Citigroup Center


The Citigroup Center is an office skyscraper in New York City, located at 53rd Street between Lexington and Third Avenues in midtown Manhattan. Built in 1977 to house the headquarters of Citibank, it is 915 feet tall and has 59 floors with 1.3 million square feet of office space.
The building is one of the most distinctive and imposing in New York City’s skyline, thanks to a 45° angled top and a unique stilt-style base. It was designed by architect Hugh Stubbins and structural engineer William LeMessurier. The building is currently owned by Boston Properties, and in 2009, was renamed 601 Lexington Avenue.

Design

The Citigroup Center is a 59-story, tower clad in glass and metal facade.
Structural engineer William LeMessurier designed the tower to be supported by four massive columns 114 feet high, positioned at the center of each side, rather than at the corners. This design allowed the corners of the building to cantilever 72 feet. To help accomplish this, LeMessurier employed a system of stacked load-bearing braces, in the form of inverted chevrons. Each chevron is designed to distribute tension loads to their center, then downward into the ground through the uniquely positioned columns.
The roof of Citigroup Center slopes at a 45-degree angle. Designers originally intended it to be terraces for apartments, and later revised it to contain flat-plate solar collectors, to produce hot water which would be used to dehumidify air and reduce cooling energy. However, they eventually dropped this idea because the positioning of the angled roof meant that the solar panels would not face the sun directly.

Notable features

The cantilever exists because the northwest corner of the building site is occupied by St. Peter's Lutheran Church. When Citicorp Center was built in the 1970s, the church allowed Citicorp to demolish the old church building and build the skyscraper under one condition: a new church would have to be built on the same corner, not attached 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. The church, at 619 Lexington Avenue with its entrance from 54th Street, has a theatre in its basement which is mainly used by the York Theatre.
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. The damper is designed to counteract swaying motions due to the effect of wind on the building and reduces the building's movement due to wind by as much as 50%. Citigroup Center was the first skyscraper in the United States to feature a tuned mass damper. In addition, in 2002, one of the columns was reinforced with blast-resistant shields of steel and copper as well as steel bracing to protect the building due to the possibility of a terrorist attack.
The building features double-deck elevators, which are separated to serve only odd or even floors. People can travel between odd and even floors using escalators.
Various plazas were included as part of the building's construction. One such plaza exists below the cantilever, where there is an entrance to the New York City Subway station at Lexington Avenue/51st Street, served by the.
From 1987 to 2009, the bank presented an annual toy train exhibition in the lower lobby.

History

Context

The northwest corner of the site was originally occupied by St. Peter's Evangelical Lutheran Church, which was founded in 1862. The original church building was sold and demolished to construct the Grand Central Terminal in 1903. In 1905, the church moved to the location of 54th Street and Lexington Avenue, where it remained until the building was purchased by First National City Bank in 1970.

Engineering crisis of 1978

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". 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 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 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 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.

Ethical questions

Architect Eugene Kremer has discussed the ethical questions raised in this case.
LeMessurier was criticized for insufficient oversight leading to bolted rather than welded joints, for not informing the endangered neighbors, for actively misleading the public about the extent of the danger during the reinforcement process, and for not informing architects or other structural engineers about the problem and solution for two decades. However, his act of alerting Citicorp to the problem in his design is now used as an example of ethical behavior in several engineering textbooks.
Kremer discusses six key points:
  1. Analysis of wind loads. Check all calculations and not rely just on building codes; these set minimum requirements and not the state of the art.
  2. Design changes. In this case change from welded to bolted connections. Changes are considered in the overall design context and by everyone involved and not a spur of the moment decision.
  3. Professional responsibility. To follow the codes of conduct for every chartered institution. LeMessurier did not consider the public safety first.
  4. Public statements. In this case the public statements issued by LeMessurier and Citigroup set out to mislead the public deliberately.
  5. Public safety. The public statement denied the public the right to ensure their own safety and to make their own critical decisions.
  6. Advancement of professional knowledge. Concealing this problem for almost 20 years prevented ethical and engineering learning that could have taken place.

    Sale and name/address changes

Former Citicorp Chairman Walter B. Wriston was reportedly behind the decision to acquire several low- and mid-rise buildings in the area, supposedly to buy out massage parlors and mom-and-pop stores in Midtown. In 1987, Citigroup sold two-thirds of its interest in the building, along with one-third of its interest in 399 Park Avenue, to Dai-Ichi Mutual Life Insurance Company for $670 million. In 2001, Citicorp sold its controlling stake in the building for $755 million to Boston Properties. Citigroup relocated its headquarters to 399 Park Avenue.
In 2008, building owner Boston Properties began the process of renaming the tower "601 Lexington Avenue". Renovation of the lobby resulted in relocation of the tower's entrance from 53rd Street to Lexington Avenue. All signage for Citigroup was removed from the building and surrounding block. The name change became effective in 2009. The company is also considering selling naming rights to the building.

Renovation plans

In March 2017, Gensler revealed designs for a major renovation around the base of the tower. The renovation will include a redesigned entry plaza and a new atrium space.

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