Citigroup Center

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This article is about the Citigroup Center in New York. See also Citigroup Centre of London and Citigroup Centre of Sydney.

Citigroup Center

Location	601 Lexington Avenue New York, New York, USA
Constructed	1974-1977
Height	279 m (915 ft)
Stories	59
Architect	Stubbins Associates, Emery Roth & Sons
Structural Engineers	Le Messurier Consultants; Office of James Ruderman
Cost	$195 million (USD)

The Citigroup Center is one of the largest skyscrapers in New York City, United States, located at 601 Lexington Avenue between 53rd Street and 54th Street in midtown Manhattan. The 59-floor, 915-foot (279 m) building is one of the most distinctive and imposing in New York's skyline, with a 45° angled top and a unique stilt-style base. It contains 1.3 million square feet (120,000 m²) of office space, and the 45-degree angle at the top of the building was originally intended to contain solar panels to provide energy (this idea was eventually dropped, however). It was designed by architect Hugh Stubbins Jr. for Citibank, and was completed in 1977.

1 History
2 Early engineering details
3 The building's engineering crisis of 1978
4 Other notable features
5 References
6 External Links

[edit] History

The site was originally occupied by St. Peter's Evangelical Lutheran Church which was founded in 1862. In 1905 the church moved to the location of 54th Street and Lexington Avenue.

[edit] Early engineering details

From the very beginning, the Citigroup Center was an engineering challenge. When planning for the skyscraper began in the early 1970s, the northwest corner of the proposed building site was occupied by St. Peter's Lutheran Church. The church allowed Citicorp to build the skyscraper under one condition: 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, because the church wanted to remain on the site of the new development, near one of the intersections. Architects wondered at the time if this demand was too much, and if the proposal could even work.

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. Each chevron would redirect the massive loads to their center, then downward into the ground through the uniquely-positioned columns.

[edit] The building's engineering crisis of 1978

Changes during construction led to a finished product that was structurally unsound. In 1978, prompted by a question from a student, LeMessurier discovered a potentially fatal flaw in the building's construction: the skyscraper's bolted joints were too weak to withstand 70-mile-per-hour (113 km/h) wind gusts at specific angles.

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 catastrophic failure due to bolt failure. 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 16 years.

LeMessurier reportedly agonised over how to deal with the problem - making it known to the wider world risked ruining his professional reputation. But with hurricane season fast approaching, he took the bold decision to approach Citicorp directly, and advise them of the need to take swift remedial action. He convinced Citicorp 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 (1978 Atlantic hurricane season), was off Cape Hatteras and heading for New York. With only half the reinforcement finished, New York City was hours away from emergency evacuation. Luckily, Ella turned eastward and veered out to sea, buying enough time for workers to permanently correct the problem.

Despite the fact that nothing actually happened as a result of the engineering gaffe, the crisis was kept hidden from the public for almost 20 years. LeMessurier's act of alerting Citicorp to the problem inherent in his own design is now used as a textbook example of ethical behavior.

After the modifications of the bolted members were completed, the building is now generally considered to be one of the most structurally sound skyscrapers in the world.

[edit] Other notable features

To help stabilize the building, a tuned mass damper was placed in the mechanical space at its top. This substantial piece of stabilizing equipment weighs 400 tons (350 metric tons) and has a volume of 255 cubic feet (7 m³). Designed to counterbalance the effects of wind by making the building sway, it is a concrete block that slides on a thick layer of oil and converts the kinetic energy of the building into friction. This mass reduces the building's movement from wind deflection by 50%. Citigroup Center was the first skyscraper in the United States to feature a tuned mass damper.

The building features double-deck elevators, which are separated to serve only odd or even floors.

It is one of the tallest buildings in New York City.

The Citigroup Center was built on the site of 53rd Street and and 3rd Avenue - which was immortalised in a song by punk rock band The Ramones. It was released February 1976. The song, written by Dee Dee Ramone, refers to a famous spot for male prostitution in New York where it is speculated that Ramone himself had worked as a "hustler".