3 ms·

2mm/year sounds like a very small number to me... compared to eg. thermal expansion during day/night and summer/winter cycles, that is practically zero, and eve

2mm/year sounds like a very small number to me... compared to eg. thermal expansion during day/night and summer/winter cycles, that is practically zero, and even after 40 years of sinking, surely there'd be massive warning signs (eg. growing cracks) before such a collapse.

tmh88j 5y ago
> surely there'd be massive warning signs (eg. growing cracks) before such a collapse. There were. I'm not a civil/structural engineer and I don't know how common this kind of thing is. > One condo owner sued the unit association for failing to fix the cracks in the outside wall of her unit in 2015, according to a lawsuit filed in Miami-Dade County. The condo owner, who could not be reached for comment, said the cracks led to water damage that cost $15,000. The court documents noted that because the cracks were a structural issue the building association was liable for the expense. > The condo owner had previously filed a lawsuit against the building association in 2001 due to a similar issue. The two sides settled outside of court, but that kind of cracking is described as “of interest” in the county’s Structural Recertification Form. I read another article about a resident complaining about cracking near the pool deck. https://www.nbcnews.com/news/us-news/cause-miami-condo-collapse-unclear-experts-say-barrier-islands-present-n1272316 https://www.nbcnews.com/news/us-news/cause-miami-condo-colla...
btilly 5y ago
The problem is that the 2mm is probably not even over the building. If one corner is subsiding significantly more than the other, eventually one corner of the building is being held up by shear forces from the rest of the building. But concrete isn't very good with handling shear stress. So then it breaks, drops, hits the ground and you have a building collapse.
deleted 5y ago
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tzs 5y ago
From aerial photographs, the building appears to be about 100m in its longest horizontal dimension. It was 12 stories tall, so maybe 40m? Assuming it was made with a steel frame, then the expansion going from that area's record low temperature to its record high temperature would be about 40mm horizontal and about 16mm vertical. Presumably it was designed to handle that, with a decent safety margin. That is indeed much more than the 2mm it was sinking each year. But wait...the temperature swings of the frame, and other interior parts of the building, is going to be a lot less than the outside temperature. That's going to cut that 40mm horizontal and 16mm vertical expansion way down. Also normal thermal expansion alternates with normal thermal contraction, so wouldn't it average out to close to zero net movement over the course of each year, with a small net expansion due to climate change (Florida average temperature is up under 0.5℃ since that building was built)? Sinking, on the other hand, is usually cumulative. 2mm/year over 40 years is 80mm. So you've got a building designed to expand and contract, staying within some range, having a net vertical movement imposed on it that is a lot larger, and probably a lot less uniform.

tmh88j 5y ago

> surely there'd be massive warning signs (eg. growing cracks) before such a collapse. There were. I'm not a civil/structural engineer and I don't know how common this kind of thing is. > One condo owner sued the unit association for failing to fix the cracks in the outside wall of her unit in 2015, according to a lawsuit filed in Miami-Dade County. The condo owner, who could not be reached for comment, said the cracks led to water damage that cost $15,000. The court documents noted that because the cracks were a structural issue the building association was liable for the expense. > The condo owner had previously filed a lawsuit against the building association in 2001 due to a similar issue. The two sides settled outside of court, but that kind of cracking is described as “of interest” in the county’s Structural Recertification Form. I read another article about a resident complaining about cracking near the pool deck. https://www.nbcnews.com/news/us-news/cause-miami-condo-collapse-unclear-experts-say-barrier-islands-present-n1272316 https://www.nbcnews.com/news/us-news/cause-miami-condo-colla...

btilly 5y ago

The problem is that the 2mm is probably not even over the building. If one corner is subsiding significantly more than the other, eventually one corner of the building is being held up by shear forces from the rest of the building. But concrete isn't very good with handling shear stress. So then it breaks, drops, hits the ground and you have a building collapse.

deleted 5y ago

[deleted]

tzs 5y ago

From aerial photographs, the building appears to be about 100m in its longest horizontal dimension. It was 12 stories tall, so maybe 40m? Assuming it was made with a steel frame, then the expansion going from that area's record low temperature to its record high temperature would be about 40mm horizontal and about 16mm vertical. Presumably it was designed to handle that, with a decent safety margin. That is indeed much more than the 2mm it was sinking each year. But wait...the temperature swings of the frame, and other interior parts of the building, is going to be a lot less than the outside temperature. That's going to cut that 40mm horizontal and 16mm vertical expansion way down. Also normal thermal expansion alternates with normal thermal contraction, so wouldn't it average out to close to zero net movement over the course of each year, with a small net expansion due to climate change (Florida average temperature is up under 0.5℃ since that building was built)? Sinking, on the other hand, is usually cumulative. 2mm/year over 40 years is 80mm. So you've got a building designed to expand and contract, staying within some range, having a net vertical movement imposed on it that is a lot larger, and probably a lot less uniform.