Citigroup CenterEdit profile
The Citigroup Center (formerly Citicorp Center) is one of the ten tallest skyscrapers in New York City, United States, located at 53rd Street between Lexington Avenue and Third Avenue in midtown Manhattan. The 59-floor, 915-foot (279-m) building contains 1.3 million square feet (120,000 m²) of office space, and 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. It was designed by architect Hugh Stubbins Jr. for Citibank, and was completed in 1977. The building is currently owned by Boston Properties, and is the tallest building that bears the Citigroup name.
The northwest corner of 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.
Early engineering details
From the 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 demolish the old church and 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.
Engineering crisis of 1978
Changes during construction led to a finished product that was structurally unsound. In June 1978, prompted by discussion between a Princeton University engineering student and design engineer Joel Weinstein, 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, which hit the building at a 45-degree angle. This oversight 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, Le Messurier 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.
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.
This knowledge, combined with LeMessurier's discovery that his firm had used New York City's truss safety factor of 1:1 instead of the column safety factor of 1:2, 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 was reported that LeMessurier agonized over how to deal with the problem, and, by making it known to the wider world, 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 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.
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.
In 2008, building owner Boston Properties began planning to rename the tower 601 Lexington Avenue. The company is also considering selling naming rights to the building.
- The roof of Citigroup Center slopes at a 45-degree angle because it was originally intended to contain solar panels to provide energy. However, this idea was eventually dropped because the positioning of the angled roof meant that the solar panels would not face the sun directly.
- 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). 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.
- The building features double-deck elevators, which are separated to serve only odd or even floors.
- The corporate headquarters of Citigroup, contrary to popular perception, are not located in the building, but across the street in 399 Park Avenue.
- 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 featured in the music video for Janet Jackson's "Runaway", in which she runs up the slanted roof.