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Big Mac: Jan 3, 2007

Away from the risk mitigation design chat, I work in construction inspection and testing, and have spent PLENTY of time working with road crews. My company has a major interstate bridge project going right now and for the next few years.
Roadwork on open traffic corridors is already stressful, and seeing that there was a road crew on the bridge at the time was absolutely gut wrenching.

# ¿ Mar 26, 2024 20:04

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# ¿ May 16, 2024 00:46

Big Mac: Jan 3, 2007

doing some quick loving around with Wolfram Alpha, I'm seeing that 115,000 tons, moving at 5 knots, is equivalent to the thrust of 82 solid rocket boosters from the Space Shuttle.

edit ah I see I'm not the only one getting posting fodder from wolfram rn

Big Mac fucked around with this message at 22:32 on Mar 26, 2024

# ¿ Mar 26, 2024 22:29

Big Mac: Jan 3, 2007

kreeningsons posted:

i did some quick loving around calculations at work but for energy absorbed, which based on my loving around research is the main way these bridge dolphins/protections systems are rated.

the sunshine skyway bridge in florida has a protection system rated for a hit from a 87,000 ton ship at 10 knots.

the ship that hit the key bridge in baltimore was reportedly 117,000 tons and traveling at 8 knots.

calculating the kinetic energy of each, the ship in baltimore had 86% of the kinetic energy as the skyway bridge protection system was rated for.

so it does seem like a protection system can withstand a direct hit from a ship this size. but key point, it has to be direct hit, not oblique like in baltimore, which looks like the ship dodged any protection system in place. this seems to be the place on the forums that this accident is getting the most technical discussion, so if any nerds know more about these systems than i do (basically just what i could learn today) then please correct me.

It does feel nice to have relatively grounded discussions about the incident and what preventative measures would look like and be capable of. While dolphins or protective islands would (presumably) be good, I also wonder about the soil typology in a large, presumably very alluvial harbor - those pilings would have to be deep and plentiful to be able to handle anything close to the lateral loading that a container ship would be capable of. I'm not familiar with the soils around Sunshine Skyway, either, or dolphin construction, or, or, or, or

I wonder, ultimately, how much of an impact any truss bridge like this could take. No question that the supports were the most overwhelmingly critical parts, but I have to presume that designing for something to impact those piles with even 1/10th the tonnage is asking for the Hoover Dam.

# ¿ Mar 27, 2024 18:40

Big Mac: Jan 3, 2007

and I have no question that the replacement bridge is going to be an incredible and shockingly resilient megastructure. How many other critical links over critical waterways are begging for maintenance, though, that are going to be largely overlooked?

dammit a terrible and nigh-unpredictable accident has got me doompilled. It'll be everything I can do not to dig through decrepit infrastructure articles for the rest of the day.

# ¿ Mar 27, 2024 18:44

Big Mac: Jan 3, 2007

Xerol posted:

One of the things I've been trying to research (unsuccessfully so far) is why they went with one of the longest truss spans in the world over any other option. I suspect the main factors were some combination of "cost", "cable stayed bridges weren't the go-to design yet", and "there's a steelworks 1/4 mile away". The channel itself is also less wide than the central span:

So it was as wide as it needed to be. Modern cable-stayed designs can have spans longer than not only the longest span of the truss (1/4 mile) but the entire width of the waterway (1 mile)*, and you can go even longer with suspension. Any serious study on the replacement bridge design is going to look at the balance between the costs of protective structures and a longer unsupported span. If they can build a replacement 1/4 mile span + sufficient dolphins for less than it costs to build the 1 mile span and no dolphins, they probably will.

Another factor is the height of the bridge itself. Not only to let tall ships pass through, but vehicles need to go up and down to get over the bridge, and you can't have that slope be too steep, and there's only so much land either side of the bridge where you can raise the highway.

*Assuming you count the causeway portion of the bridge complex as land

I'm far from an engineer, or a historian, just a concrete guy
In that era, truss systems were the thing. It was a tried and true technology, and in use and active construction for interstate bridges across the country. It's light (as bridges go), forgiving in construction, repairable, and has been proven to be able to span critical waterways at crazy heights.
Concrete cable stayed bridges are tough. They're generally constructed by building the piers to their full height, and then building the decks outwards from the piers evenly on both sides. For cast-in-place concrete, this is no small undertaking, especially using 70's era concrete mixing and pumping equipment. Modern cable-stayed bridges didn't come about until after the Key bridge was built - the replacement Sunshine Skyway bridge has a main span only 45 meters longer than the Key bridge's was, and it was opened 15 years after the Key. They Skyway also cost an inflation-adjusted $775,000,000. The Key bridge came in at half that.

# ¿ Mar 27, 2024 19:48

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# ¿ May 16, 2024 00:46

Big Mac: Jan 3, 2007

kreeningsons posted:

Thanks, that Reddit thread is pretty good. The day of the collapse, the attitude in various online places was that nothing could possibly be built to protect a bridge from a ship of this weight. Which is not really true, systems have been built that are rated to protect bridges from ships weighing this much. There was an attitude elsewhere that the ship was simply too large to be stopped by any man made device, which again isn�t true, the ship was stopped by the bridge itself after all, just after suffering catastrophic damage.

Now the conversation has shifted to two questions.

Are bridge protection systems rated for a direct hit or just a glancing hit? I�ve found some specifications online for structural dolphin systems, but nothing more recent than the mid 20th century. Protection systems function by berthing the ship. There are calculations for determining how far the ship will ride over the dolphins before coming to rest, which seems to be as much as half the length of the ship. These calculations can include tidal flows, thrust of the ship, momentum of the water behind the ship, etc. So the follow up question is, would the dolphins meaningfully function if the ship could still continue for half its length in the direction of the bridge? I don�t know.

The other question is, could a retrofit even have been built in the harbor considering its space constraints and the composition of its bottom? The water under this bridge is very tight and the composition of the bottom is not favorable, so the attitude among the experts seems to be that building one would be possible but not practical. There�s not a lot of information presented to back up that claim, just a few tweets and a line in the NYT. So I don�t know either.

My own biases lead me to remember I�ve heard these same sources conceal their pessimism as pragmatism by stating many other objectively good state projects are �possible but not practical� while the state spends mind boggling amounts of money, effort, and creativity on objectively evil things. I�m cynical, but also, Baltimore is a great city, the same forces that made the city a decaying rust belt nightmare also make it possible for beautiful things to exist that couldn�t exist in any other city, and I�m heavily invested in it.

Beauty is a very real consideration when it comes to things like bridges, especially. I'm not a fan of the aesthetics of the concrete cable-stayed bridge, but I'll go to bat for them over the precast girder bridges that so many places have defaulted to.
Baltimore also having to deal with hurricanes can't help make designing for beauty easier.

# ¿ Mar 28, 2024 18:09

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