Due to a design oversight and changes during construction, the building as initially completed was structurally unsound.
For his original design, LeMessurier calculated wind load on the building when wind blew perpendicularly against the side of the building—wind from due north, east, south, or west—all that was required by New York building code.
Such winds are normally the worst case, and a structural system capable of handling them can easily cope with wind from any other angle.
Thus, the engineer did not specifically calculate the effects of diagonally-oriented "quartering winds" (northeast, northwest, southeast, or southwest).
In June 1978, prompted by discussion between a civil engineering student at Princeton University, Diane Hartley, and design engineer Joel Weinstein, LeMessurier recalculated the wind loads on the building, this time including quartering winds.
This recalculation revealed that with a quartering wind, there was a 40% increase in wind loads, resulting in a 160% increase in the load at the chevron brace connection joints.

LeMessurier's original design for the chevron load braces used welded joints.
However, during construction, builder Bethlehem Steel was approved to use bolted joints to save labor and material costs.
LeMessurier's firm approved the change, although this was not known to LeMessurier himself.
The original welded-joint design had ample strength to withstand the load from straight-on wind, with enough safety margin to withstand the higher loads from quartering wind; however, the load from a 75 miles per hour (121 km/h) hurricane force quartering wind would exceed the strength of the bolted-joint chevrons.
The bolts could shear and the building could collapse.
Wind tunnel tests with models of Citigroup Center revealed that the wind speed required to bring down the building would occur every 16 years on average.

For occupant comfort, the building has a tuned mass damper, which also negates much of the wind load.
The damper is electrically activated, so if power failed, for example during a hurricane, the damper might not turn on, and a much lower-speed wind would suffice; wind of this speed occurs on average once in 16 years.
LeMessurier also discovered that his firm had used New York City's truss safety factor of 1:1 instead of the column safety factor of 1:2.
These factors, combined, put the building in critical danger.
The problems were discovered in June, the beginning of hurricane season, and had to be corrected quickly.

LeMessurier reportedly agonized over how to deal with the problem.
If the issues were made known to the public, he risked ruining his professional reputation.
He approached the architect (Hugh Stubbins) first, and then Citicorp.
He advised them to take swift remedial action.
Ultimately, he persuaded Citicorp to repair the building without informing the public, a task made easier by a then-ongoing press strike.

For the next three months, construction crews working at night welded 2" steel plates over each of the skyscraper's 200 bolted joints.
They worked during the night, after each work day, almost unknown to the general public.
Six weeks into the work, a major storm (Hurricane Ella) was off Cape Hatteras and heading for New York.
With New York City hours away from emergency evacuation, the reinforcement was only half-finished.
Ella eventually turned eastward and veered out to sea, buying enough time for workers to permanently correct the problem.
As a precaution, Citicorp did work out emergency evacuation plans with local officials for the immediate neighborhood.

Because nothing happened as a result of the engineering gaffe, the danger was kept hidden from the public for almost 20 years.
It was publicized in a lengthy article in The New Yorker in 1995.
Due to a design oversight and changes during construction, the building as initially completed was structurally unsound. For his original design, LeMessurier calculated wind load on the building when wind blew perpendicularly against the side of the building—wind from due north, east, south, or west—all that was required by New York building code. Such winds are normally the worst case, and a structural system capable of handling them can easily cope with wind from any other angle. Thus, the engineer did not specifically calculate the effects of diagonally-oriented "quartering winds" (northeast, northwest, southeast, or southwest). In June 1978, prompted by discussion between a civil engineering student at Princeton University, Diane Hartley, and design engineer Joel Weinstein, LeMessurier recalculated the wind loads on the building, this time including quartering winds. This recalculation revealed that with a quartering wind, there was a 40% increase in wind loads, resulting in a 160% increase in the load at the chevron brace connection joints.
LeMessurier's original design for the chevron load braces used welded joints. However, during construction, builder Bethlehem Steel was approved to use bolted joints to save labor and material costs. LeMessurier's firm approved the change, although this was not known to LeMessurier himself. The original welded-joint design had ample strength to withstand the load from straight-on wind, with enough safety margin to withstand the higher loads from quartering wind; however, the load from a 75 miles per hour (121 km/h) hurricane force quartering wind would exceed the strength of the bolted-joint chevrons. The bolts could shear and the building could collapse. Wind tunnel tests with models of Citigroup Center revealed that the wind speed required to bring down the building would occur every 16 years on average.
For occupant comfort, the building has a tuned mass damper, which also negates much of the wind load. The damper is electrically activated, so if power failed, for example during a hurricane, the damper might not turn on, and a much lower-speed wind would suffice; wind of this speed occurs on average once in 16 years. LeMessurier also discovered that his firm had used New York City's truss safety factor of 1:1 instead of the column safety factor of 1:2. These factors, combined, put the building in critical danger. The problems were discovered in June, the beginning of hurricane season, and had to be corrected quickly.
LeMessurier reportedly agonized over how to deal with the problem. If the issues were made known to the public, he risked ruining his professional reputation. He approached the architect (Hugh Stubbins) first, and then Citicorp. He advised them to take swift remedial action. Ultimately, he persuaded Citicorp to repair the building without informing the public, a task made easier by a then-ongoing press strike.
For the next three months, construction crews working at night welded 2" steel plates over each of the skyscraper's 200 bolted joints. They worked during the night, after each work day, almost unknown to the general public. Six weeks into the work, a major storm (Hurricane Ella) was off Cape Hatteras and heading for New York. With New York City hours away from emergency evacuation, the reinforcement was only half-finished. Ella eventually turned eastward and veered out to sea, buying enough time for workers to permanently correct the problem. As a precaution, Citicorp did work out emergency evacuation plans with local officials for the immediate neighborhood.
Because nothing happened as a result of the engineering gaffe, the danger was kept hidden from the public for almost 20 years. It was publicized in a lengthy article in The New Yorker in 1995.
Citigroup Center