Ferry Dock Collapse 13

[dik]

"At least seven people were killed and several others injured Saturday after part of a ferry dock collapsed on Georgia’s Sapelo Island, according to the Georgia Department of Natural Resources.

It happened as crowds gathered on the island for a celebration of its tiny Gullah-Geechee community of Black slave descendants.

At least 20 people were plunged into the water when a gangway collapsed on the visitor ferry dock shortly before 4 p.m., Georgia DNR Capt. Chris Hodge said at a Saturday night news conference. A McIntosh County commissioner previously said a boat hit the dock but a DNR spokesperson later told The Associated Press there was no collision and it is unclear why the dock collapsed."

https://www.cnn.com/2024/10/19/us/sapelo-island-georgia-boat-dock-collapse/index.html

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

-Dik

 

[Sym P. le]

... I can't see how thi2 railing is 5 feet high ...

Scaling off the 4" rub rail or 6" bottom chord, I get a height in the order of five feet. Also, the worker holding the top rail is barely taller than the height of the gangway. I also see some camber in the structure as it sits on the barge though it's difficult to measure. Unfortunately, the additional foot in height over the 60' brochure design also makes the pony truss more slender.

 

[Shu Jiang PEng SE]

Top chord out-of-plane buckling could be the reason. vatical hanger and bottom chord do not provide enough lateral support stiffness to the top chord. What the unsupported length of the top chord is in you FEA simulation?

Here is a post on LinkedIn which may help understand why the collapse happens: Application of equations A-6-2a/A-6-2b of AISC360 in Pratt truss bridge design.

 

[Oops409]

The two images below are lifted from Crescent Aluminum Docks and Gangways Web Site, where they advertise these images as examples of their residential grade gangways and commercial grade gangways.

First is the image Crescent advertises as an example of Commercial grade.

The Second is the image Crescent advertises as Residential grade.

Based solely on these two images, it appears that Georgia DNR installed a residential type gangway, and not a commercial version designed for public use and loads? Note Commercial Grade example adds lateral bracing to sides of U-shaped truss.

Third image below from Crescent's web site, says all designs "have been analyzed and stamped by licensed engineers and proven by years in the field".....

We have no idea what design load specification Georgia Docks procured this gangway under?

Point of post is the clear difference pictured between Comnercial Rated vs Residential Rated on Crescent's Web Site. What failed looks more like the residential example than the commercial example.

 

[LittleInch]

Oops409

The images you post are not the gangway that collapsed

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

 

[Oops409]

LI,

Correct, as I stated in my post, the images were from Crescent's web site. I added more verbage to make it clear the images I posted are examples of residential vs commercial grade on Crescent's Web Site.

PS: human909's images posted above also were not of the collapsed gangway, rather were examples of better laterally restrained designs from Crescent's Facebook Page.

 

[FacEngrPE]

A local report with some additional shots of the post collapse condition.

Sapelo Gangway Damage Minorcan Mullet Oct 22 2024
Image from video above. Definitely a sideways kink in the handrail, close to the sharp bend in the walking surface.

 

[Oops409]

What caused the Dock Collapse on Sapelo Island. Witness relays story
12K views · 3 days agoSAPELO ISLAND
...more

Minorcan Mullet
44K

6 minute point he recants eye witness account of what they saw. Once gangway listed, lots of lateral senior citizen loads on truss.

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

 

[Sym P. le]

Scaling off the 4" rub rail or 6" bottom chord, I get a height in the order of five feet.

By the same method, the truss section interval is 48".

 

[Sym P. le]

From the Minorcan video, the mainland dock appears to be of the same vintage as the island dock, i.e. they were both renovated as part of the same project. A significant difference though is that the pony truss on the mainland side is a Pratt design whereas the failed structure was a Howe design. This distributes forces differently as portrayed in the following images (garrettsbridges.com). The Pratt design develops higher compressive loads in the top rail compared to the Howe design whereas the Howe design develops compressive loads in the slim diagonals. It's interesting that the Howe design is the one that failed, seemingly by buckling of the top rail. I wonder why the two different styles.

Edit: I see now that the Howe design develops higher tensile stress in the bottom chord, which brings weld failure back into play.

garrettsbridges.com/design/pratt-truss/

garrettsbridges.com/design/howe-truss/

Mainland ramp (Pratt design)

 

[Sym P. le]

From news article here

A resident of Southeast Georgia's Sapelo Island in McIntosh County said that he warned the Georgia Department of Natural Resources last summer that the agency's ferry dock gangway on the island was in poor condition, months before it collapsed Saturday, killing seven people.

JR Grovner told GPB that he notified a DNR captain three or four months ago that he thought the aluminum gangway's structural integrity was compromised.

“I said, ‘This dock is going to collapse;’ I sure did,” Grovner said. “I was walking on it, and it was bouncing. So, I stood in the middle of it and bounced up and down for the captain. And he said, ‘Ah, it ain't going nowhere.’ Yeah. Look what happened.”

The DNR, which operates the dock and the two ferries serving Sapelo Island, declined to comment on Grovner's allegation, which he also aired Sunday at a news conference held by the agency about the fatal incident.

In an interview with GPB on Saturday night shortly after the gangway collapsed at approximately 4:30 p.m., Grovner said, “The captain told me tonight, right after it happened, he said, ‘JR, you told me.’ He said, ‘You were right. You said it.’"

 

[Shu Jiang PEng SE]

Top chord out-of-plane stability is key for design this type of structure. Here is one example I prepared before. Different load capacities could be derived with different unsupported compression lengths (out-of-plane) specified in FEA.

The structure is not same, but design principle is. In this case, bottom chord and vertical hanger should provide lateral restraints to the top chord to prevent it losing out-of-plane stability.

—————————————————————
Shu Jiang, SE (California, Nevada, Oklahoma). PE(Michigan, South Dakota,Missouri,Oregon), PEng (Ontario)

 

[Oops409]

JACKSONVILLE, Fla. (AP) — Regina Brinson heard a crack before the metal walkway gave way beneath her feet, plunging her into the water beneath the state-operated ferry dock on Georgia's Sapelo Island.

The Georgia Department of Natural Resources, which operates the dock, is leading the state's investigation into why the aluminum gangway failed. The dock was built in 2021. Crump said he doesn't trust the state to investigate itself.

Crump and some family members Tuesday questioned whether the gangway may have collapsed under the weight of too many people. Officials have said it held about 40 people when the metal snapped.

In a statement to The Associated Press, the Georgia Department of Natural Resources said engineers calculated that the 80-foot (24-meter) gangway should have been able to support the weight of 320 people.

The dock was rebuilt in 2021 after Georgia officials settled a lawsuit by Hogg Hummock residents who complained the ferries and docks failed to meet federal accessibility standards for people with disabilities.

The disaster happened on a day when 700 people visited Sapelo Island for a fall festival celebrating its tiny Gullah-Geechee community of Black slave descendants.

40 people times 200 lbs average yields 8000 lbs.
320 people times 200 lbs average, yields 64,000 lbs.

One article I read states people were jam packed together to board the last ferry from Sapelo Island that day, which creates more of a concentrated point load, and perhaps torsional load if biased to one side. I also read article that stated at least one of the gangways was installed by Georgia DNR, and not Crescent.

I have seen term "re-built" used as well to changes made as a result of litigation.

Was 80' gangway originally designed for that location and application?

The interface to dock at hinge looks underdesigned for that span and 7' wide double width gangway.



64,000 lbs sounds like a lot for this aluminum gangway to support?

https://www.wral.com/story/grieving...ollapse-say-tragedy-was-preventable/21684224/

 

[MintJulep]

(80 ft x 5 ft)/320 people = 1.25 people/ft²

That's mosh pit density.

No way that spindly thing would hold that.

 

[Oops409]

New safety signs erected after gangway collapse on remaining gangway. Now no more than 8 individuals equally spaced or 4 individuals with 4 carts!

https://www.yahoo.com/news/sapelo-island-tragedy-safety-signs-030415820.html

 

[Sym P. le]

I also read article that stated at least one of the gangways was installed by Georgia DNR, and not Crescent.

Can you source that article?

Given the fundamental design differences, that would make sense. The typical Crescent designs are Pratt trusses so it's quite likely the failed structure is the state provided gangway. All the more reason to have outside investigators.

I don't know if it's just me but listening to the original press conference, the media handlers were condescending to the questioners and trying to tone down the difficult political questions. Fortunately they largely failed and the audience had their say.

 

[Oops409]

Symple, I will look. I did find this article and news video concerning history of gangway. It actually cites contract documents as source of information via Georgia DNR statements.

https://www.msn.com/en-us/weather/t...land-in-coastal-georgia-collapses/ar-AA1szgo2

Found it. It was NYT article.

https://www.nytimes.com/2024/10/20/us/georgia-dock-collapse-gullah-geechee.html

 

[Sym P. le]

Thanks. It doesn't look like an installation problem but I'm still curious as to the procurement and design anomaly.

 

[Oops409]

Symple, Great catch on Pratt vs Howe Truss, and that is very interesting variation in design from one local manufacture. Hard to understand why one is Howe and one is Pratt?

I found this specification from a Florida Aluminum Gangway Manufacture. NOTE DIFFERENT STATE AND DIFFERENT MFG. but specification is for aluminum gangway product.

PART 3 – ENGINEERING
3.1 Uniform Live Load Selection
 A. Gangways 4 feet wide and less shall be designed for a uniform live load of 50 pounds per square foot.
 B. Gangways over 4 feet wide shall be designed for a uniform live load of 85 pounds per square foot.
3.2 Maximum deflection of structure shall be calculated using L/180 where “L” is the length of the gangway
in inches.
3.3 Deck material shall be designed for a concentrated vertical load of 300 pounds distributed over a one
square foot area.
3.4 Handrails shall be designed for a horizontal load of 20 pounds per linear foot.

So this manufacture in Florida would use 85 lbs/SF live load for a 7' wide gangway and L/180 (L in inches). 20 lbs per linear foot lateral load on handrails. Yet point load limited to 300 lbs distributed over a 1 SF area.

85 lbs/SF * 7' * 80' = 47,600 lbs live load

L/180 Deflection Limit 80' x 12inches/LF / 180 = 5.33 inches

20 lb/LF x 80' = 1600 lbs total handrail lateral load


EDIT: Design Requirements Article on Sapelo Gangway. 8' width by 80' per quote.

“Paragraph 2.2, Design Criteria in Specification Section 053300 – Aluminum Marine Structures calls for the 80-foot gangways to be designed for a 100 pound per square foot live load. The total square footage of the 80-foot gangway is 640 square feet. Therefore, the gangway was specified to be designed, constructed, and installed to withstand an evenly distributed loading of 64,000 pounds. With these numbers and conditions as a prerequisite, assuming an average weight of individuals at 200 pounds, and an even distribution of mass, this would equate to 320 persons.”

I would like to see Crescent's Shop Drawings and City Approval on those.

https://www.wsbtv.com/news/local/sa...weight-320-people/XB3UXBUUSNGCNHYQCAKJGID3FQ/

 

[Oops409]

Could it be the fabrication shop mistakenly started welding diagonals in Howe configuration, even though the verticals were sized for compression and the diagonals sized for tension? I could see that this could have easily been done, and workers if they even realized that, may have thought, 'ah that won't matter' as you have same number of diagonals and uprights either way.......

This one key detail that Symple discovered, appears to be the manufacturing flaw. No way the skinny diagonals should be in compression with the bigger verticals in tension as they are in Howe Configuration.......

This would also explain why the gangway was so "bouncy" as pointed out by the frequent passenger to Captain.

Bouncy means faster fatigue of aluminum. 700 passengers that day, vs normal day of 100, so a lot more cycles that day.

So did everybody sign off on this reversal of the diagonals or was this detail even noticed by anybody till now???

Would like to see shop drawings and orientation of diagonals vs what we see as-built on failed truss vs not failed truss....

I can't believe they did NOT use the same design for both gangways....

I wish I could give more than one star to Symple on this one........

 

[human909]

Personally I'm surprised this is still being talked about as an engineering disaster. There are so many flaws are so many here that one questions whether anything involved in the design and supply of this gangway counts as engineering. As SocklessJ said "Remember this was likely "pre-engineered". In other words NOT engineered."

The mistake here was almost certainly NOT having an engineer involved.


(Sure there are incompetent engineers out there, but I think even an incompetent engineer would be able to achieve something better than this.)