How one design flaw almost toppled a skyscraper Alex Gendler
In 1978, Diane Hartley was writing
her undergraduate architecture thesis
when she made a shocking discovery.
Her paper focused on the Citicorp Center,
a skyscraper in midtown Manhattan.
And after weeks poring over
the building’s plans,
she’d stumbled on a potentially
deadly mistake.
An oversight that threatened to topple
the 59-story tower
into one of New York City’s
most densely populated districts.
When it was built two years earlier,
Citicorp Center was one of the
world’s tallest buildings.
Its sloped roof was unique
in the city skyline,
but its more distinctive feature
lay at the base.
Since the construction site was already
occupied by St. Peter’s Lutheran Church,
the new skyscraper had to be built
on columns supporting it, like stilts.
Using stilts on a building’s corners
wasn’t unheard of,
but because the church stood
at the corner of the block,
these stilts had to be placed
at the center of each side.
While this novel design worried some
of the building’s backers,
chief structural engineer
William LeMessurier
took numerous precautions to ensure
the building’s stability.
The outside would consist
of v-shaped chevrons,
forming a strong exoskeleton
to support the skyscraper.
This external structure also made
the building much lighter,
meaning there’d be less weight
to support overall.
This design did leave the building
vulnerable to strong winds.
But LeMessurier had another
state-of-the-art solution—
a tuned mass damper.
This 400-ton counterweight was controlled
by computerized sensors
designed to counteract any swaying.
With these structures in place,
calculations showed that each side of the
building could withstand powerful winds.
And with all safety issues resolved,
the building opened for business in 1977.
But when Hartley was studying the tower
a year later, she noticed something odd.
It was true that each face of the building
could endure powerful winds.
And since a building’s broad sides
catch the most wind,
these would typically be the strongest
winds a building encounters.
However, the towers unique base
meant that winds
blowing on the building’s corners
were actually the bigger threat.
And since traditional designs didn’t
warrant safety calculations
for corner winds,
it seemed to Hartley that the threat
had gone unaccounted for.
When Hartley contacted LeMessurier’s
firm about the issue
they assured her the building was strong
enough to handle these winds.
But checking the plans again, LeMessurier
noticed an alarming detail.
A change approved without his knowledge
had replaced the exoskeleton’s
welded joints
with cheaper and weaker bolted joints.
This alone wasn’t enough to topple
the tower thanks to the mass damper.
But if a storm knocked out
the building’s power,
it would deactivate
the counterweight’s sensors,
leaving the building vulnerable to winds
of just 112 kilometers per hour.
Given available weather data,
a storm this strong had
a one-in-sixteen chance
of hitting New York City
every single year.
LeMessurier never told Hartley
what she’d uncovered.
In fact, everything he did next
was top secret.
After filling in the architects
and executives at Citicorp,
LeMessurier’s team worked with city
officials to craft a confidential plan.
Without warning the residents,
construction crews began
a string of night-time shifts
to reinforce the bolted joints.
This delicate work began
in mid-August 1978,
and was only halfway complete when
Hurricane Ella
approached the city in September.
City officials and Citicorp executives
planned an emergency evacuation
for a 10-block radius,
but at the last minute,
the hurricane veered out to sea.
These secret evacuation plans
were never used,
and the reinforcements were completed
just a month later.
Typically, it would’ve been impossible for
this covert construction to go unnoticed.
But the press was occupied
with a newspaper strike
spanning the length
of the reinforcement project.
In fact, the public didn’t learn how close
they’d come to disaster until 1995,
when a New Yorker article revealed the
story to the city, and to Diane Hartley.
Like LeMessurier, the article failed
to give credit where it was due,
but at least Hartley knew
that her homework had saved lives.