Which NYC skyscraper was the topic of the PBS (Nova?) program regarding the wind-related design flaw discovered by a grad student after construction and then fixed over a period of a couple of year in the late 70's or early 80s?
The building isindeed the Citicorp Center at Lex. and 53rd.
The design flaw stems from a fundamental change in the building's design. The site of the tower had been occupied by the St. Peter's Lutheran Church. When Citicorp purchased the land and air rights of the church in the early 70s, the plan was to construct a building with a normal base that contained the main structural supports of the tower at all four of its corners.
Citicorp realized, however, that its new building could be taller than the zoning laws allowed through a process called "Bonuses" a method made popular by the zoning variances on Mies' Seagram Building. The idea of Bonuses is to give your building greater floor space (i.e. height) than the law allows by agreeing to build certain public or community ammenities on your site. Citicorp hence decided to build a new church for the St. Peter's congregation at the corner of 53rd an Lex.in order to meat Bonus requirements. Citicorp and its structural engineer David LeMesurier would then cantilever the building over the church. The four corners of the building were removed from the first 10 stories and the main structural supports of the building were shifted to the middle of each outer side of the square floor plan. Though ingenious, this plan had one crucial shortcoming: LeMesurier forgot to recalculate the diagonal wind loads on the building's four corners (which had been significantly weakened given the redesign of the tower). Even more astounding than the elementary design mistake made by one of America's most respected structural engineers, was the fact that the flaw had been noticed by an engineering undergraduate at Columbia University who had been studying the structural layout of the building as part of a class assignment. Once the flaw was discovered, LeMesurier and Citicorp hastily evacuated parts of the building and began massive structural reinforcement of the building. The fact that the Citicorp Center could have toppled over (possibly killing tens of thousands of people) was only revealed, albeit very quietly, at the beginning of the 90s. Even today, such a story seems hard to believe.
One minor correction: the corrective work took place at the end of construction. The building was not yet occupied.
Actually, the building had been occupied for a year. The story about the student actually discovering the error is not completely correct either. For an accurate accounting of this incident go to http://onlineethics.org/moral/lemessurier/index.html
Actually, the flaw was not in the design. According to the NOVA program, the contractor second guessed the plans call for rivetting the supports, and instead welded them, to save money. The weaker connection was the problem, not the initial design.
According to my Pearson/Prentice Hall engineering textbook, the original design was for welded joints, but after the approval of LeMessurier's subordinate engineers they were instead bolted together. Four years later a student noticed the flaws, which were repaired in the middle of the night during a newspaper stike at a cost of $8million, $2mil of which came from maxing out LeMessurier's malpractice insurance. The final calculations of the structural integrity showed that a storm of magnitude great enough to topple the bolted Citicorp Center occurs about every 16 years in NYC.