Miami building collapse: What could have caused it?
As the search for survivors continues, questions are being raised about what caused a 12 storey apartment complex to collapse in Surfside, near Miami in Florida.
Experts gathering information at the scene will have to consider a range of possible causes - from structural defects to environmental influences - and whether a combination of factors may have triggered the sudden collapse of the 40-year-old block.
What were the issues raised in the 2018 inspection?
The 2018 inspection highlighted "a major error" in the original design of the apartment block.
The engineer, Frank Morabito, warned that the ground floor pool deck was not sloped to drain, so any water "sits on the waterproofing until it evaporates".
The report said the waterproofing below the ground floor pool deck and decorative planters was "beyond its useful life" and causing "major structural damage to the concrete structural slab below these areas".
Failure to replace the waterproofing in the near future would, it said, cause the extent of the concrete deterioration to "expand exponentially".
Mr Morabito's report also referred to "abundant cracking… of columns, beams and walls" in the basement parking garage below the complex.
However, the report didn't suggest the 40-year-old building was at any imminent risk of collapse but urged that the concrete repairs be carried out in "a timely fashion".
Earlier this year, the president of the condominium association warned residents that the damage in the parking area had got "significantly worse" and repairs needed to the whole building had risen from US$9m (£6.6m) to more than US$15m.
Jean Wodnicki told residents the concrete deterioration was accelerating, and the roof situation was also much worse, according to the letter published by the New York Times.
Morabito Consultants have said that at the time of the collapse, roof repairs were under way, but concrete restoration had not yet begun.
How could concrete damage affect the building?
Structural engineer, Greg Batista, who has carried out thousands of inspections of buildings in Florida, said damage to the concrete was one possible explanation for the collapse.
"These buildings up and down the coast are susceptible to what we call spalling," he told the BBC.
"Basically, the reinforcing steel inside the concrete, it basically rusts and it expands up to seven times its volume. The concrete that surrounds it breaks and that causes significant deterioration in the structural integrity of that particular member, whether it's a beam or a column. All it takes is one beam or one column to fail and it causes a domino effect."
Did land movement contribute?
A study ________ researchers at Florida International University published last year found that the building was sinking at a rate of about two millimetres per year in the 1990s.
Lots of movement underneath buildings can cause cracking and contribute to structural problems.
Professor Shimon Wdowinski said the study did not focus on Champlain Towers South in particular, but the building stood out as one of the places that showed the most subsidence.
He said it was not much, but the study was more than 20 years ago and "we do not know what happened after 1999, at what level it continued to sink, if it continued to sink, and how this may have affected its foundations".
"Maybe a point was reached where the structure couldn't hold the load and collapsed. But this is a structural problem. They are not things that I study," he told BBC Mundo, the BBC's Spanish language news service.
"What we do know is that the building that collapsed in Miami has been sinking for decades, but that alone does not explain the collapse."
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Which is the incorrect alternative?