Citigroup Center Refrences: 1. 2.

The Center (formerly Citicorp Center and now known as 601 ) is

one of the ten tallest skyscrapersin , United States, located at between Lexington Avenue and in midtown . 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. and structural engineer William LeMessurier for , and was completed in 1977. The building is currently owned by , and in 2009, was renamed 601 Lexington Avenue.


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 and Lexington Avenue. [edit]Early engineering details

Night View of Citigroup Center from GE Building.

From the beginning, the Citigroup Center was an engineering challenge. When planning for the began in the early , 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 would make the proposal unfeasible. 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 invertedchevrons. Each chevron would redirect the massive loads to their center, then downward into the ground through the uniquely positioned columns. [edit]Engineering crisis of 1978 Changes during construction led to a finished product that was structurally unsound. In June 1978, prompted by discussion between a 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, 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. 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.

Base of the Citigroup Center

View from the street

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 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 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 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 in 1995.[2] LeMessurier was criticized for insufficient oversight leading to bolted rather than welded joints, for not only not informing the endangered neighbors but actively 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.[3] 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. [edit]Name change In 2008, building owner Boston Properties began planning to rename the tower 601 Lexington Avenue.[4] Renovation of the lobby resulted in relocation of the tower's entrance from 53rd Street to Lexington Avenue.[5] All signage for Citigroup was removed from the building and surrounding block. The name change became effective in 2009.[6][7] The company is also considering selling naming rights to the building.[8] [edit]Notable features

Public lobby

 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%.[9] Citigroup Center was not the first skyscraper in the United States to feature a tuned mass damper. That distinction belongs to the John Hancock Tower in Boston, MA.  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 .  The building is visible in numerous television shows and movies (often as part of a wider panoramic shot of New York City), notably during the opening credits of the long- running NBC police procedural and legal drama Law & Order.  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.[10] [11]  From 1987 to 2009, the bank presented an annual toy train exhibition in the lower lobby. [edit]In Popular Culture

A season one episode of the TV show NUMB3RS, "Structural Corruption," involves a fictional building with faults almost exactly paralleling the crisis of the Citigroup Center. Like the Citigroup Center, a college student studying the fictional Cole Center finds the building to have inadequate strength when subjected to quartering winds. However, the insufficient welds in the Cole Center lie in the foundation, and a tuned mass-damper (not present in the original construction) is added to make the building safe.

CITY CROP failure


A fatal flaw was discovered only a year after the completion of the Citicorp building by the structural engineer himself, William J. LeMessurier (pronounced La Measure). The The fifty-nine story building could only withstand a sixteen-year-storm instead of the fifty-five-year storm it was designed for, every year there was a one in sixteen chance that the building would experience total collapse. It was the summer of 1978 and hurricane season was fast approaching, a severe storm could topple the building just a year after its completion. There were many contributors to the inadequate structural design of the Citicorp building, including changes made to the original design of the connections. William J. LeMessurier had three options, silence, suicide, or setting the events in to motion that would ensure the safety of thousands of building occupants in tandem with sacrificing his reputation and career. In July of 1978 LeMessurier mobilized an effort to repair all two-hundred joints of the steel-frame structure and employed subject matter experts to oversee the quality of welds, maintain the tuned-mass damper, provide weather forecasts three times a day, and monitor strain gauges on the building. Two-thousand Red Cross workers were also brought in to assist with the evacuation plan if needed. Two months later the repairs were complete making the Citicorp Building one of the most structurally sound in the world, able to withstand a seven-hundred- year storm. This secret retrofit valued at twelve-million dollars U.S. wasn't made public until almost twenty years after its completion and LeMessurier's reputation remained unscathed. (Morgenstern, 1995)

Figure 1: Citicorp Building, New York, New York

Innovations On its completion the Citicorp building was the seventh-tallest building in the world. The building site located on the east side of Lexington Avenue between Fifty-third and Fifty-fourth Streets shared the block with St. Peter's Lutheran Church. Citigroup wanted the whole block, and as part of the land acquisition negotiation Citigroup agreed to demolish and build a new church on site in exchange for the air-rights above the church. This meant that the Citicorp building was now allowed to horizontally project itself over the church. LeMessurier repositioned the columns so that they were located at half-way points under the exterior walls. The Citicorp building was raised up on 9-story stilts, with a 72 foot cantilever over the new church. (Morgenstern, 1995)

The base columns were made robust to handle the shear loads caused by wind forces and to provide a more aesthetically pleasing proportion; the columns were much larger than structurally necessary.It occurred to LeMessurier to make efficient use of these columns and run them up the length of the building therefore using them as a direct load path to the building's foundation. To direct the forces to the columns LeMessurier designed diagonal braces that carried the member forces to the center of each exterior wall; where the main column was located. (AR, 1976)

Another issue had to be addressed by the engineer; for a building of its height Citicorp was relatively lightweight making it more dynamically excitable. Several methods were considered to address the excessive deflections that would be caused by wind forces; they included increasing the stiffness of the building and semi-isolating the building from the ground by anchoring it on flexible moorings, but were deemed unfeasible. Le Messurier felt that the best option was to use a Tuned-Mass-Damper (TMD), a tuned- counterweight of 400 tons. This made the Citicorp building the first building to have mechanical aid as part of its structural design. (Gannon,1985)

Structural Design of Citicorp At the time the Citicorp building was designed, the New York Building Code only considered the effects of diagonal winds on tall structures because this type of loading was critical for the traditional column layout; columns on each of the four corners. Until adoption of a new code in 1968, New York had required that all structures be designed "to resist, in the structural frame, horizontal wind pressure from any direction." In the early 1970s the prevailing standard of care was to design tall structures considering the effects of quartering winds.Tall steel frame buildings had been around for over half a century by the 1970s and the technology of making rigid connections with steel members was fully developed. (Kremer, 2002)

The steel-frame structure was comprised of a system of stacked load-bearing braces (V pattern) which redirected loads to the center and down the four columns the extended up the entire length of the building. These 48 braces are arranged in a six-tiered pattern on each side of the building and pick up the loads of gravity in increments and guide it down the column by diagonal in compression. These braces span 8 stories in height and were very conservatively designed to be erected in pieces joined by full penetration welds. These braces were also cantilevered seventy-two feet from each corner to support the edges of the tower floors. The central service core supporting the tower was comprised of four-massive 114 foot (35 meters) high columns. (ASCE, 2007)

During the 1970s, the building was considered lightweight in comparison to buildings of similar size. Building deflections were a structural concern and therefore wind tunnel testing was performed on the structure by one of the world's leading wind-tunnel-testing facilities of its time. The area modeled in detail was 4000 feet in diameter; roughly four city blocks in each direction. With the TMD included in the model the building achieved a 4% damping mechanically, this was meant to reduce the building's movement due to wind by as much as 50%. Located 59 stories above the sidewalk, the damper room was an enormous 80 feet by 80 feet. Dominating the center of the room was a concrete block twenty-nine feet squared and eight feet thick floated on preassurized oil bearings. The TMD cost was $1.5 million, less than 1% of the total building cost. This computer- controlled damper starts working when the outside wind velocity reaches about 25 mph or when the building sensors (accelerometers) detect a sway larger than 3 milli g's (three thousandths of the acceleration of gravity). When this happens, a 50-horsepower oil pumps thumps into action, and slowly the block begins to rise. For minutes later, the block has lifted 3/4 inch. Now it's standing on 12 hydraulic footpads the size of manhole covers, each separated from the 30-square-foot polished steel floor beneath the block by a near-frictionless layer of oil. The block's movement is inhibited by two sets of pneumatic gas springs designed by Neil Pattersen. The springs are made of pistons that could go +/- 3 1/2 feet and with a force of 150 kips to work in both directions. The block and stiffness of the springs have been tuned to the natural oscillation period of the building, 6.7 seconds. The block moves 90 degrees out of phase, to absorb the energy of the swaying structure, this out-of-phase movement damps the building sway. The damper produces an improvement, estimates LeMessurier, equivalent to quadrupling the building's structural bracing. That's a saving of nearly $4 million. (Gannon,1985)

The Failure While working on a new building design in the spring of 1978, LeMessurier planned once to again implement diagonals. During a meeting with the steel fabricator the question came up on the connections of these diagonals; field welded full-penetration welds are expensive. LeMessurier called his associate to confirm the success on the field-welded connections used on the Citicorp building and was told that the welded connections were replaced by bolted connections designed by Bethleham Steel for a savings to the bank of $250,000. This was common practice for steel erectors at the time and the new joint design was based on the loads provided by LeMessuriers' Cambridge office. There is a discrepancy in the account of events, whether or not LeMessurier was aware of the change from bolts to welds. McNamara claims LeMessurier knew of the switch to bolts, this statement has some credibility since LeMessurier's Cambridge office was the one that provided the design forces to .

In June of 1978 LeMessurier received a call from an architectural student from New Jersey who was assigned the Citicorp building as a project. The student told LeMessurier that his professor says columns should always be put on the building corners.LeMessurier explained the site restrictions of the project and told the student that due to the building's innovative column layout, it was particularly resistant to quartering or diagonal winds. After hanging up he gave this final statement some thought and chose to further investigate. In conventional buildings; columns on the four corners, the worst loading case is when the wind is pushing straight on the building. Upon investigating he discovered that the worst loading case on the Citicorp building was not when the wind is perpendicular but when the wind is on the diagonal . LeMessurier calculated the forces in the diagonals by method of virtual work and determined that with diagonal wind the stresses on half the diagonals vanish and double on the other half, a "very peculiar behavior." (LeMessurier, 1995)

This critical design flaw may not have been as crucial if the original full-penetration welds had been used but since the New York design firm ignored quartering winds when choosing bolts over welds for the connections, these connections were now undersized. By going through the shop drawings, LeMessurier also noticed that the New York contractors exempted many diagonal braces from load-bearing calculations by interpreting the building code in a certain way. LeMessurier's firm used New York City's truss safety factor of 1:1 instead of the column safety factor of 1:2. The code states that since columns in tall buildings are put in tension from wind when bldg is leading over, you should only subtract from the tension of the wind 3/4 of the dead load. The engineers said that the diagonals aren't like a column in a bldg, but instead, more like a diagonal of a truss therefore you can subtract the full dead load. What this did was reduced the required number of bolts by one-half. Each bolt was good for 100 kips load, this meant that with the assumptions made by the engineer, each connection needed a total of four bolts instead of eight bolts. The original design connection consists of high strength, 1 1/4 to 1 1/2 in. bolts, through the holes. (ENR, 1978)

(2000k tension wind) - (1600k dead load) = vs. (2000k tension wind) - [3/4(1600k dead 400k load)]=800k

bolts good for 100k each... 4 bolts bolts good for 100k each... 8 bolts

Too few bolts used in the first place without even considering diagonal winds. According to LeMessurier's calculations, diagonal winds increase member load by 40%. Le Messurier reran