responsibility.

The joint between the A frames / wishbones and the rest of the arch would seem to be the key area of interest and likely failure location.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

"potentially catastrophic"

Yes. But it wasn't. It fell down, went boom.


spslso

Perhaps we need a special forum for word games, but my Oxford dictionary defines "catastrophe" as:  Sudden or widespread or noteworthy disaster;  event subverting system of things;  disastrous end, ruin.  It then elsewhere defines "disaster" as:  Sudden or great misfortune;  calamity;  complete failure.  [My emphasis]

So, definitely catastrophic.  And potentially fatal.

Complete and sudden failure is what "catastrophic" means in engineering yeah. The forum isn't called catastrophes, anyway, and this is definitely an "engineering failure". Can I have membership of the word game forum?

It surely was a "complete failure". Beyond the thing falling down and needing to be cleaned up eventually, it appears most of the catastrophe was the damaged bridge railings.

I wonder what the mayor would have called it if several someones were killed in the process, or one or two bridges taken out. Would "mega" or "super jumbo" have been appended?


spsalso

We're nitpicking someone's words for amusement, but if my house were to burn down it would be catastrophic to me, irrespective of any damage to anyone else's property.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Re: your house burning down.

If the town burned down, that would be a catastrophic event.

This seems more like having the decorative fountain in your front yard fall over and break, after you paid good money for it.

However. I have noted that I am unimpressed with the mayor's choice of terms. Others may not have that reaction.


spsalso

Quote (If the town burned down, that would be a catastrophic event.)

Depending on the town, it might not be.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Hickory Council Comments (wbtv.com)

Quote:

In a city council meeting, the City of Hickory says they will not be conducting an independent investigation into the collapse of two arches over the City Walk pedestrian bridge over Highway 27 on Feb. 18.

Officials say the city never accepted ownership of the arches. Officials say the project was technically incomplete until the DOT signs off and the city formally takes ownership.
....
The North Carolina Department of Transportation owns the bridge, according to Hickory City Council. Therefore, the city does not have the authority to remove the arches, so other parties will have to remove them.
....
The city says multiple groups are investigating and developing reports on why this happened. The city is not one of those groups investigating. Officials say the insurance company advised the city not to get involved in the investigation.

"The city’s focus is recouping the investment that taxpayers made. This now involves multiple contractors, multiple insurance companies, and multiple lawyers."

Oh boy - That will take years and have a value several times more than the Bridge.

I was once involved in a project which had 5 sets of parties involved between sub contractors, main contractors, operator, designer and owner - all different, each with their own set of insurance companies and lawyers.

As no one could agree on how much compensation to pay for the the damaged tank (in my example) they decided to repair it. Repair costs were easily 5 times what the tank was possibly "worth" and due to its location could not be feasibly demolished and re-built or a new one substituted. Was a great experience for me, but utter madness in terms of value for money.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

The arches are to be cleaned up and removed this weekend per the city...

https://www.hickorync.gov/nc-127-road-closure-removal-city-walk-arches

This gentleman has some excellent photos that he took while visually documenting various stages of the arches construction.

They are some of the clearest images of the design choices in question that I have come across, including the (possibly infamous) 'pin' joint.

I do not want to directly link his images or upload them here (without permission), but the album in its entirety can be found and viewed on his flickr page:

https://www.flickr.com/photos/raddad/albums/72157719596386082/with/51332009377

The joint that is key to this is actually the one at the top of the wishbone I think.

That's where artistic impression of continuous strands of wood gave way to engineering reality and where the failure almost certainly came from

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I think so, too... it's interesting that the connection at the very top can likely take the forces; they should'n be huge... and it looks so flimsy, almost a little out of proportion. From a load point of view, it likely works.

I thought it was attractive; now that they know what not to do, can they rebuild it?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Mighty interesting photos--wish there were more.

The extreme closeup (on the ground) seems to be one of the two pieces of the top connection. I note that the cable anchor is conspicuously missing, and assume it was added after the arch was assembled, and likely cut to length. It kind of messes up the "purity" of the single bolt connection.

In the photo shot directly from one end, you can look up towards the near "outer" connection and see three steel plates assembled together. What is weird is that they're not flat and flush with each other--"bent" would be the word I would use. I could wonder if the person who designed this structure spent much time on figuring out how it would be assembled.


spsalso

Quote (spsalso (Electrical) 5 Mar 22 20:46)

three steel plates assembled together

You are right. As I see it, each laminated timber member is fitted with a weldment which slots into the members end and extends proud of the member to allow for bolted connections to adjacent members. These individual members are then connected to each other with an additional weldment configured to accept the various members. At the connection to which you refer, you see two end plates and one junction weldment between them.

At the peak junction, you will find an end plate on each member and each of which is bolted to a mount plate. The mount plates then connect with a single bolt.

The following image shows just how minimally, the timbers are connected at the wishbone:

I would suggest this might not have collapsed had Hickory used hickory. (Well, that and some better way of connecting the joints.)

By the way, were the bent members fully stable, including through temperature variation? i.e., did they have any tendency to unbend?


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Quote (LittleInch)

The joint that is key to this is actually the one at the top of the wishbone I think.

Quite right. I was horrified by the concept as soon as I saw the detailed pictures, and certainly didn't need to do any calcs to back up that impression.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

A model, built with pipe-cleaners even, would have identified the weakness of those joints to lateral flexing. There are even lateral triangles in the design, but rendered immediately useless by balancing them on their tips. It is almost anti-engineering.

STEAM !!

The problem with sloppy work is that the supply FAR EXCEEDS the demand

The test model that was apparently never built should have been done to a scale of 1:87.1. At 178 fee long, it would be a nice 24" long!

That way, after proving the design (should that have actually happened), it could have been given to the city of Hickory on a platform with the adjoining bridges, and an HO scale train on the railroad track.

That way, it could even have been incorporated into the upcoming visiting (proposed) modular model railroad coming to town on April 1 and 2.

Would'a looked pretty cool if it had visually recreated the boo-boo, also. And saved some money and embarrassment.


A missed opportunity all around, I'd say!


spsalso

Well, somebody got a good supply of firewood out of it all.
(We'll overlook the toxic preservatives it contains.)

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Not if it's pressure treated, or unless they have a suitable wood burner.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Just thought I'd post that it appears the local bulldogs of public interest have not been able to find any more information on this topic. Well, that they wanted to share with the world.

A whole month. Maybe they had the vapors.


spsalso

$750,000 project to Neill Grading, Dane Construction and Western Wood Structures.

They are going to have another go?

They're having a go at suing them apparently....

https://hickoryrecord.com/news/local/hickory-sues-...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I actually quite like the look of it. If they can get it to stay up.

No chance they will do that again. Maybe a nice steel sculpture?

Steel sculpture? Maybe they can hire the guy who made this for Council Bluffs, IA:

The name of the town is Hickory - with a name like that you can't replace wood with steel.

I think it would be nice if the wood arch was replaced with another similar structure that was designed and assembled properly.

I kinda liked it, before it fell down.



spsalso

The Elders of this Hick(ory) town Pined Fir a way to Spruce up a Trunk Route. They twigged on to the warped idea of a Red-wood arch. When the arch felled itself (tim-berrr!) they could not beleaf it. They could knot leaf the lumber there, so to get the town out of the woods it was replaced by a material with more structural integritree.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

It was good as these things go visually. It had different features depending on the angle.

Per say I think it could be engineered to work. Might not be as clean on the joint points but 95% of it would be the same.

But now the political legal wheels are turning it won't happen.

I even liked the Florida pedestrian bridge...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

It wouldn't take that much to get I viable either. I agree it did look good.

Taste is individual. I found both of those structures to be dishonest structurally, so not my cup of tea.

I liked the wood one because it was the fish rune and changed with the aspect that you looked at it.

I suspect that it was a short cut by the building contractors that killed it.

I saw what you did there Alistair.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

I think what it needed was for one of the spars to be complete and unchanged and then the other two timbers bolted to it giving the impression of a wooden single cross piece. Doing it as four pieces all connected together in one location was the error there IMHO. That is after the person who drew it without giving it a thought as to how it was going to be made....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

That doesn't address the fact that there is an "axis of failure" running through the middle of it.

Quote (Sym P. le)

That doesn't address the fact that there is an "axis of failure" running through the middle of it.

Given the increasing frequency of invasive species, perhaps it was done in by an 'Axis of Weevil'?

Sorry.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Weevils wobble but they don't fall down.

!!!

Update on the situation...

One of the engineering firms brought on to investigate the collapse (by the legal defense team representing the contractor that assembled the structure) has released a preliminary report of their investigation into the failure.

Greensboro News & Record has published an article here on the topic:
https://greensboro.com/news/state-and-regional/why-did-the-hickory-arches-fall-report-cites-faulty-design/article_ca281b3b-9cd6-5978-a6e6-3b8668592606.html

And included the report in PDF form available here:
https://bloximages.newyork1.vip.townnews.com/greensboro.com/content/tncms/assets/v3/editorial/4/f3/4f3c3a1d-152e-5e27-9bc4-3390ac47eeb7/62a51df1b05e4.pdf.pdf

The report is an interesting read and is harmonious with a lot of the structural and design observations made within this discussion thread.

There's something else at play here... I would not normally expect this to be a cause...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

According to page 8, the contract/design called for 35PSF while Western Wood Products only designed for a '32.16' PSF load. I'm not sure it specifics anywhere what the practical cable strength is/was, that I can see anyway. Edit: I kind of doubt a beefier cable would have saved this design regardless...

disappointed in the report...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Ditto but might just be my cynicism. Report attached below.

So the lower arch cable attachment is suspect. The cable attachment bracket was attached asymmetrically with one bolt in the center of the lower arch. What?!?! The cable itself became frayed but stayed together while the welded bracket failed (not much weld visible either). There is no information regarding wind events that this structure experienced during its brief two month lifespan though a dump of local significant events for 15 years prior is provided. Did this cable fray solely in this event or was the damage initiated in prior events?

Edit: Was the bracket attached/welded only on one side ???? Answer: Not likely

Edit 2: I see now how the second bolt hole was utilized. The north side of the timber bracket is bent underneath the debris and would have been provisioned with a second attachment point. My comment on the welding is still in play. It seems like there is ample linear space for a robust weld connection so it brings into question the quality of the weld. Also with regard to the bolt connection, why the slots and did the bolts work themselves loose?


Photograph 14


Photograph 19


Photograph 20

• https://files.engineering.com/getfile.aspx?folder=ee64d2fb-142c-4604-97e6-f

Obviously it shouldn't matter, but keep in mind this particular report was specifically initiated to hold harmless the Neill Grading and Construction Company. I believe there is at least one if not two other groups doing an investigation as well for the other parties involved.

Regarding the welds, I'm kinda surprised the confidence level is high enough to claim they failed while in the air vs the damage in question being the result of the twist and then impact. The area(s) in question clearly had a rough landing.

Quote (gusmurr (Structural) (OP) 22 Feb 22 02:23)

Wow! If this "connection" isn't the cause of the collapse, I will eat my hat.

You're safe. Sorta. They made a worser one.

Quote (Electronbelt (Industrial) 19 Jun 22 19:49)

...claim they failed while in the air vs the damage in question being the result of the twist and then impact.

Key point.

Edit: The snap back would argue for the damage occurring while the structure was still erect.

Quote (According to page 8, the contract/design called for 35PSF while Western Wood Products only designed for a '32.16' PSF load. I'm not sure it specifics anywhere what the practical cable strength is/was, that I can see anyway. Edit: I kind of doubt a beefier cable would have saved this design regardless...)

Out of curiosity, what was the load on it at failure?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Quote (Sym P. le (Mechanical) 19 Jun 22 19:55)

Edit: The snap back would argue for the damage occurring while the structure was still erect.

Very good point if that final position is indeed from a snap back.

Quote (dik (Structural) 19 Jun 22 20:24)

Out of curiosity, what was the load on it at failure?

I do not see that indicated anywhere in the report. Just that the wind speed around the time of the incident was 18mph with 33mph gusts. The report also states that prior to this the structure: "...had not had high wind loads on it prior to the collapse." See page 6.

A re-occurring theme of the report is that the EOR only modeled a single arch (the upper) for the design/load calculations, and not the two arches tied together as a single structure. It also claims the EOR modeled the guy wires as solid rods in place of cables for the load calculations, possibly influencing the numbers by introducing support in compression. See page 9.

Quote (A re-occurring theme of the report is that the EOR only modeled a single arch (the upper) for the design/load calculations, and not the two arches tied together as a single structure. It also claims the EOR modeled the guy wires as solid rods in place of cables for the load calculations, possibly influencing the numbers by introducing support in compression.)

As noted earlier in this thread, a possible cause for collapse was due to the geometry. The effects of the cables became negligible, and the forces in them would increase substantially. If the project were mine, I'd likely have modelled it as an upper arch and looked at the effects this would have on the base, including the effects of the geometry. I don't find that to be at fault. I find the reliance on the FEM program to be distressing. For those wind speeds and gusts, I would think the failure wind pressures would be in the order of about 20 psf. The effects of gusts are local and not not on the entire structure at the same time. I didn't like the report. [Added] This precludes any dynamic effects of the gustiness.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Another source of additional tension on the lower arch cable may have been from the cable crossing the railing on the bridge though the railing may not have much strength to offer resistance and I have not checked the geometry of such an occurrence. IPSO, the structure was so flaky, it fell over. No further assistance required.





Also, a sharp arrow may have pierced the cable.

I think everyone who has commented here on this topic would have said:

"Well. That's a VERY unusual design. Since we don't have a lot of history with something like this, let's go real conservative on both design and testing."

I will note that it does not appear that anyone involved in this project said that.



spsalso

"Another source of additional tension on the lower arch cable may have been from the cable crossing the railing on the bridge..."

Looking at the first photo posted in this topic, it appears to me that, by the time the cable could have moved to intersect the bridge railing, it was all over.


A strange thing: I just did a search on Bing for "hickory north carolina bridge collapse" in images. There were none for this event. I wonder how that came to be. Google came through.


spsalso

which is to say that the cable failure was not a cause of the collapse but a result of collapse --> behind door number 2

Quote (Sym P. le (Mechanical) 19 Jun 22 19:27)

A revision to my previous post copied here.

Edit 2 - I see now how the second bolt hole (in the lower cable bracket connection) was utilized. The north side of the timber bracket is bent underneath the debris and would have been provisioned with a second attachment point. My comment on the welding is still in play. It seems like there is ample linear space for a robust weld connection so it brings into question the quality of the weld. Also with regard to the bolt connection, why the slots and did the bolts work themselves loose?

I find the comment that someone decided the cables were rods, and could then work in compression pretty hilarious. Just how much compression can you put into a 7/8" rod that is maybe 50' long before it fails and bends? Or bends and fails.


spsalso

It is difficult, perhaps impossible, to define exactly which elements failed first, and which were destroyed by the impact, etc. In addition to the crazy looking joints, another thing struck me. A lot of those 'anchor bolts' just pulled cleanly out. The concrete bits seem not to have suffered much, but the bolts popped right out. Straight bolts, no anchorage. Were they chemical anchors or expansion anchors?

Edited and corrected again as I misidentified the orientation of the second image.

Birdcaging of the wire rope at the socket may be a clue to the sudden release of tension or indicative of other issues? It appears more pronounced in the south rope which had significant tension failure. Of course when the structure fell, the north rope would go completely slack so perhaps this is evidence of the structure flapping in the wind prior to its collapse or cycling during its short life span.

The upper arch south north cable also exhibits significant birdcaging at the upper socket which would indicate that it was abused as well likely prior to the fall and in another image it is seen to be sharply kinked upwards at the socket so it likely sustained this damage from the fall.


Photograph 20


Photograph 17 - south cable at the bottom and north cable at the top. The north cable is kinked at the socket but it is not readily apparent in this view.

Quote (hokie66 (Structural) 20 Jun 22 04:44)

Were they chemical anchors or expansion anchors?

Threaded rod epoxied in place but I doubt I can call it accurately from a photo.

So far, all of the anchor bolts I've seen have pulled out of the drilled holes rather than pull up any concrete so it would seem that they did not develop their intended strength. This may be a moot point given the base was entirely more stable than the upper portion.


Photograph 9

Looks like a bunch of stacked shim plates under a relatively thin base plate, with a bit of grout to make it look pretty. Not a base detail to engender confidence.

They are so lucky that they didn't maim or kill anyone. I bet they are still doing their laundry, if they didn't just burn it.

The base plate installation is pretty interesting.

The photo 9 one has some shim plates surrounded by grout to support the base plate. The photo 11 one does not. It is extremely unlikely that the designer would spec two different styles of mounting, one with and one without.

So it appears that the mounting shown in photo 9 came up 3" short (photo 5), and filler was added. I suspect discovery of the need for this modification was made during the physical installation of the arch itself. It does not reek of prior planning.

Study of the photos shows that the hold-down bolts were not an additional 3" longer where the shimming was used. From photo 9, it appears the exposed hold-down bolt was engaged in about 3" of grout and 2 1/2" of concrete.

I have little experience in gluing bolts into concrete, but I do wonder whether the "glued-on stuff" (of about a 2 1/2" length) is concrete dust that was never blown out of the hole.

I question whether the grout is "Structural Grout". Which means that, structurally, it isn't there. So it would contribute nothing to pull out strength for the bolts. In addition, it would provide nothing for lateral movement. As I said, structurally, it isn't there.

I am sympathetic to doing an install, and finding the "bridge" is 3" shorter than the needed span. But this looks incredibly poorly designed. As an experienced electrical worker, with all the knowledge that that implies, I would have at least made the steel shims the full size of the base plate, and cut bolt holes through them. I would have added additional large steel blocks on all four edges to minimize lateral movement of the stack. And I certainly would have used longer bolts.


I am SO looking forward to finding out how this one "solution" turned out to be needed, and was so designed. I note that the Dara Thomas, the investigating forensic professional engineer, made no mention of the above in the report.


One additional point: for the base plate in photo 4, I question having two of the hold-down bolts located so far away from the location of the landing of the wood beam. Works OK for lateral, but pull-out???



spsalso

Kupla more:

I do see that the reason for this report was so that the PE could "...express my opinions, to-date, about the catastrophic failure...". So perhaps the thing about the design quality of the baseplates was not an opinion he wanted to express. Fair enough, I guess. I do suspect that the design of the baseplate is not the primary reason for failure, so perhaps it can be left out.

I AM interested, however, with what the design of the baseplate illustrates as to quality of design and installation.

It struck me, when I read that this installation cost $750,000, that that number seemed low (not that I have experience in judging). If it IS low, that could account for a certain style of approach to the task.


spsalso

Quote (That's a VERY unusual design. Since we don't have a lot of history with something like this, let's go real conservative on both design and testing.)

I do a lot of stuff that is unusual... I thrive on it. I take great care in understanding the system to start with, and try to keep up with materials. Although not common, there was nothing distinctly different from a normal structure. Something was likely overlooked and not taken care of. I suspect it was the geometry... but don't know for sure.

Quote (which is to say that the cable failure was not a cause of the collapse but a result of collapse)

The cable and geometry may have contributed. The effect of the cable may not have been as good as inticipated.

It would be neat to see actual detailed drawings of the arch and connections.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

The breaking strength of 7/8" steel cable is 31.1 tons.

The 1:5 vector ratio means that 6.2 tons of sideways force at the top anchor would break it:

12,400 pounds.

Subtract from that the "pre-tensioning" done by tightening those turnbuckles at the bottom. I wonder what that "pre-tensioning" figure was supposed to be and how it was achieved.


Once one cable is caused to snap, presumably by wind load, then the "pre-tensioning" load on the other side will add into the wind load that just caused the other side to snap.


Doesn't sound good!


spsalso

It is a bit alarming to me that nearly every picture containing steel exhibits welded joints that have pulled or cracked right at the root/HAZ.

If that rusty line on the inside edge of the loose twin socket bracket was where another piece of steel was butted prior to galvanizing, it does not not look like there was any fillet penetration whatsoever.

Edit: Not implying this is a trigger/cause. Perhaps a quiet indicator of a much larger issue...

I agree, welding is poor everywhere. As for the connector, it looks like it was continuously welded on one side only, possibly so that protruding weld did not interfere with the fit, lest costs were incurred for further prep or grinding. The counter argument is that when the structure collapsed, these connections were going to be destroyed one way or another.


Photograph 21 and 14

I realize now that I've just presumed the frayed cable to be the south stay. I can't find any evidence to confirm that. (i.e. photos to trace the cable to its origin)

I really wasn't impressed with that "forensic report". If that really was the extent of the work done then it really isn't going to do it for me.

The thing that was interesting to me what that she didn't really look into that joint that failed and how it was modelled and designed.

The SAP model and the report don't say exactly how nodes 4 and 22 were modelled. With the relative stiffness and strength of the lower wishbone compared to the rather flexible section above it that joint design is where the failure probably lies IMHO.

I've said before that maybe a better design was to have one solid section going through that joint with two angled connections to the joint so in the SAP model 3,4,5 and 21,22,23 would be a single solid piece of timber and 28 to 4 and 29 to 4 would be bolted connections to the main solid spar.

To not even try to model this herself in SAP is a real budget job of a report IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

In the photos shown here:

https://m.mooresvilletribune.com/photos-arches-col...

you can see the south cable is not broken, and is stretched taught. Note the indent on the pedestrian bridge southern handrail (see shadow in photo) made by the cable. The other railing also has a dent, though lesser.

It appears it's the north side cable that is frayed. Which is a surprise to me, as mentioned/implied in a previous post. With the direction of the fall, there should have soon been no load on the north side cable.

Also, you can see most of the sideways movement of the structure happened above the pivot points called nodes 4 and 22.



spsalso

Quote (spsalso (Electrical) 21 Jun 22 16:02)

... you can see the south cable is not broken, and is stretched taught.

There are two cables on the north and two on the south. One each side for both the upper and lower arches. The arches are two separate structures, unconnected. The cable which remains tight is for the upper arch whereas the frayed cable is for the lower arch and runs a random path along the ground which is easily confused with its counterpart.

From a design aspect, the report touches on an interesting aspect, modelling for wind load. It appears that wind load was considered for each arch standing separately but without consideration for any proximity to adjacent structures, in this case a twin arch immediately adjacent.


Page 8

I wouldn't be the least bit surprised if the engineer who ran those calculations learns of the '35psf on the entire profile as if it's a solid wall' requirement from the forensic report. I would be even less surprised if they said they concluded the design was completely infeasible with that wind load within other parameters of the project, and so chose to reduce it based on engineering judgement. I get it...modeling it as a solid wall is a bit silly...but it also doesn't fit nicely into any of the categories for wind analysis that structural engineers typically use. So to reduce it without a more rigorous testing method (such as a scale model in a wind tunnel) isn't the best move. No doubt they felt the budget pressures...wind tunnel testing on a $750,000 structure? When pigs can fly...

Sym P. le,

Thank you for pointing out the four cables, as opposed to two. I do, however, wonder if the two arches WERE connected, after installation. If they were not, why wouldn't one, only, tip over? I supposed what could have happened is, when the lower arch tipped far enough, it started bearing on the north side cable for the upper arch. If it keeps going, it then also pulls the upper arch down, using the cable.

So, I believe you're saying one of the south cables snapped (the lower arch one). And the other south cable followed the structure down, as it tilted over.

There would then have been over 31 tons tension on the snapped cable, when it happened, to make the cable fail.

And the snapped cable is on the windward (south) side.


spsalso

Virtually impossible to say for the cable what is pre and post failure damage IMHO. The report is more than a little bit one sided and tries too hard to find a smoking gun.

Think this is a much smaller version of the FIU bridge. Very odd design, strange construction, no proper models or proven design methods. Probably very little independent review

Result, it fell down.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Quote (There would then have been over 31 tons tension on the snapped cable, when it happened, to make the cable fail.)

The jury is still out about the failure being precipitated by the failure of the cable... They likely don't have a clue about what caused it yet. The problem with the report that's out there is that it is poor and may not be correct. I hope the professional association is a little more on the ball... why lodge a complaint using such poor information?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Quote (spsalso (Electrical) 21 Jun 22 19:33)

I haven't seen any indication that the two arches are connected. With the lower arch nested under the upper arch, between its two stays, the collapse of either one would take the other with it.

One of the lower arch stays is badly frayed down to a few strands. It's as though it was taken near to its limit but just prior to failure the load was released. I think that lends to the argument that it was the victim of another failure. As I alluded to earlier it's strongly suspected that it's on the windward side but we need confirmation.

Quote (LittleInch (Petroleum) 21 Jun 22 19:46)

... smaller version of the FIU bridge.

I think from a structural design standpoint, this is far worse than FIU. This design didn't have a chance and the failure was sudden. There was no opportunity to protect the public or remedy the problems. FIU could have been salvaged had they taken appropriate precautions and undertaken to understand the as built structure (not what the final report said or the intended design).

I disagree... there were fatalities with the pedestrian bridge.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

If you're talking about the outcome, of course FIU was worse. It didn't have to be. With Hickory, it was just dumb luck that it wasn't worse.

Yes, the FIU bridge gave days of warning that it was in trouble. This thing seemed OK and then just dropped.

The failure of this arch structure has similarity to the FIU pedestrian bridge failure for the following:
1. Computer modeling appears to have been relied upon without having a complete analysis model.
2. The computer model appears to have been simplified for factors/scenarios that basic hand calculations would have called in to question the results presented by the computer analysis. Quite surprising, if the report by Dara Thomas is correct, that the design analysis only looked at the top arch and no moment control of the structure was employed during the analysis. Was the expectation the guy wires with very narrow included angles connected to relatively thin pinned joints at the arch apexes were going to provide all the necessary moment control?
3. The EOR has a stellar record and many accolades and has been an innovator in his field of design. Possibly, the EOR's reputation caused less scrutiny of the design analysis? Did the reputation of Western Wood Structures cause less scrutiny of the design?
4. During construction, failure modes were noted and remediation done but there may not have been a reassessment of the design analysis to account for the unexpected behavior of the partially completed structure.

This failure and the FIU pedestrian bridge failure are reminders to me to not get too casual or comfortable with my design expertise.

I read through the forensic report and I think the author found a significant modeling error but didn't really follow up on it. That is the cables were modeled as rods and not tension only cables. I am very familiar with STAAD and not SAP2000 and I am assuming they work in a similar fashion for compression elements. That is there is no buckling in the analysis and the compression rod could have taken quite a bit of load because in the model it would be a stiff element. This could be confirmed by looking at the compressive stress in the rod.

Assuming I am correct its not surpassing the structure was under designed for overturning since the rod would be like a kick stand on a bike.

So someone actually ASSUMED a 7/8" diameter rod that was approximately 50 feet long would accept any significant compressive load.

And who might this blindingly overqualified engineer be?



spsalso

...so, you can't push on a rope. Actually you can, you just reduce it's effective prestress!

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Quote (dik)

...so, you can't push on a rope

I’m learning so much on this thread!

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

I have not read through the forensic report, but I certainly have used rods to mimic cables in RISA models. A key thing is to make sure to set them to tension only, so that the analysis program will turn the member off if it sees any compression (and also re-run the model without that member). I can't speak to what the EOR did, but the idea that you would use a rod to model a cable isn't crazy.

I'd be surprised if that were the issue, but of course I'd have to see the model. In all 3-D analysis software I've used, even if it doesn't simulate buckling affects on the overall model, it'll still flag the rod as "failed" when it compares the stress to the calculated allowable - which in this case would be essentially zero.

Now if the user thought "well, of course it fails in compression!" and ignored it, then yes...that could be a big problem. That would be so many levels of big problems. None of them surprising, sadly, but still big problems...

Quote (I’m learning so much on this thread!)

Glad to help... that used to be (50 years back) one of the three laws of engineering.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

Quote (I have not read through the forensic report,)

I did... and, it's not well written. Don't waste your time...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

I was going back the forensic report page 9, second to last paragraph where they state the rods were not set for compression only.

I agree on the rod vs cable comment, I do they same thing but set them for tension only.

Cable, in tension, is slightly less stiff than solid rod in tension.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik