It is one of the most distinctive and imposing in New York's skyline, thanks to a 45° angled top and a unique stilt-style base (stilts are poles, posts or pillars used to allow a person or structure to stand at a distance above the ground).

The stilt style was necessitated as the church earlier existing at proposed site of construction allowed Citicorp to demolish the old church and build the skyscraper under the condition that a new church would have to be built on the same corner, with no connection to the Citicorp building and no columns passing through it.

To cater to this requirement, Structural engineer William LeMessurier set the 59-story tower on four massive 114-foot (35-m)-high columns, positioned at the center of each side, rather than at the corners. This design allowed the northwest corner of the building to cantilever 72 feet (22 m) over the new church.

To accomplish these goals LeMessurier designed a system of stacked load-bearing braces, in the form of inverted chevrons (which have inverted V-shaped pattern). Each chevron would redirect the massive loads to their center, then downward into the ground through the uniquely positioned columns

The Blunders:

a) In June 1978, prompted by discussion between a University engineering student and design engineer, LeMessurier recalculated the wind loads on the building. In the original design, the engineer calculated for wind loads that hit the building straight-on, but he did not calculate for quartering wind loads (i.e. force applied 45 degrees [quartering] to the main axis of the building), which hit the building at a 45-degree angle. This recalculations done by LeMessurier revealed that quartering wind loads resulted in a 40% increase in wind loads and a 160% increase in the load at all connection joints. While this discovery was disturbing, LeMessurier was not overly concerned because the original design was padded by a safety factor (which in most cases was 1:2) and the design allowed for some leeway.

b) Later that month, LeMessurier met for an inquiry on another job where he mentioned the use of welded joints in the Citicorp building, only to find a potentially fatal flaw in the building's construction: the original design's welded joints were changed to bolted joints during construction, which were too weak to withstand 70-mile-per-hour (113 km/h) quartering winds. While LeMessurier's original design and load calculations for the special, uniquely designed "chevron" load braces used to support the building were based on welded joints, a labor- and cost-saving change altered the joints to bolted construction after the building's plans were approved.

The engineers did not recalculate what the construction change would do to the wind forces acting on two surfaces of the building's curtain wall at the same time; if hurricane-speed winds hit the building at a 45-degree angle, there was the potential for failure due to the bolts shearing. The wind speeds needed to topple the models of Citigroup Center in a wind-tunnel test were predicted to occur in New York City every 55 years. If the building's tuned mass damper went offline, the necessary wind speeds were predicted to occur every 16 years.

Tuned mass damper  also known as an active mass damper (AMD) or harmonic absorber, is a device mounted in structures to reduce the amplitude of mechanical vibrations

c) 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.

Response:

The discovery of above engineering blunders meant that the building was in critical danger. The discovery of the problem occurred in the month of June, the beginning of hurricane season. The problem had to be corrected quickly.

It is reported that LeMessurier agonized over how to deal with the problem. If he made it known publicly, he risked ruining his professional reputation.

He approached Citicorp directly and advised them of the need to take swift remedial action, ultimately convincing the company to hire a crew of welders to repair the fragile building without informing the public, a task made easier by the press strike at that time.

For the next three months, a construction crew welded two-inch-thick steel plates over each of the skyscraper's 200 bolted joints during the night, after each work day, almost unknown to the general public. Six weeks into the work, a major storm (Hurricane Ella) was 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.

Because nothing happened as a result of the engineering gaffe, the crisis was kept hidden from the public for almost 20 years. It was publicized in a lengthy article in The New Yorker in 1995. 

LeMessurier was criticized for insufficient oversight leading to bolted rather than welded joints, for misleading the public about the extent of the danger during the reinforcement process, and for keeping the engineering insights from his peers for two decades.

However, his act of alerting Citicorp to the problem inherent in his own design is now used as an example of ethical behavior in several engineering textbooks.