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Engineers have begun to consider the potential cause of last week's apartment building collapse in Miami.
The 12-storey Champlain Towers, located in Surfside, Florida, collapsed on Thursday morning.
John Pistorino, a Miami structural engineer who evaluated the Florida International University bridge collapse, has now been hired to investigate the cause.
Surveillance video
He told the Palm Beach Post that he would need to carefully inspect the evidence before making any pronouncements. However he said he was struck by a surveillance video that showed the collapse started in the centre of the building, which pulled down the entire north section of the building.
"The video is very tell-tale," he said. "It shows the centre section collapsing all the way down, and then the tower on the northeast still standing, then it collapsed and pancaked."
However, Virginia Tech Department of Civil and Environmental Engineering professor Roberto Leon said that it is difficult to come to any real conclusions from the video.
He said: “By itself you can’t begin to draw any conclusions from a video. We have all of these surveillance cameras now, so we get to see what happened. But it takes very careful examination of these videos — almost on a pixel-by-pixel basis — to determine the collapse initiation.
"What you can see in the video and what a careful analysis of the video might show later on are entirely different things. So, it is useful, but we’re not going to establish the cause of the collapse based on a video alone.”
Leon also added that the way the building collapsed could provide a challenge in the investigation.
“The kind of remnants that you have in this pancake type collapse unfortunately destroy a lot of the evidence,” he said.
“The columns and walls — all the vertical elements that stabilize a structure — appear to have been damaged or destroyed. The lack of intact structural elements will likely affect the pace of the investigation.”
Salt impact
Generally, Pistorino said buildings along the ocean are vulnerable to salt and saltwater intrusion, making regular inspections and maintenance important.
Drexel University engineering professor Abi Aghayere also told news channel 6abc that the structure's location near a body of water raised the question of whether this could have affected its integrity.
Similarly, South Florida construction company National Home Building & Remodeling Corp founder Gary Slossberg told Fox Business that salt in Miami's coastal air could potentially have facilitated the corrosion of steel.
However he said he was unsure how long it would take for salt to corrode a building's materials to the point of collapse.
Structural issues
Slossberg emphasised that he hasn't heard any specific leads on the cause of the collapse but suggested a number of other possibilities. He said that if the building were constructed with a "post-tension slab" (or a concrete slab that has cables running through it) and one of those came loose, this could have contributed.
Another possibility, he said, is that the balconies may have had some construction issues with many Miami-area buildings built with concrete balconies that are "back-pitched". This means they don't allow water to escape properly after it rains.
"There's a lot of concrete restoration going on, and this is where you see a lot of that rust and rebar coming through the slab between the water sitting there and the salt air — it's just not a good combination," he said. "But again, I don't know if that would take down the whole building. We just don't know what happened."
Foundational failure
West Palm Beach engineering firm WGI specialty structures director Jeffrey Bergmann also cautioned against jumping to conclusions. However he said that buildings five stories and higher are built using deep pilings in their foundations. The pilings are driven into the limestone, which in Miami-Dade County can just be a few feet below the surface, he said.
Aghayere also suggested that a cause could be a foundation failure.
"The foundation could fail because there might be a sinkhole, nobody knows, but that needs to be investigated," he said.
Review process
Built in 1981, the apartment block was due its standard 40 year review. According to officials, it was undergoing its "recertification" process and required repairs.
Experts who studied the apartment complex last year warned that it was unstable, with one study from a Florida International University researcher finding that the building had been sinking at a rate of 2mm per year in the 1990s.
Professor Shimon Wdowinski told BBC Mundo that 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".
“It was probably designed to the standards of the time," Leon added. "But our standards have changed quite drastically over the last 40 years or so. With the benefit of modern computer analysis, there could be some design issues that we see today that weren’t clear then."
Slossberg added: "With every hurricane, new construction codes come out. New engineering codes," he said. " … This is [40] years later [since the building was constructed]. The codes have changed at least a dozen times. I know they have. So, some of these older buildings are not really built to withstand the type of same weather conditions as when they were built originally."
Overall, Leon said it is “entirely too early to tell what happened".
He added: “This is going to require quite a bit of investigation, and in the end, we might not come to a complete understanding of what happened here.”
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At the risk of pointing-out the obvious, whatever was the point of weakness that initiated the collapse, it is clear that the structure’s design lacked robustness against progressive collapse. It seems to have been riddled with points of weakness/brittleness. The wreckage reveals numerous inadequate slab-to-column connections, and some of the columns seem to lack enough links.
The recent events in Miami are a stark reminder of the consequences of sub-standard engineering and construction. Although 40 years old the condo block should have been good for another 80 at least so I hope the investigators discover the true cause and not sweep it under the carpet of “climate change ” and sinkholes. When you look at the perfectly symmetrical mode of collapse of the first part of the block, it was as though an expert demolition contractor had put charges in each of the column bases. And the second part came down pretty much in the same way. This suggests to me that the columns all failed due to bursting stress. This would have been caused by failure of the shear (stirrup) reinforcement which surrounds the main vertical reinforcement in the columns, followed by the buckling of the main rebar either quickly afterwards or over a period of a few days/weeks. The shear rebar probably corroded, expanding and pushing the cover concrete off so that the area of concrete column available to withstand the compressive stress from the weight of the building was much reduced. So why did the shear rebar fail? I think the clue lies in the location of the building. My guess is that either the concrete aggregate was washed with seawater or worse still, seawater was used to mix the concrete. This happened a lot in the Middle East back in the 60s and 70s. The sodium chloride reacts with the steel causing corrosion.
This is conjecture on my part but it was the sudden and symmetrical way the building collapsed which suggests it wasn’t a foundation (below ground) issue.