OT: Speculation as to reason for the Florida high-rise collapse.

I remember with the Ronan Point partial collapse, the papers had picked up on the use of high alumina cement (cheaper) causing instability.

ISTR that gas supplies to high-rise blocks was banned after that collapse.

On salinity migration. A paper concerning the aquifer under the Nile Delta .

"A 50 cm rise in the seawater level is expected to cause about 4.5 km inland migration of equiconcentration line"

The gas explosion didn't help.

Not many buildings can survive an explosion, but system-built premises are probably worst for falling to pieces, since the explosion endangers more than one household. Gas supplies were promptly forbidden in similar blocks and no more have collapsed in similar circumstances.

I thought also that the general construction method was responsible. In that when one flat collapsed, so did all those above.

The construction was meant to have the concrete panels bolted together. But apparently that was easy in theory, but difficult in practice. So, many of the bolts were omitted.

That is true, and exactly the reason why a gas supply should never have been allowed near the place (and other built in the same way).

Lift-slab construction is quite popular these days, I wonder how they reliably lock each slab in position after jacking them up , to avoid them all collapsing back down again, aftr say localised fire damage to one slab.

I wonder if Roman-era concrete might have proved more durable. Bearing in mind there are plenty of examples still in use 2000 years later.

...and plenty more of it in ruins!

Seawater and Roman concrete forms the most durable of all hydraulic concretes apparently. Shame the likes of the Collosuem were not below seawater level.

Florida is notorious for sink-holes suddenly appearing.

Quote "Zone 1 (Yellow): This region consists of exposed or thinly-covered carbonate rocks. Broad and shallow sinkholes are common in this area. Cities in the zone 1 region include Miami, Coral Springs, Hialeah, and Hollywood".

At this stage I think there's so little known about the root cause of the collapse that speculation is pointless. Wait for the full report.

"Three years before the deadly collapse of the Champlain Towers South condominium complex near Miami, a consultant found alarming evidence of ?major structural damage? to the concrete slab below the pool deck and ?abundant? cracking and crumbling of the columns, beams and walls of the parking garage under the 13-story building.

The engineer?s report helped shape plans for a multimillion-dollar repair project that was set to get underway soon ? more than two and a half years after the building managers were warned ? but the building suffered a catastrophic collapse in the middle of the night on Thursday, crushing sleeping residents in a massive heap of debris.

The complex?s management association had disclosed some of the problems in the wake of the collapse, but it was not until city officials released the

"The building was just entering the recertification process for aging structures that have endured the punishment of coastal Florida?s hurricanes, storm surges and the corrosive salty air that can penetrate concrete and rust the rebar and steel beams inside." ...

"At the ground level of the complex, vehicles can drive in next to a pool deck where residents would lounge in the sun. Mr. Morabito in 2018 said that the waterproofing below the pool deck and entrance drive was failing, ?causing major structural damage to the concrete structural slab below these areas.?

The report added that ?failure to replace the waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially.? The problem, he said, was that the waterproofing was laid on a concrete slab that was flat, not sloped in a way that would allow water to run off, an issue he called a ?major error? in the original design. The replacement would be ?extremely expensive,? he warned, and cause a major disturbance to residents.

In the parking garage, which largely sits at the bottom level of the building, part of it under the pool deck, Mr. Morabito said that there were signs of distress and fatigue.

?Abundant cracking and spalling of varying degrees was observed in the concrete columns, beams, and walls,? Mr. Morabito wrote. He included photos of cracks in the columns of the parking garage as well as concrete crumbling ? a process engineers refer to as ?spalling? ? that exposed steel reinforcements on the garage deck.

Mr. Morabito noted that previous attempts to patch the concrete with epoxy were failing, resulting in more cracking and spalling. In one such spot, he said, ?new cracks were radiating from the originally repaired cracks.?

The report also identified a host of other problems: Residents were complaining of water coming through their windows and balcony doors, and the concrete on many balconies also was deteriorating." ...

"Researchers analyzing space-based radar had also identified land that was sinking at the property in the 1990s. The 2020 study found subsidence in other areas of the region, but on the east side of the barrier island where Surfside is, the condo complex was the only place where the issue was detected."

Indeed! This is going to be a lawyers' paradise.

There's been speculation on this thread about saltwater seepage into the fabric, and it's worth noting that the part of the block that collapsed was the part nearest the sea.

Concrete cracking and spalling had previously been noted, salt will be distributed fairly liberally during the hurricane season.

But given the scale of the collapse a sink-hole sounds like a plausible option.

I was only half listening, but I thought I heard in a BBC report concerning the structural survey of 3 years before, he had detected/surveyed a lean of the building. I cannot find that reference though. Much more significant, indicating foundation problems , rather than presumably cosmetic cracks that can be rectified by epoxy.

True, but the numerous missing bolts didn't help.

There is quite a mound of debris above subterranean car park level and surrounding ground level, would have to be a pretty small sink-hole. I'd like to know how much water pumping is required to keep the 40 foot deep access trench open, and if fresh,brackish or full salt water being pumped

No steel reinforcement to rust.

Progressive collapse is the rule, not the exception, with high rise. As you can see massively demonstrated by demolition videos on you tube etc.

All you have to do is take out enough structure to start it.