Possible Failure Point Emerges in Miami Building
Collapse
Some engineers looking at the failure of a 13-story
condo tower in Florida said the collapse appeared to have begun somewhere near
the bottom of the building.
By James
Glanz, Anjali Singhvi and Mike Baker
June 27,
2021
The
investigation into what may be the deadliest accidental building collapse in
American history has just begun, but experts who have examined video footage of
the disaster outside Miami are focusing on a spot in the lowest part of the
condominium complex — possibly in or below the underground parking garage —
where an initial failure could have set off a structural avalanche.
Called
“progressive collapse,” the gradual spread of failures could have occurred for
a variety of reasons, including design flaws or the less robust construction
allowed under the building codes of four decades ago, when the complex was
built. But that progression could not have occurred without some critical first
failure, and close inspections of a grainy surveillance video that emerged in
the initial hours after the disaster have given the first hints of where that
might have been.
“It does
appear to start either at or very near the bottom of the structure,” said
Donald O. Dusenberry, a consulting engineer who has investigated many
structural collapses. “It’s not like there’s a failure high and it pancaked
down.”
The early
examinations came as rescuers on Sunday spent a fourth day pushing through the
enormous heap of debris created when half the 13-story building, Champlain
Towers South, fell away early on Thursday. The death toll climbed to nine as
additional remains were found, and more than 150 people remained unaccounted
for.
While a
number of bridges, overpasses and buildings under construction fail each year,
the catastrophic collapse of an occupied building — absent a bomb or an
earthquake — is rare, and investigators are struggling to understand how it
could have come with so little urgent warning.
“It would
be like a lightning strike happening,” said Charles W. Burkett, the mayor of
Surfside, Fla., where the collapse occurred. “It’s not at all a common
occurrence to have a building fall down in America,” he said. “There was
something very, very wrong with this situation.”
The
National Institute of Standards and Technology, a federal agency, was sending
scientists and engineers to do a preliminary review, hoping to identify and
preserve materials that might help understand the collapse. Officials said they
expected a number of local, state and federal agencies also to be involved in
the inquiry, though it was not clear which agency would lead the effort.
The search
for an explanation comes with a sense of urgency not only for sister buildings
near the complex but also for a broad part of South Florida, where a necklace
of high-rise condos, many of them decades old, sits on the edge of the Atlantic
Ocean, enduring an ever-worsening barrage of hurricane winds, storm surge and
sea salt.
Structural
engineers were shocked that a building that had stood for decades would
abruptly crumble on an otherwise unremarkable summer night.
But three
years before the deadly collapse, 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 building.
While no
definitive conclusions could be drawn from the surveillance video, which was
shot from a distance and reveals only one perspective of the disaster, some of
the engineers reviewing it last week said it seemed to suggest that the failure
began at a specific point near the bottom of the structure — perhaps as far
down as the parking garage beneath the building, or on the first few floors.
What the
2018 Engineer’s Report Found
The
structural survey pointed out damage in multiple areas of the below-ground
parking level. Some issues noted in the report are shown in italics.
From what
can be seen in the video, part of the structure first slumped, seemingly
falling vertically in one giant piece, as if the columns had failed beneath the
southern edge of the center of the building, not far from the pool. Like a
nightmarish avalanche, the failure quickly spread and brought down the entire
center of the building. Seconds later, a large section to the east also
toppled.
Mr.
Dusenberry, whose impressions matched those of several other structural
engineers who examined the video, said such a failure “would suggest a
foundation-related matter — potentially corrosion or other damage at a lower
level.” But he said it was not certain that corrosion was the culprit, and
added that “you certainly can’t rule out a design or construction error that
has survived for 40 years.”
One other
clue that a problem started at the bottom of the building: Immediately before
the collapse, one of the residents saw a hole of sorts opening near the pool.
Michael
Stratton said his wife, Cassie Stratton, who is missing, was on the phone with
him and was looking out through the window of her fourth-floor unit when, she
told him, the hole appeared. After that, the call cut off.
Rick De La
Guardia, an engineer based in Miami with experience in forensic investigation
of building component failures, said that the collapse could have also started
higher than the foundation, possibly on the second floor, based on his cursory
review of the columns in the floor plans and his review of the video.
Explanations
for an initial failure at the bottom of the building could include a problem
with the deep, reinforced concrete pilings on which the building sits — perhaps
set off by an unknown void or a sinkhole below — which then compromised the
lower columns. Or the steel reinforcing the columns in the parking garage or
first few floors could have been so corroded that they somehow gave way on
their own. Or the building itself could have been poorly designed, built with
substandard concrete or steel — or simply with insufficient steel at critical
points.
Evan Bentz,
a professor of structural engineering at the University of Toronto, said that
the best evidence so far had come from the video and some simple reasoning —
pointing a finger of suspicion at the supporting columns in the underground
parking garage.
“The
primary purpose of all the columns in the basement is to hold the structure up
in the air,” he said. “Because the structure stopped being held up in the air,
the simplest explanation is that the columns in the basement ceased to
function.”
The extreme
rarity of major building collapses in the United States deepens the mystery,
engineers said, especially considering that Champlain Towers South had remained
upright for four decades and had no obvious failure before much of it tumbled
to the ground.
“It stood
for 40 years and it collapsed relatively suddenly,” said Glenn R. Bell,
director of Collaborative Reporting for Safer Structures, a program in the
Structural Engineering Institute of the American Society of Civil Engineers.
“Why did it collapse at that moment?”
The 2018
report from the consultant, an engineer hired by the condo owners’ association
to examine the building, helped set in motion plans for a $12 million repair
project that had been set to start soon — more than two and a half years after
the building managers were warned about the structural damage.
The
corrosion of reinforcing steel identified in that report could have been a
critical issue if it occurred on or near the supporting columns and was
pronounced enough, Mr. Dusenberry said.
“If I were
an investigator, I would check this as an issue,” he said.
But other
structural engineers said that some level of corrosion was common in old
buildings and was unlikely to bring a building down on its own.
A crucial
clue is still unknown, Professor Bentz said, and “that would probably be found
under the debris where the collapse seemed to start.”
The
structural fiber of the building was largely reinforced concrete. That means
the floor slabs upon which apartments sat were made of concrete that was poured
around horizontal lengths of rebar, or stout steel rods, that provided critical
strength when the concrete dried. Likewise, the columns that held up the slabs
were created by pouring concrete around vertical stretches of rebar.
The
corrosion of the rebar in the slabs, as revealed in the 2018 report, was
probably significant only if it occurred in places where the slabs joined with
the columns, Mr. Dusenberry said. Corrosion there could have weakened the
connection to the columns, potentially leading to a failure, he said.
The same
idea holds for the reinforced concrete pilings — deeply buried, vertical
supports on which the entire building sat, said David Peraza, a structural
engineer at Exponent, an engineering and scientific consulting firm.
A previously
reported academic study showed that the entire coastline in the area of the
building has been settling, or sinking, at the rate of a couple of millimeters
a year. But the deep piles would have provided stability, Mr. Peraza said.
Danger
would emerge only if there had been something like a void or a sinkhole that
had caused one or several piles to settle downward and left the others
unchanged. That could have threatened the structure that sat atop those piles:
columns in the underground parking garage.
“Whether
there’s something geologically under the building that caused this, that’s
definitely something that’s got to be investigated,” Mr. Peraza said.
Another
possibility is improperly installed piles, he said.
One last
theory under consideration is the possibility that heavy construction next door
in 2019 could have damaged the Champlain Towers building. An email released by
the city on Sunday revealed that a member of the condo board had gone to the
city for help at the time, expressing “concerns regarding the structure of our
building.”
Town
officials declined to intervene, suggesting that the residents hire someone to
monitor any impacts.
Clues to
any of those problems will be clear only after the rescue operation ends,
recovery begins and engineers dig all the way to the bottom of the debris pile,
Mr. Peraza said. He added that investigators should also examine construction
documents that describe exactly how the piles were built.
Gregg
Schlesinger, a contractor and lawyer in Florida, said that cracks and a kind of
crumbling in the concrete known as “spalling,” also identified in the 2018
report, should have been a “red flag” if it seemed serious at the time. If that
were the case, he said, engineers should have dug deeper to find out what was
causing the deterioration.
“There are
questions that are relevant around spalling where the concrete falls off of the
structural elements,” Mr. Schlesinger said. “But no real research was done into
why this stuff was coming off the wall and what was causing that.”
The weeks
ahead will involve a meticulous dig to unearth clues that Mr. Dusenberry
likened to an archaeological excavation.
Engineers
will record each layer photographically, possibly with drones, before moving on
to the next.
Collapsed
portions of the building will most likely be taken apart piece by piece and
reassembled at another location where experts can assess them. They will also do
“petrography” on the concrete — studying it chemically and microscopically to
test its strength and quality. They will measure the thickness of the slabs and
columns and the positioning of the steel to see if it all matches the design
drawings.
Donna DiMaggio
Berger, a lawyer who represents the condo association, said that members of the
association board — the ones who survived — had been left dumbfounded and
hoping for answers. Nothing in the 2018 assessment presented to the board had
suggested that the building was at risk of collapse, she said, and the board’s
“deliberative” approach to doing necessary repairs had been based on the
assumption that there was time to do it right.
“The sense
of urgency is directly tied to the wording used and the consequences outlined
in the report,” Ms. Berger said. “All boards can do is rely on the advice of
the professional advisers that they engaged.”
Patricia
Mazzei and Joseph B. Treaster contributed reporting.
James Glanz
is a reporter on the International Desk. He was previously on the
Investigations Desk, and before that was the Baghdad bureau chief. He has a
Ph.D. in astrophysical sciences from Princeton University and joined The Times
as a science writer. @jamesglanz
Anjali
Singhvi is a graphics editor. She is a trained architect and holds a master's
degree in urban planning/urban analytics from Columbia University.
@singhvianjali
Mike Baker
is the Seattle bureau chief, reporting primarily from the Northwest and Alaska.
@ByMikeBaker
A version
of this article appears in print on June 28, 2021, Section A, Page 1 of the New
York edition with the headline: Emerging Clues Suggest Collapse Began at
Bottom. Order Reprints | Today’s Paper | Subscribe
Sem comentários:
Enviar um comentário