Keeping a Cool Head
This week, my professor brought up an ethics case involving the Citicorp building in Manhattan. The building looks odd to anyone who is not a Structural Engineer (or Structural student). The building was constructed on four main beams, each of which is located in the center of each side of the building, not on the corners.
The reason for the odd base arises from the plot Citicorp was able to purchase. With a church located close to the plot (bottom left in image above), the company in charge of the project had to find a unique way of designing the building. As is required in design, head Structural Engineer William LeMessurier considered wind loading on the structure. He designed the building with chevron bracing to handle the wind loading seen. This loading calculation was based upon a facial load. He had also calculated the loading from quartering winds. His initial conclusion was that the load perpendicular to a face of the building was greater than the load seen by quartering loads. Upon a second analysis, he found that with the correct quartering loads, two chevrons at each floor saw nearly no load, and the other two saw nearly double the load seen with the calculated perpendicular facial loads. With Bethlehem Steel using bolted connections rather than welded connections for the chevrons, the members could be subjected to loads large enough for failure. With a large enough wind load (70 mph +) over a significant period of time, the entire building was capable of toppling over.
With hurricane season coming, LeMessurier had to act fast. Two methods of fixing the problem were developed. The first solution was a 400-ton computer controlled block of concrete. The concrete damper reduced the swaying induced loading on the building by a half. When the building experience wind loading, the block “relocated” the center of mass closer towards the center of the building; this allowed the members of the chevron to hold less force. Also, under complete secrecy, construction workers replaced the bolted connections with welded ones. This was possible because the chevron braces were internal to the building and could be accessed with ease.
All of this work was completed four months after realizing the problem. With the new welded connections, the building is capable of withstanding any storm and is one of the safest buildings in New York. For LeMessurier to remain so calm and co-operative over this period amazes me. As engineers, it seems that we are almost destined/required to be calm during stressful times. This man is indeed a fine example of a calm, cool headed individual, who not only followed a proper code of ethics, but prevented a major catastrophe as a result of his problem solving abilities.
