Condo Wreckage Hints at First Signs of Possible Construction Flaw

[Leroy N. Soetoro]

https://www.nytimes.com/2021/07/03/us/florida-condo-collapse-steel-
rebar.html

Engineers studying the collapsed Florida tower said there seemed to be
less steel reinforcement in certain areas than would have been expected
from the 1979 design drawings.

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.

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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.”



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