Surfside Condo Wreckage Hints At First Signs of Possible Construction Flaw

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[Nytimes] Engineers who have visited or examined photos of the
wreckage of the Champlain Towers South condominium complex have
been struck by a possible flaw in its construction: Critical places
near the base of the building appeared to use less steel
reinforcement than called for in the project’s original design
drawings.

The observation is the first detail to emerge pointing to a
potential problem in the quality of construction of the 13-story
condo tower in Surfside, Fla., that collapsed last month, killing
at least 24 and leaving up to 121 still unaccounted for.

Reached by phone, Allyn E. Kilsheimer, a forensic engineering
expert hired by the town of Surfside to investigate the collapse,
said the investigation was still in its early stages. But he
confirmed there were signs that the amount of steel used to connect
concrete slabs below a parking deck to the building’s vertical
columns might be less than what the project’s initial plans
specified.

“The bars might not be arranged like the original drawings call
for,” Mr. Kilsheimer said in an interview. He said he would need to
inspect the rubble more closely to determine whether in fact the
slab-to-column connections contained less steel than expected.

R. Shankar Nair, a member of the National Academy of Engineering
and former chairman of the Council on Tall Buildings and Urban
Habitat, was among the other engineers who reviewed photographs and
saw inconsistencies between the design and the steel that remained
visible in the columns.

The investigation of the collapse could take months, so
preliminary observations and findings could change. Some engineers
said the possible shortfall in steel rebar in the relatively small
part of the building they had examined should be seen not as a
cause of the collapse, but it could potentially have been one of
several factors that allowed whatever initiated the problem to
accelerate into a catastrophic failure.

In raising questions about the amount of steel reinforcement in
the building, engineers pointed to three damaged columns in a
western section of the building that remains intact.

Those columns were part of an exterior deck that served as a
ground-level parking area adjacent to a pool plaza. It is a key
point of interest, because at least two witnesses have said they
saw part of the deck collapse in the minutes before the building
toppled.

The condo tower’s 1979 design drawings, provided by the town of
Surfside and reviewed by structural engineers and The New York
Times, indicate that the vertical columns in many parts of the
building were supposed to provide a critical structural connection
to horizontal slabs, embedded with eight rods of reinforcing steel
— four in one direction, four in the other — near the tops of the
slabs. But the reinforcing rods in the parking area, left exposed
in many places after the parking deck slab slammed down to the
lower level, appear to be fewer in number.

On most faces of the exposed slab, only two pieces of rebar can
be seen, half of what would be expected. Three bars are visible on
another.

Concrete can support tall buildings only when it is bolstered
with steel reinforcement. Concrete is a strong material “in
compression,” as engineers put it — in supporting weight above it,
for example. But it is far less effective “in tension,” or holding
things together when forces would tend to pull the concrete apart.
Embedding steel adds that essential tensile strength, and sound
design calculations, backed up by building codes, specify how much
steel is needed, depending on the type, size and other features of
a building.

Mr. Kilsheimer cautioned that it is common in construction for
the final product to differ from drawn designs.

He also said more analysis would be needed to assess whether the
amount of steel reinforcement used was a factor in the collapse. He
is working on computer modeling to address that question, he
said.

Some of the engineers who said there was less steel visible in
the wreckage than would have been expected based their judgment on
a close reading of the design documents for the building.

Mr. Nair, who has over 50 years of design experience, said it
was clear from images of the three damaged columns that the
structures did not appear to contain the expected amount of
steel.

The design, he said, called for running a number of steel bars
through the slab of the parking deck, at least four of which would
thread in each direction through each of the columns that held both
the garage and the rest of the structure on their shoulders. But
photos show far fewer passing through the columns, he said.

“There does not appear to be enough steel connecting the slab to
the columns,” Mr. Nair said. “What we see out there seems
inconsistent with what the drawings show.”

Engineers cautioned that investigators would need to examine the
components more thoroughly to rule out the possibility that
additional rebar was present but was sheared off in the fury of the
collapse. Mr. Nair and other engineers also said it was possible
that some bars could have been placed very close to the columns’
edges and left little or no trace that they had been present.

John Pistorino, a consulting engineer in the Miami area who has
been in the industry for decades, said that in general, a city
building inspector or someone hired by the builder would have
examined the placement of steel before concrete was poured.

It is not clear from immediately available records whether the
original design of the building met or exceeded building code
standards at the time of construction in 1981. Most codes around
the country have been strengthened since then, requiring more steel
reinforcement.

Florida’s inspection standards were bolstered in the 1980s after
the collapse of a building under construction in the city of Cocoa
Beach, near Orlando. The standards now require the hiring of an
independent inspector for larger buildings.

The development of the Champlain Towers South project was led by
Nattel Construction, a firm that has been defunct for 20 years. The
company’s leader, Nathan Reiber, died in 2014.

In recent years, the Champlain Towers South complex had been in
line for extensive renovations, with a 2018 engineering report
identifying rusting steel, crumbling concrete and a flawed pool
deck design that would cost millions to fix.

So deteriorated was some of the concrete that during an initial
repair project in October, crews were unable to complete scheduled
work around the pool. Deterioration had penetrated deep into some
components, and excavating it could have affected the stability of
that part of the structure, according to a report from the
engineering firm Morabito Consultants.

During the collapse, the slab in question at Champlain Towers
South, which was suspended in part on the three damaged columns
that are still visible, fell to the parking level, one floor below
ground level. Inadequate steel “could explain a slab failure — the
slab letting go and dropping down,” Mr. Nair said. But he said he
did not believe the tumbling slab could have been the initial cause
of the larger collapse.

Still, attention has focused on the area around the parking
garage slab and pool deck. Video of the collapse suggests that the
initial failure occurred somewhere near the bottom of the building,
where the garage and deck are located.

In addition, other video from outside the building, taken
shortly before the collapse, shows gushing water and what appears
to be rubble near the entry ramp to the parking garage.

Gabriel Nir, who lived in Apartment 111, on the first floor of
the portion of the building that collapsed, said he came home
before 1 a.m. and noticed water pooling at the base of the entry
ramp, although that was not uncommon. He did not see any rubble at
that point, he said in an interview.

When he and his mother entered their apartment, they heard
banging noises that sounded as if they were coming from above.

“We thought people were doing construction,” Mr. Nir said. They
initially shrugged it off, but it continued for several minutes,
growing more intense.

Mr. Nir said his mother went to the lobby to complain about the
noise. He was in the kitchen making food when he heard loud
rumbling and saw a cloud of dust coming from the area of the pool
deck. He and his sister hurried to the lobby to join their mother
and encouraged a security guard to call 911.

Then, walking outside, Mr. Nir saw that a part of the
ground-level portion of the parking garage and pool deck — the part
that engineers are now scrutinizing — had dropped. He also saw
pipes bursting, he said, followed eventually by the collapse of
about half the building. He and his family members ran from the
site, surviving even as their condominium was crushed at the base
of the collapse.

Engineers said it seemed unlikely that having less rebar would
trigger a collapse in and of itself, even factoring in significant
deterioration over many years. An inherent safety factor built into
most projects would mean that a slight reduction in steel content
would not necessarily lead to disaster.

The rebar in the Champlain Towers South slabs is a complex mesh,
with crisscrossing sets of long, parallel bars set into a plane
near the top of the slab and more reinforcement close to the
bottom.

Because it is difficult to say which of the lower segments of
rebar would have gone through columns, and would therefore be
visible in the wreckage after the slab dropped, some engineers have
focused so far on the upper bars. The most important clues to what
happened are probably still buried in the rubble.

“We have a whole bunch of issues that we think might be part of
or the trigger of what happened,” Mr. Kilsheimer said.

The unexpectedly low amount of rebar visible after the collapse
of the parking slab was not the only potential problem with steel
reinforcement that engineers noticed in their initial reviews.

Dawn E. Lehman, a professor of structural engineering at the
University of Washington, noted that rebar could be seen dangling
from parts of the remaining structure, pulled clean from the
concrete. She said that could indicate that in some places, the
concrete was damaged and the steel might not have had a sufficient
bond with the concrete. This could have several explanations, she
said, including corrosion, concrete deterioration, shear damage to
the concrete or the use of a type of reinforcing rebar with weaker
bonding properties.

Mr. Kilsheimer said he hoped to get a closer look at the
remainder of the building in the coming days, to better assess its
components. There have been concerns that the remaining structure
is a hazard. Mr. Kilsheimer said a computer analysis suggested that
the northern part of it could be at risk of collapse in high
winds.

Concrete and steel in the building will eventually be tested,
Mr. Kilsheimer said, and investigators will go below ground to
examine the soil and test the area with borings. They will model
the building with computers and piece together components recovered
from the rubble at an off-site storage facility.

Solving the mystery, he said, is like starting with several
puzzles, “throwing them up in the air, mixing them up with a broom,
and then trying to figure out which piece goes to which
puzzle.”