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

 

[LittleInch]

It's the long rather slender walkway going out to the outer pontoon.

Sounds like there were about 20 people in it, mainly elderly waiting for the ferry.

Not clear what the failure is, but looks to have collapsed in the middle. Only a year old apparently

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

 

[human909]

The span to depth ratio looks about right. But there seems to be very little to prevent lateral torsional buckling of the truss frames. The top handrail members are in compression without any lateral restraint. For something that length you normally want out riggers or some pretty stout verticals with moment connections under the walkway to provide the stiffness necessary to restrain lateral movement.

Also it is kinda odd that the designer chose a Howe truss, but I suspect that isn't relevant to the cause of failure.

 

[FacEngrPE]

I wounder
[ul]
[li]What was the design load? Did the design condition consider people sometimes stand packed as tight as possible, or stand to one side of a gangway?[/li]
[li]Was a load test performed at construction?[/li]
[/ul]
Gangways are not buildings, there is essentially no dead weight. Buildings are typically at least 50% dead weight, which indirectly provides some margin on the load factors (safety margin).
While ASCE 7 still applies, it becomes a starting point for determining the real requirements.

 

[3DDave]

With a concentration on elderly, it could have also had the weight of powered wheelchairs or seated scooters on it. If they were powered chairs the occupants could have been wearing seatbelts.

If either or both of the sidewalls buckled inward they would have trapped people.

 

[zeusfaber]

it could have also had the weight of powered wheelchairs or seated scooters on it. If they were powered chairs the occupants could have been wearing seatbelts.

I'd never considered that before, but that's the second time this week it's come up. Report here on an accident where two powered-wheelchair occupants drowned when the boat they were passengers on capsized.

 

[LittleInch]

I wonder if there were two lanes on the gantry. One to queue to get on and the other left for those arriving? Hence a rotational load which might not have been fully considered?

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

 

[3DDave]

LittleInch, excellent guess. There was even a center railing to ensure keeping to one side.

 

[TugboatEng]

For clarity, it wasn't the gangway that failed. It was this span.

 

[LittleInch]

I call that a gang way...., but yes it's that bridge / span between the floating Dock and the Duke m fixed height.

Anyone an idea of the state of the tide at the time of collapse??

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

 

[3DDave]

I agree, a gangway. One end is hinged to the shore and the other end follows the dock as it goes up and down.

There must be thousands of these. Perhaps someone didn't assemble the erector set using all the pieces.

 

[Sym P. le]

Aerial view on Bing Maps. Google is outdated.

Anyone an idea of the state of the tide at the time of collapse??

Low tide was 4:52 p.m. The incident occurred approximately 4 p.m. (Oct 19)

 

[bugbus]

Interesting thought from reading the comments...

Is there any situation in which a pony truss like this has less capacity with 50% live load distributed on one half vs 100% live load applied everywhere?

My instinct would tell me no, but I would be interested to muck around with some simple truss bridge models and check the relative capacities. I say no because even with 50% live load on one side of the bridge, the truss on the loaded side still only carries ~3/4 of half the live load (3/4 x 50% = 37.5%, which is less than the 50% live load if the entire thing were loaded). So I don't think that buckling of just the top chord could be worse in this situation.

If this sort of loading arrangement were indeed the more critical one, then the lesser capacity would have to come from some kind of global instability of the whole bridge with the torsion effect of having the live load on one side only.

I suspect the answer is probably much simpler though. It might just come down to a single connection or loss of support where the gangway attaches to the dock. Alternatively, the top chord look pretty slender for such a long span. I guess the span is somewhere in the 20m/60' range, but the top chords appear to be ~100mm/4" tubes. This thing being only 1 year old, I wonder if this was the biggest load the gangway ever saw and it just happened that it was all on one side.

The Australian bridge design code does not strictly require pattern live loading to be considered on pedestrian bridges (though it is often checked in situations where there is a clear potential for instability or similar). I believe it is similar for other bridge design codes around the world.

 

[3DDave]

Typically there are wheels on the end that rests on the dock so it is unlikely to be a failure at the place, unless you mean the hinge failing at the shore.

 

[bugbus]

I ran a quick model to see what the difference would be for the two load patterns.

The case with full live load on the entire bridge is the worst case.

I made a guess at the dimensions based on the photos:
*Span = 60'
*Width = 8'
*Height = 5'
*20 bays
*Top chord: 4" aluminium tube
*Bottom chord: 4x6" aluminium tube
*Diagonals/verticals: 2x4" aluminium tube

Interesting that the elastic buckling load is so small in the model. Granted, all these dimensions are a rough stab in the dark, but an elastic buckling load of ~2 does not bode well for this gangway.

 

[3DDave]

Grovner (senior mate on one of the ferries) said he complained to one of the ferry captains about four months ago that the gangway to the ferry didn’t seem sturdy enough, but nothing happened. Rabon (Georgia Department of Natural Resources Commissioner) said he wasn’t aware of any complaints being made.

Also a larger problem:

Resident Reginald Hall was among those who charged into the water, where an outgoing tide created a strong current that was pulling victims toward the ocean.

Hall said he was handed a 2-year-old child and passed her along a chain of bystanders to shore, roughly 60 yards (55 meters) away. He then helped carry blanket-wrapped bodies.

“It was chaotic,” Hall said. “It was horrible.”

JR Grovner loaded an injured woman into the back of a pickup truck and drove her to a field where a helicopter was evacuating victims. The ground was thick with tall grasses that camouflaged holes dug by wild boars, he said.

Sapelo Island residents in 2015 sued McIntosh County and the state of Georgia in federal court, arguing they lacked basic services including facilities and resources for medical emergencies. In a 2022 settlement, county officials agreed to build a helicopter pad on the island.

JR Grovner, Hall and Watts all said that still hasn’t happened. Patrick Zoucks, the McIntosh County manager, did not immediately respond to an email message seeking comment.

The ferry dock was rebuilt in 2021 after Georgia officials reached a settlement in the same lawsuit, in which island residents complained that state-operated ferry boats and docks failed to meet federal accessibility standards for people with disabilities.

https://apnews.com/article/ferry-do...apelo-island-c4fadec943d8ca46f70ef8dc3541da21

 

[3DDave]

bugbus, what happens to the buckling if some people are leaning against the top chord, say 5 pounds each for 10 people?

 

[bugbus]

3DDave, it makes some difference, that 2.5 factor goes down to 2.3-2.4ish.

 

[3DDave]

Another question - what if the gangway is supported only on opposite corners, like if part of the hinge failed?

 

[bugbus]

3DDave, I think the bridge would be flexible enough in torsion that it could just twist slightly and find support on 4 corners again. That's just my guess.

From what I have seen, it is very common for pedestrian bridges with ~20m/60' spans to have a top chord of a 150mm/6" or even 200mm/8" steel square hollow section.

Comparing this to a 100mm/4" aluminium tube, the bending stiffness (E*I) could be up to 20x more for the steel square hollow section. Examples below:

I admit I'm not familiar with the requirements for gangway loads, probably they are less onerous than for bridges, but maybe someone can enlighten us.