Pittsburgh bridge collapse 32

[Besserat]

It just happened

https://www.post-gazette.com/local/...int-breeze-squirrel-hill/stories/202201280075

 

[bradw1128]

Bradley Wilder, P.E.
Construction P.E. (KY), MBA
Bridge Rehab, Coatings, Structural Repair

 

[FacEngrPE]

The cables may have been an attempt to stabilize the structure after the bracing became unreliable. Not something I would consider, except for an out of service structure, while awaiting or during a rehabilitation.

https://www.wonkette.com/why-did-joe-biden-blow-up-this-pittsburgh-bridge[/URL]]Let's all take a few minutes to remind ourselves that government is what we do to make sure the roads don't fall apart under our buses, and that if a disaster like that does happen, there'll be rescue crews and ambulances and hospitals to help. Wonkette (a Pittsburgh paper) January 28, 2022 03:15 PM

 

[bridgebuster]

- looks like weathering steel from the photos, but that's not to say weathering can't corrode.

Anyway, this will give Pennsylvania another opportunity to be on "Engineering Catastrophes"

 

[JedClampett]

There's an old saying, "There's never time (or money) to do it right, but there's always time to do it over." This is the exact opposite. Someone made a decision to build a road, a bridge, a sewage collection system, etc. and pay for it. But then no one expected to ever maintain it? I just don't get it.
In the 50's and 60's the US made a decision to build an interstate highway system. It's one of the engineering wonders of the world. Note that it had military undertones, but so what, it got done. In the dollars of the time, I'm sure it was a massive expense. It has been a driver of a massive economic boom. And now it's falling apart, because we're too cheap to take care of it.

I don't disagree with closing substandard bridges. But a little story is in order: In Arizona we have seasonal washes. They only flow after rainstorms. And rainstorms are not all that common. But sometimes we get a bad one. Well, one of these washes needed a bridge, but it's traffic would be low and Arizona only builds infrastructure for politically connected developers. So a couple of years ago a family tried to cross the wash when it flooded and three of their kids drowned. The parents are going to jail as it was 95% their fault. But closing bridges has cascading effects that must be recognized.

Another story: A farmer was bragging to his neighbor, "I've got a mule and I stopped feeding him. And guess what, he works just as hard." So a couple of weeks later, the neighbor saw the farmer, and asked him, "Where is your mule?" The farmer just grumbled, "I don't have him anymore." In the 80's we decided it was time to stop feeding our mules.

 

[bones206]

Note that it had military undertones, but so what, it got done.

And now it's falling apart, because we're too cheap to take care of it.

It's the same dynamic now as it was back then. We aren't too cheap- we continue to spend all our hard-earned money on military expenditures. Just happens that roads and bridges aren't that strategically important to the military as they were back then. We probably spent more rebuilding infrastructure in Iraq and Afghanistan over the past 20 years than we did on our own infrastructure. The military industrial complex is a giant black hole sucking away society's resources. Sorry to get on a soapbox but it's such a frustrating situation. My son's future elementary school is a mess - cobbled together with temporary trailers - while we spend trillions on F-35 fighter jets. How many bridges could we have repaired for the cost of that single F-25 jet that's sitting at the bottom of the South China Sea right now?

 

[dik]

and they keep trying to land them in the water, it would seem.

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

-Dik

 

[Burloak]

Can't figure out how to reply to a post, so to @Milliontown: "With a few inches of snow and the bus, that was well over the posted weight limit. Interesting to see how this might affect public transportation policy if our vehicles are too large for our aging infrastructure."

26 tons would be the weight allowed for A vehicle on the bridge, not ALL vehicles on the bridge. It's indicated that the bus is 23 tons, which should be ok. Two inches of snow is not heavy, well less than 0.5 kPa (10 psf). The background lane loading that would be allowed with the 26 ton truck would be about 3 kPa. Essentially the load posting says a 26 ton truck can be on the bridge along with almost bumper to bumper car traffic. At least according to the Canadian Bridge Code.

So I don't think the bridge was overloaded beyond what the sign allowed.

 

[dik]

That was when it was certified... maybe things had deteriorated in the last decade or two. Things don't usually get better on their own.

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

-Dik

 

[digger242j]

My understanding of bridge weight limits is that vehicles over the limit will cause further deterioration--not immediate failure. You should be able to drive 26 ton vehicles over that bridge all day long without it falling down. Somebody please correct me if I'm wrong.

(Maybe I should phrase that differently--Vehicles *under* the limit will not hasten the deterioration. Obviously for any given bridge you could find something heavy enough to kill it.)

 

[timeforchili]

the east side of the bridge appears to have ended up much further from its abutment than the west side. for reference, the 2018 twitter photo was of the west side, but you can the same similar cable remediation on the east side as well in the streetview from the trail below bridge, taken in 2020.

in this photo, the east side is on the left with the bus and the west side is on the right, using that gazebo at top right for reference. the broken section the bus is on appears to be the only section that ended up underneath another section of bridge. 35 mph zone so the bus is taking about 7 or 8 seconds to get that far across the bridge.

Could that be enough of a clue to indicate which side failed first?

are either of these plausible failure modes?

-East side (bus side) columns fail first as the bus load reaches them, deck breaks at the column and the bus section kinda pivots as it falls due to some resistance still there at the columns, then slides down the hill before stopped by other debris, everything else just falls mostly straight down as its way too much load for the columns to provide any resistance now.

-West side column fails first, falls more straight down and pulls rest of bridge down towards it as it fully collapses, causing east side to be so far away

lot of good photos at the trib

https://triblive.com/local/photos-bridge-collapse-in-pittsburghs-frick-park/

 

[bridgebuster]

The Bridge Hunter website confirms that the bridge was constructed from weathering steel. There are also some notes speculating about the cause of failure. It's not clear if the person commenting is from PennDOT

Link

I noted on the Bridgeport site that the bridge was only 21 years (1991) old and rated in fair condition and in 1995 was rated in "Poor" condition. apparently some work was done between 1995 & 1997 to up the rating to "Fair".

I'd be interested in seeing the bearing details. bradw1128 wrote that the bearings were rated "Poor". Maybe thermal stress from frozen bearing at one of the piers is the cause? According to AccuWeather the Friday's temperature was between 15F & 30F with wind chills of -15F. perhaps it was something else: There could have been a fatigue failure or a brittle fracture. Poor detailing may have contributed to the failure. Weathering steel does corrode.

Forgot something: Could it be a geotechnical failure? On an episode of "Engineering Catastrophes" about a landslide in Pittsburgh someone mentioned the area has pockets of funky soils.

I'd like to the actual inspection reports, particularly from 2009 and 2011 when the bridge went from "Fair" to "Poor". Until PennDOT or the County releases this information, everything is just speculation.

 

[digger242j]

I've been curious as to why it would have ultimately failed when so lightly loaded. (One bus and a handful of personal vehicles.)

Temperatures Wednesday night into Thursday morning were into the minus single digit range. Is it plausible to think that something could have fractured during that period, and not ultimately failed until a bus crossed it 24 or so hours later? The straw that broke the camel's back, so to speak.


I think that's due to the fact that a good sized portion of the deck folded up into a vertical position. (The back of the bus is resting against it.) Had that eastern end of the deck landed flat on the ground beneath, that distance might look more similar to the other end.


Looks to me like it's the cross bracing on the west end that's missing. The gazebo, while hard to see, is discernable between the column and deck, at the left side.

 

[bridgebuster]

I was speculating about a thermal load, which can get pretty high even with a modest change of temperature.

 

[dik]

There can be a problem with weathering steel, if the environment is aggressive... it can 'corrode' faster if the protective coating is removed. This is what happened to a local floodway with the S Beam guiderails.

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

-Dik

 

[FacEngrPE]

As add for weathering steel states

https://usbridge.com/what-is-weathering-steel/[/URL]]The Cons of Weathering
Although weathering steel may sound ideal, there are a few factors that should be considered before construction. Certain weather and climate conditions can lead to issues with durability and corrosive resistance.

If unpainted, weathering steel depends on a patina to provide corrosion protection. If the patina is compromised the resulting rust blisters will contain corrosion cells, defeating the steels corrosion resistance. Rust blisters appear to be present in some of the pictures above. Road salt is one of the potential drivers of weathering steel corrosion.

https://www.metalsupermarkets.com/what-is-weathering-steel/

Question - do the bridge inspections compare observed section loss to design section loss (corrosion allowance)?

 

[bridgebuster]

Maybe, depending on the state. In NY, the DOT requires an additional 1/16" (min.) on the design thicknesses for weathering steel plates. Typically, in the biennial inspections in NY, primary members are supposed to be re-rated if section loss is observed.

Forgot something, on the news this morning, there was a story that the NTSB is supposed to release its initial findings in about two weeks. The final report will be issued in the next 12 to 18 months.

 

[CARunderscore]

Early morning failure under modest traffic indicates that thermal self-straining load was a contributing factor. It could be that bridge was probably trying to contract with frozen bearings, and the bus finished it off. Cold temperatures also may have increased fracture susceptibility in existing fatigue cracks.

That doesn't account for the column issues, though. Maybe the ineffective bracing meant the columns weren't doing much to begin with? If the inadequately-braced columns were shedding load to the girders, maybe the failure initiated in the girders, causing them to fracture and result in two cantilever spans that immediately collapsed.

My understanding of bridge weight limits is that vehicles over the limit will cause further deterioration--not immediate failure. You should be able to drive 26 ton vehicles over that bridge all day long without it falling down. Somebody please correct me if I'm wrong.

(Maybe I should phrase that differently--Vehicles *under* the limit will not hasten the deterioration. Obviously for any given bridge you could find something heavy enough to kill it.)

Theoretically. And they DID run 26-ton vehicles over this bridge all day long. As we've established, these buses with regular routes across the bridge were up to 20+ tons, and probably reached 26 tons when full. Random trucks in excess of 26 tons was probably a daily or weekly occurrence. I'm also going to go out on a limb and say the reasoning behind the actual posting was probably imprecise at best and objectively incorrect at worst, so I wouldn't be surprised if fatigue due to a too-high posting was an issue.

Structurally non-redundant ones should be candidates for closure

But what we really need to do is shrink the system, as the Iowa DOT commissioner says some years ago. If we can't generate the revenue to maintain our roads and bridges, maybe we need to get rid of some

This is the position I take. I see so much infrastructure, especially car infrastructure, that is obviously overbuilt compared to the number of cars on the road, even without taking "induced demand" into account. Glad to hear similar opinions from a traffic engineer. I like to take it a step further and say we also just need to dial back the number of cars on the road. We need more transit, more public thoroughfares where pedestrians and cyclists don't get turned into little pink clouds, and stricter driver licensing requirements.

Even as we speak, this new infrastructure bill is going to pour money into projects we don't need but that we'll never be able to tear down once they're built. My favorite is lock and dam upgrades on the eastern end of the Ohio River. As far as I can tell, the financial justification for these projects is mainly that the coal industry needs them (an assertion which I actually find dubious to begin with). Sometimes it takes the feds 20 years to finish lock projects. Who says the coal industry will even still be around in 2042?

And, yeah, we spend to much on the military, too.

EDIT: Oh, shoot, speaking of infrastructure being maintained indefinitely with a forgotten purpose, I just read an article about how flood control dams "need" melting winter snow to fill their reservoirs for boaters and fishermen in the summer. No! That's backwards! We need the dams to protect us from the melting winter snow. Maybe. A lot of those were built basically as make-work for unemployed people during the Great Depression, so some of their benefit-to-cost ratios could probably also stand to be re-examined.

 

[dik]

Thanks CAR...

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

-Dik

 

[debodine]

Does it seem reasonable that perhaps the governing authority reviewed the bus weights and decided to post the bridge limit high enough to allow the buses without having to do a detailed analysis of the actual limits that should have been imposed? I can see a bean counter providing a cost/benefit analysis that would "...keep the buses running..."

 

[ATSE]

For a minute, I thought this was an engineering forum.
"... Without getting political, there's a real downside to Reaganomics."
"... The military industrial complex is a giant black hole."
"... we spend to much on the military, too." (sic)

Most of us recognize infrastructure requires public funds. And civil structures require maintenance. No argument here. But when approx 1/3 or less of many federal spending "infrastructure bills" goes to actual civil construction, I don't think blaming the military is rational.

Anyway, thanks to the others (engineers?) for the engineering discussion. Thermal strains and secondary loads routinely get ignored in most civil structures.