Ferry Dock Collapse 9

[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]

The difference between the bridges in the photos and the failed gangway is the heft of the railing stock. The lightweight gangway railings do not even pretend to be structural (at the ends, the diagonal doesn't even align with a design node). Also, the gangway (servicing a ferry) is intended to regularly handle load surges rather than random traffic.

 

[3DDave]

The hinge is over the water. If it fails there isn't a return to 4 points of contact.

In most of the catalogs I looked at there were ratings of 60 pounds per square foot for residential units to 120-150 pounds per square foot for public use units.

 

[SWComposites]

the hand rails look way smaller than 4" diameter
and the bottom chords look more like 2x4" than 4x6"
the verticals look like 1.5 or 2" squares not 2x4"

 

[TugboatEng]

I notice that on both ends of the span, the last web does not tie in directly with the cord. Is this normal?

 

[Sym P. le]

Roughly, the span is 75 feet. Twenty sections of Howe truss (verticals in tension). The rail stock still looks inadequate. It only took 20 people to bring down this structure, only 1 per truss section. That's some bad math.

Tug, no, it looks like another eclectic computer wiz design though this design feature may not be as weak as the overreaching span. Edit: Chckle, It looks like the end section is a tad shorter than a typical section so they just truncated the diagonal???

 

[TugboatEng]

There is zero deformation of any visible sections of the structure which leads me to believe the structure failed at the very end far from what we can see.

 

[Sym P. le]

That would be consistent with the gangway sloping down to the dock during low tide. The image only shows six of twenty truss sections.

 

[bugbus]

Scaling off some (kinda) known dimensions, I agree with SWComposites, these are smaller than 4" tubes, they appear to be 3" instead.

The width is also around 7', not 8'.

 

[SWComposites]

It appears that the high tide flow tore off one of the hinges. It is not failed in the photos at low tide.

 

[bugbus]

Also, I initially thought the 73' span by Sym P. le seemed a bit long, but it seems to be closer to 80' even.

The height of the doorway should be 8' exactly, given there are 16 slats at 6" spacing as can be seen in other photos.

 

[Sym P. le]

So we're good at 75 to 80 feet? That works for me. I was thinking 20 truss sections at four feet each would equal 80 feet as an outside estimate. Also, the gangway has to be longer than the span for tidal height variations.

 

[bugbus]

My last post on this because I don't want to clog up the thread with my ramblings.

I've tweaked the model somewhat to reflect the more realistic geometry:

*80' span
*7' width
*5' height
*Top chord = 3" tube (generously assumed to be 0.25" thick, which seems to be the thickest available)
*Bottom chord = 6x3" (again 0.25" thickness)
*Cross-girders = assumed same as above
*Verticals = 4x2" (0.25" thickness)
*Diagonals = 2" tube (0.25" thickness)
*Loading includes self-weight of aluminium truss, plus 0.25 kPa (5 psf) decking, plus 40 average sized adults as per the news report (at 75kg / 165 lb each) concentrated along one side

The elastic buckling factor is 2.0, without any additional load factors considered, nor imperfections, nor people leaning on the handrails, etc. etc. etc. This thing seems to have been woefully under-designed.

 

[LittleInch]

Does it make a difference if the mass (humans) were concentrated at one end? Say in the last half or third?

Sounds like it was close to low tide so at quite a slope and people tend to queue quite closely.

Also I can't see how thi2 railing is 5 feet high. More like 4 foot / 1.2m when you look at the photo and the size of the man or even count the 6 inch planks.

There is a hand rail at 900 visible 3/4 way up the span.

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

 

[MintJulep]

This photo from Google maps seems to show the dock maker's logo.

I haven't been able to get it readable.

 

[MintJulep]

Crescent docks and gangways

https://m.facebook.com/crescentdocksandgangways/

 

[Sym P. le]

From Wayback Machine - http://crescentdock.com/]crescentdock.com[/url]

These screen grabs are from a promotional brochure (attached below). The failed structure differs somewhat in that the diagonals are inverted and the depth likely 48" or more. I suspect the landing truss section mirrors the upper detail, unlike the open section detail in the drawing.

 

[Sym P. le]

Document referred to above

 

[SocklessJ]

Apparent cause:
Failure by buckling of pony truss due to grossly inadequate 2"x1 1/2" horizontal deck brace.

Root cause:
Lack of enforcement mechanism to prevent "manufacturers" from practicing engineering without a license.

Remember this was likely "pre-engineered". In other words NOT engineered, unless:
1. The design was part of a permitted project where a licensed SE was involved.
2. The SE was aware of the gangway, it was in his scope, and he delegated the design.
3. The contractor actually read the drawings and noticed the delegated design provisions.
4. The contractor purchased the gangway with the requisite engineering.
5. The engineer was an SE, and not an ME unfamiliar with the phenomenon of pony truss instability.

In my experience, manufacturers will talk about Engineering on their website, but they know that once an SE gets involved, he will increase the size of all the members and triple the cost of the project.

So they bid non-engineered designs and wait for the contractor to say something. "Oh, you want stamped drawings? Hey Bill did they buy this one with a stamp? That's gonna add a lot of cost and push us out 12 weeks."

 

[LittleInch]

Interesting that the drawing has a title gangway 60ft x 3ft.

We're looking at 80 ft x 5 ft.

You wonder if someone just scaled it up??

This sounds a bit like they designed a 60ft one years ago.... From their website.

"All designs have been analysed and stamped by licensed engineers and proven by years in the field. Our production team of AWS certified welders have gained a reputation in the industry for meticulous attention to detail and quality. We pride ourselves on building a safe and well engineered product that looks great and will stand the test of time."

The pontoon deck looks very uch like one of theirs as well. So a package supply - give me a pontoon dock and a gangway please.

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

 

[LittleInch]

From this video

https://abcnews.go.com/US/7-dead-mu...gangway-collapses-georgias/story?id=114960648

You get this snap shot. Looks like they've lifted it onto a barge.
Other reports do say up to 40 people onthe ganway and that it "bounced" when people ran up and down it with fewer people.

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