Completed in 1977, the Citigroup Center is a major skyscraper on the Manhattan skyline [a]. Even before construction began, the building faced challenges. A church owned the land in the corner where the tower was intended to go. As such, the entire building was lifted over the church and each of the four corners of the building was cantilevered. With this design, the church was able to sit beneath one corner of the tower yet head the structural engineer William LeMessurier placed the four columns supporting the structure in the center of each façade instead of on the sides [b].
In order to accommodate this placement of the columns, the forces of the building had to be redirected. In a common skyscraper, the forces are directed out and travel down the corners of the building. In the citigroup center, once forces travel to the perimeter, they are directed down an eccentric structure using lateral bracing to the center of each façade and down into the columns. This can be seen in Figure 1. Each of these lateral bracings are eight stories tall and as such there are a number of critical connections where each floor meets the lateral bracing [c]. This is where the building’s failure lies.
The original structural design by LeMessurier called for these connections to be welded and as such fixed. Yet in the transition from design plans to construction plans, a subcontractor suggested a change in these connections from welds to bolts, effectively changing the connections from fixed to pinned [c]. This was approved without much examination and the building was built.
It was only after construction was complete that LeMessurier took a further look into the nature of the structure. After analyzing the final structure, he realized that if a constant high speed wind held for five minutes hit the building from the right direction, the building would completely topple over. A wind of this magnitude was estimated to occur approximately every sixteen years. This failure would have wiped out a significant portion of Manhattan causing a domino effect of buildings and inevitably causing the deaths of thousands of people [c].
To correct the issue, construction crews worked under the cover of night to weld in reinforcement plates at each of the susceptible connections. Media was kept in the dark to the issue yet the Red Cross and local authorities were preparing for the worst case scenario. This scenario almost came to fruition as Hurricane Ella approached New York in the summer of 1978. At the last minute though, the hurricane switched paths and headed out to sea [d]. Luckily, the reinforced connections were completed before disaster could occur within about two months and with great cost to Citigroup. This obviously led to a string of litigation crossfire, yet the building was able to be stabilized and disaster was averted [e].
[c] How New York Escaped Tragedy http://www.youtube.com/watch?v=TZhgTewKhTQ
[e] PBS: Building Big http://www.pbslearningmedia.org/resource/phy03.sci.phys.mfw.bbskyscraper/citigroup-skyscraper-design-problem/