The Citigroup Center seems to defy gravity, standing on four stilts placed at the center of each of its sides rather than at its corners. This distinctive building owes its ability to stand without support directly under its corners to the design of its internal steel skeleton. Diagonal beams form giant, eight-story-tall Vs that transfer weight from the building's corners to strong columns running the full height of each side. The force of gravity is not so much defied as redirected.
Another force, besides gravity, that is critically important to skyscraper design is wind. As part of the design process, structural engineers consider the force of wind on a building's foundation, columns, beams, and joints. The large surface area of a building is perfect for catching wind, and the resulting pressure can bend and stress a building's parts, possibly to the point of collapse.
Citigroup Center's engineers used scale models and wind tunnels to analyze the effect of wind on their design. Unfortunately, they only measured the effect of wind striking the building perpendicular to one side, all they were required by law to test. However, diagonal winds, which strike a building's corner and two sides, typically place a much greater load on the parts of a building, especially its joints.
Months after its completion, when the building was already occupied, the structural engineer who designed the Citigroup Center learned that workers had used bolted joints in the construction of the tower, rather than stronger and more expensive welded joints. Generally, bolted joints are considered strong enough for most building applications. However, subsequent wind tunnel tests of the Citigroup Center's design showed that diagonal winds of 70 to 80 mph could tear the tower's bolted joints apart, and cause the building to collapse. Realizing this, engineers ordered workers back to the building to perform secret emergency repairs. Working for several weeks under cover of darkness, they welded strong steel plates over each of the building's 200 joints, strengthening the building's skeleton and averting a possible disaster.
In engineering, there are always tradeoffs. What were some of the tradeoffs in the construction of the Citigroup Center? Should builders have to tell the engineers every change they are making in their plans? How can a builder know which changes are important and need to be told to others and which don't matter? What do you think about the fact that the emergency repairs were kept secret? Discuss how a student's input led to a re-analysis of the work that was done.