A few years ago I worked for Citigroup. When the main building, Citigroup Center was sold for the second time in it's history (Citigroup hasn't owned it since the early 80's when they sold it to a Japanese company with the provision that they can name it and retain some office space in it, who in turn sold it to a local investor who partnered with Boston Properties), one of my co-workers relayed the building's strange history to me.

When it was constructed in 1977, Citicorp Center (as it was then called) was the 7th tallest building in the world. William J. LeMessurier was the structural engineer who designed the steel skeleton of the building. The block Citicorp was to build it's skyscraper on was partially occupied by St Peter's Church.. Citicorp agreed to rebuild the run down church with a newer, more modern structure, if they would grant permission to build in the air above it.

The church agreed, but this left Citicorp with a strange problem. They wanted their building to occupy the whole block LeMessurier's ingenious solution was to put the building on four stilts, each nine stories tall. These stilts, however, aren't built at the corners of the building, they're at the center of each of side.

In order to compensate for this, diagonal support girders were placed throughout the building, and according to LeMessurier's design, these were to be welded together. These full-penetration welds are extremely strong, but are expensive and time consuming. They would funnel the weight of the building away from the corners and towards the middle of each side.

The following year, an engineering student studying the tower called LeMessurier to ask him about the oddity of the building's support columns being in the center rather than at the corners. LeMessurier explained about the system of braces used to compensate for wind.

Since the building supports are in the center of each side rather than at the corner, it would be particularly susceptible to quartering winds - winds coming in at a diagonal. Without the corner braces, it would be much easier for wind to topple the building. LeMessurier assured the student that the design would work, but it did cause him to revisit the design. Quartering wind would increase strain on half of the support braces by about 40%. More than he'd originally thought, but well within the margin of safety.

LeMessurier's design was to be used again for a building in Pittsburgh, and he was called in to review the plans. While reviewing the plans for the Pittsburgh building, he asked one of his associates about the technique used to weld the joints. He explained that the process was too expensive, and they'd decided to use bolts instead of welds.

LeMessurier was nervous. The bolted joints would not be as strong as welded joints, and the wind braces would not be as effective. He'd hoped the design team considered this and used a special kind of bolt, but they did not - the building code didn't require it.

Even worse, the building team had defined the diagonal wind braces as trusses, evading stringent safety specifications requiring braces to be of a certain strength. The trusses were built using fewer bolts than was necessary.

Technically, the building was up to code, but New York had never had a building like this. LeMessurier was never contacted because architects and structural engineers are never contacted for the day to day changes that go on in a construction project. It would endlessly mire the project down, and often these changes are minor and have no affect on the structural integrity of the building.

LeMessurier issued a report called Project SERENE - Special Engineering Review of Events Nobody Envisioned. He called in a Canadian expert to review his data using wind tunnel analysis from the original design. The tests results confirmed his fears - under the right conditions, a sustained light wind in the right direction could cause the building to vibrate like a tuning fork. Next, he checked the weather data for New York. Not only did this condition happen, on average, every sixteen years, but such a hurricane was on it's way to New York.

A light breeze could cause the building to topple and damage buildings as far away as Bloomingdales. (About six blocks.)

Unlike his television counterpart, LeMessurier contacted the architect, the architect's lawyers and Citicorp executives. He explained the problem and described the repairs that would be necessary to fix the building - they had to strengthen the joints where the building was vulnerable.

Citicorp issued a press release about how they were strengthening the building as an extra precaution, and hired welders to work overnight adding two-inch-thick steel plates to the joints.

The hurricane passed veering off course out towards the ocean, and the building is now stronger than it's original design specified.

The tuned damper described in the episode does exist, but is part of the building's original design. It's housed in the angled top part of the building. The building is composed of unusually light weight materials and tends to sway more in the wind than other buildings. It's not particularly dangerous, but residents on the top floors may get seasick from the motion. To counter this, a heavy concrete slab is put on the top of the building and it floats in oil. If the building starts to sway, the concrete slab is released and starts swaying in the same direction as the building. By the time the building starts swaying back in the opposite direction, the concrete slab is moving in the opposite direction, stabilizing the building. Citicorp center was the first building to use such a design, but it's much more commonplace now.

Now you know the "real" story behind the most recent episode of Numb3rs. Re-arrange a few of the facts (the overnight welders weren't saving the building, they were cheap labor, the student is the one who knew the problem but nobody would listen to him, etc.), and you have the plot for a TV show.

While TV shows like this have existed for a while, I think it represents a new breed of television show that concerns itself with the science behind everyday life. Medical Investigations, Crossing Jordan, and a dozen or so other shows that I'm not even aware exist.

Personally, I hope the trend continues.

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