The questions started in the immediate aftermath of a partial building collapse in Surfside, Florida, June 24, that has left more than 100 people unaccounted for. “How could this happen?” “Why?” “What went wrong?”
But those questions will likely take a long time to answer, according to two of ASCE’s most prominent forensic structural engineers.
“I think it is way too early to tell,” said Roberto Leon, P.E., F.SEI, Dist.M.ASCE, the D.H. Burrows Professor of Construction Engineering in the Charles Edward Via Jr. Department of Civil and Environmental Engineering at Virginia Tech.
“It’s going to require a very careful forensic approach here, because I don’t think the building collapsed just because of one reason. What we tend to find in forensic investigations is that three or four things have to happen for a collapse to occur that is so catastrophic.”
Champlain Towers South, a 12-story condominium building along the Atlantic Ocean just north of Miami Beach, collapsed suddenly in the early morning of Thursday, June 24. Search-and-rescue work continued into the weekend, with forensic investigation set to begin soon thereafter. It’s the kind of investigative work that Glenn Bell, P.E., S.E., F.SEI, Dist.M.ASCE, has helped pioneer throughout his career.
“We have to be careful not to speculate too much on the potential causes, but I can say that one of the things I’m sure the investigators will be focusing on is: why, after standing for 40 years, did that structure collapse apparently without warning? That’s pretty extraordinary,” Bell said.
“Most catastrophic collapses happen either during construction or early in the life of the structure, indicating there were systemic, inherent structural problems. The fact that this happened with the building 40 years out will lead you to potentially different sets of causes and things to think about.”
The key questions to ask, Bell said, are:
- Did the structure change in any way over 40 years?
- Did something extraordinary happen in terms of the forces or the environment imposed on the building in that moment of collapse?
Bell referred to the investigation’s “melting ice cubes,” the priority evidence to note now before it’s gone – the material samples, eyewitness interviews, any video, and of course, the debris pile.
“The investigators will want to look at the debris pile very carefully, how things are piled up, how they’re stacked, what’s broken where, the juxtapositions of the different pieces,” Bell said. “That can help to give you some clues into where the failure might have initiated and how it progressed.
“I tell people training in forensic engineering to think about if you can imagine seeing a video of the structure collapsing in a certain sequence that’s along a certain failure hypothesis that you have. Would it wind up in a pile that you’re seeing on the ground here? Or is that pile not consistent with your theory?
“You look at the evidence. We like to say, to the trained eye, the structure will talk to you.”
The progressive nature of the failure, a so-called pancake collapse, also presents questions. Leon noted the importance of examining the parts of the building that remain standing, as well as examining every detail of the debris.
“As the building is pulled apart, they’re going to be looking at anything that is in there, trying to see if, for example, the reinforcing bars are in the correct location,” Leon said. “It matters very much in some structures of this type. If you have a 6-inch slab, let’s say, and the plans call for the rebar to be 5 inches from the bottom, but the rebar is instead at 3 inches, it actually might impact the strength. You can get a very substantial decrease in the shear capacity of the structure, which is critical to flat-plate structures of this type.
“It boils down to little details. And one has to be so careful.”
What is clear is that the investigation will take time.
“They run months at a minimum, often over a year, sometimes two years or more,” Bell said. “In this particular case, since it’s so complex, there are a lot of things to sort through and a lot of hypotheses to chase down.”
And for the people of Surfside, the recovery may take even longer.
Read ASCE’s statement about the building collapse.
Many years ago, I remember there was a building issue down in FL in which salt water from the ocean had been used to mix concrete. Although it seems very unlikely that this was the case here, could the presence of chlorides in the concrete slowly reacting with ambient moisture and humidity absorbed by the concrete over 40 years create sufficient corrosion of reinforcing steel that it finally led to collapse?
La falla, no tuvo que ver sólo con el concreto, que no está diseñado a la tensión, sino con el acero. El hecho que la sección de corte esté prácticamente uniforme, casi como cortada con un cincel, habla de una falla de cimentación.
This structure has a 40 year history with corrosion issues but never signs of major structural failure. From photos of this incident, there appears to have been a large movement (settlement) of the oceanside building which “dropped”. Do the building design borings show cavities that would have been flooded with ocean (salt) water? Has there been a rise of ground water elevation due to rising sea water level? Did the pile cap or if so designed, the footing, drop due to a cavity failure?
I probably have some memory lapse about what I remember about an unusual collapse of two precast concrete schools in England in the early 1970’s. I recall they were both twenty years old, across town from each other, the same design, and collapsed on the same day within hours of each other. Due to precast concrete details I think. So, although maybe somewhat unusual for this to happen after 40 years, but not an isolated case. Do you have any files on the two schools I referenced. ?
I am a retired Civil/Structural Engineer, and a Fellow of the ASCE. My heart goes out for the victims and their loved ones for the tragedy which came upon due to the partial collapse of the Champlain Towers South building . As an engineer, I feel I am obligated to share my thoughts as to what may have led to the disaster, and whether it will be helpful in any related investigation.
I tend to think that the building, which stood and inhabited upon for 40 years, was designed and constructed reasonably well enough for its intended purpose. Other than some red flags, related to certain major components of the building’s structure, apparently having been raised in an inspection report about 3 years ago there has been no serious concern or warning raised for such an impending disaster. I am neither a forensic scientist nor a forensic engineer. However, upon viewing the collapse remotely via media, and reading various news commentaries, it seems to me that the root cause for the disaster cannot be visible above ground. I believe it has something to do with the foundation, which had been slowly losing its strength over a period of time and suddenly gave away.
A question arises. Is it possible that the subsoil beneath the footprint of the building may be underlain by limestone? Caverns of various shapes and sizes are often present in the continuum of limestone bed. How deep is such bed? A 12-storied building is expected to have been built on pile foundations, possibly end-bearing type. If so, the piles must have been resting on the limestone bed. Limestone often undergoes a leaching process, and if caverns were originally present, it is likely that over a period of 40 years since the building was built, the caverns grew larger thereby weakening the limestone bed’s bearing capacity, more beneath the northeast side of the building . Weakening might have begun a few years ago, and finally became significant. If this phenomenon is considered to be a viable cause, other properties in the area may also be vulnerable. Involvement of a qualified geotechnical engineer during investigation will, in my opinion, be useful. This investigation will take months, may be years. As part of the investigation, testing of existing piles, either individually or as a group if pile caps are found undamaged, may be considered in order to compare their present capacity against design capacity.
Within 24 hours of the collapse, I reached the same conclusion based on one additional detail. The presence of standing water in the lower building levels and inadequate water proofing referenced in the 2018 report. I used to investigate sinkholes in a previous job in Maryland. I think water seeping into the foundation over 40 years has left voids around at least 4 of the deep pile supports. Water intrusion into the subgrade will erode the underlying limestone whether salt water or freshwater. I say four pile supports because that is what would be needed to double the load on the remaining columns and exceed the normal factor of safety in the design. Considering the number of similar buildings along the SE Florida coast, I don’t believe residents can wait 2 years (?) for answers. Stephen Baluch, PE (FL, MD, VT).
Lots of issues could happend salinity in water , differential settlement of piles due to consolidation or ice melting. Proper investigation should give clear ideas of failure mode! It obviously will take time.
We ,Structural would love to learn the from this type of collapse! It’s too early to say what we’re the causes of failure!
I am a Ph.D. structural engineer with more than 40 years of experience, and noticed that the type of building is a flat slab, no beams. However, don´t know if there were shear-walls, essential components in this type of structure. I didn´t saw any drop cap at the columns locations. It seems that most columns felt staying straight and in a pattern showing they belong to the same vertical axis. These observations could imply that an extra load on some upper floor, punched the slab through a column, transmitting the load to the next column producing the same failure, and so on. After the firs floor collapses, the floors below had the same effect in a magnified form up to reaching the ground. The other part of the building that collapses immediately after, was do to the fact that became unstable because the lack of vertical supports (columns).
I guess, the cause for settlement/failure/collapse was due to untreated wood/timber piles under pile caps were decayed/deteriorated/broken, possibly if the existing foundation is on timber piles.
I look forward to read the case study/fact findings by experts.
Also I feel that the remaining part of the structure should be shored up as a safety precaution, I’m sure officials/experts might have thought about it.
I have read these 8 comments with interest. Have all these comments based on review of all construction drawings and specifications from the designers, shop drawings, geotechnical engineer’s reports, material testing reports, inspection reports during construction by the engineer of record and a site inspection by the commentaters? It would be interesting to read their comments after they have done that .
Rightfully correct! Though these are obvious hypothesized theories, that distinction could be more clear. What is fact is the building failed suddenly.
I would hope that the investigation would look into the possibility of collateral damage due to construction of other nearby structures. Residents of the collapsed building has complained about vibration due to pile driving(?) on construction of nearby buildings. Surfside Building Officials claimed no jurisdiction as the location was in another City.
Many years ago the company I worked for did structural engineering design work for a very large Theme Park in Florida. Members of the design team including the Architect, our firm, and other design consultants went down the Florida to present plans and specifications to the permitting agency. I didn’t go.
When they came back the engineer from me company said that the Agency apologized saying it would take them one or two weeks to review the documents, a tiny fraction of the time it would take in Southern California where we were located. This caused us to view, rightly or wrongly, the Agency as a bunch of laid back good old boys.
Florida’s requirement for a 40 year evaluation is great and it was done. However, apparently, there were NO strong actions on it, including from the HOA and the responsible governmental AGENCY. In my opinion this probably would not have saved the building but evacuations could have saved many lives.
The photos show precast columns with visible rust thru the open crack in the column. Some photos show the parallel white lines joints on the soffit of one-way slabs. Has an Engineer investigating the collapse mention prestress?