@Ron - Great interview. Seems I dimly recall collapse of one or more 1920s beach front similar buildings in the Miami area, maybe in the 1970s or 80s. The cause was rebar corrosion; sea water had been used for the concrete (no fresh water available at the beach in the 1920s, I suppose). If my memory is correct, those 1920’s building would have been about 40 years old at the time of collapse. Interesting parallel.
I know sea water would not have been used in 1981 construction, but perhaps coastal flooding provided the salt this time.
@SlideRuleEra ….I agree. I’ve tested a lot of concrete on the coast for chlorides and have designed remediation of corroded rebar on a lot of coastal concrete structures.
You’re right about the use of seawater for concrete. It was used for buildings and bridges early on.
@Latexman …
Yes, that’s important. It is another symptom of multiple issues that will likely come to light as the investigation gets moving.
I’ll quote the part that is interesting, “Early Thursday morning, Mike Stratton awoke to the sound of his cellphone ringing. It was his wife, Cassie Stratton, on the other end, speaking frantically about their condo building shaking. She told him she saw a sinkhole where the pool out her window used to be. Then the line went dead.”
So, was this a traditional sinkhole as we know it, or did the slab under the pool deck fail catastrophically somewhat earlier (minutes?) than the condo and give the appearance of a sinkhole? My gut says the latter, because the pool is actually still there, though it appears it lost it’s water. Cassie must have been speaking of the pool deck area between the condo and the pool. Granted, she would have been in a panic with all that was going on at the moment.
@Ron Can you provide, or point me to, a theory/hypothesis, of why the rebar stripped out of the concrete is so clean? There are a lot of folks wondering about that.
@Latexman …
The building was post-tensioned, so a lot of what is sticking out are sheathed tendons. They are probably not corroded. When rebar gets stripped from concrete, depending on how fast it gets stripped and this was fast, it powders the bonded concrete and friction cleans the rebar a bit. The rebar that might have caused the failure is likely in the columns, not the slabs, so can’t be seen in photos yet.
I’m not sure the 2018 Engineering Report even addressed the lowest concrete (substructure).
Looking through the 1979 plans, I see the building is on driven 14" square “precast ( probably prestressed) concrete piles” driven to 50 tons bearing value. Prestressed piling should have no problem with sea water corrosion over a 40 year time span. Unless the piling were not driven correctly to start with, it is unlikely they “settled” in the 1990s. I would expect that settlement reported in the 1990s came from deteriorated concrete / rebar that supported the piling supported… perhaps the columns Ron mentioned.
General notes on concrete compressive strength are revealing, different values given on various pages of the 1979 plans:
2500 psi (Absolute garbage)
3000 psi (IMHO, inappropriate for use in a corrosive [sea water] environment.
4000 psi (For floors through the 8th story, with 3000 psi for floors above the 8th story)
About the the Contractor interviewed in the video just before Ron spoke. I discount every word the Contractor said. In 1981, the subsurface (piling) would have been just as saturated with groundwater as it is today… maybe saltier because of sea level rise, but not wetter. Ron’s explanation is superior.
Mike - Would have to see the geotechnical report to get an idea of pile driving conditions. Driving to a certain bearing value (50 tons) is a performance specification, not to a certain pile tip elevation. Bearing value is determined by using a suitably sized impact hammer with a traditional mathematical pile driving formula (typically, Modified Engineering News Record). Pile tip elevation can vary, usually not much in sand. Concrete pile are displacement (bulky) piling. They can work through either point bearing or skin friction (often a combination of both).
Close to the ocean, at the depth where pile would be driven, ground water is probably all brackish instead of a specific sea/fresh line. How brackish will depend on subsurface conditions.
I saw that the National Institute of Standards & Technology (NIST) will be investigating this collapse. They get involved only rarely, when the future risk to public safety is potentially overwhelming. This will be only their fifth investigation of a structural failure in their history.
@SlideRuleEra …thanks for the links…those are good.
As for the foundation, I agree completely. This is a common foundation system in the area and should have been able to achieve 50 tons/pile without much difficulty. The oolitic limestone formation there is very shallow and continues to deeper formations. You are correct that most of the interface with the foundation will be brackish as the interface with saline/non-saline will not be clear, nor is it an issue.
For those reasons, if I were doing the investigation, I would concentrate on the exposed concrete, particularly the columns and the observed deterioration.
And as I said before, the attitude of the powers to be in Florida is at the root of a very large problem here… Follow the money…
For DeSantos to say that the condo had “problems from the start”, the “establishment”, local government in particular, has known of this for many years.
@MSQUARED48 …I think that his job might have left him rather than what was reported!
Yep…DeSantis is at the root of quite a few issues in Florida. The legislature has to address this recertification issue. 40 years is too long, particularly along the coast. I think 20 years is more reasonable, maybe even every 10 years after 20.
I saw another report today that some other buildings in the area are being evacuated. Is that the case now? If so, do you know which ones?
I wonder if the powers to be are now seeing the money trail with all the fingers pointing to their jobs, with possible criminal charges, let alone the litigation?
@MSQUARED48 …Mike…yep, at least one large condo about 5 miles to the north of Champlain was ordered to be evacuated. It was based on an engineering report showing similar issues. The attorneys for the Condo Association engaged another engineer to do an inspection who said all was OK…the municipality rejected the second one and ordered the evacuation. Correct move.
We have gotten several requests for inspections by condos along the coast since the Champlain collapse. They all seem to think that all it requires is a “home inspection” approach. I have explained to them that in order to do an adequate inspection it will require a lot of stuff and it won’t be cheap. For an adequate inspection, it will require multiple engineers and technicians, some nondestructive testing and some sampling/laboratory testing. Could easily run upwards of 20-30K.
