Workers placed steel columns that will make its unfinished skeleton a little over 1,250 feet high, just enough to peak over the roof of the observation deck on the Empire State Building.
The milestone is a preliminary one. Workers are still adding floors to the so-called "Freedom Tower" and it isn't expected to reach its full height for at least another year, at which point it is likely to be declared the tallest building in the U.S., and third tallest in the world.
Those bragging rights, though, will carry an asterisk.
Crowning the world's tallest buildings is a little like picking the heavyweight champion in boxing. There is often disagreement about who deserves the belt.
In this case, the issue involves the 408-foot-tall needle that will sit on the tower's roof.
Count it, and the World Trade Center is back on top. Otherwise, it will have to settle for No. 2, after the Willis Tower in Chicago.
"Height is complicated," said Nathaniel Hollister, a spokesman for The Council on Tall Buildings and Urban Habitats, a Chicago-based organization considered an authority on such records.
Experts and architects have long disagreed about where to stop measuring super-tall buildings outfitted with masts, spires and antennas that extend far above the roof.
Consider the case of the Empire State Building: Measured from the sidewalk to the tip of its needle-like antenna, the granddaddy of all super-tall skyscrapers actually stands 1,454 feet high, well above the mark being surpassed by One World Trade Center on Monday.
Purists, though, say antennas shouldn't count when determining building height.
An antenna, they say, is more like furniture than a piece of architecture. Like a chair sitting on a rooftop, an antenna can be attached or removed. The Empire State Building didn't even get its distinctive antenna until 1952. The record books, as the argument goes, shouldn't change every time someone installs a new satellite dish.
Excluding the antenna brings the Empire State Building's total height to 1,250 feet. That was still high enough to make the skyscraper the world's tallest from 1931 until 1972.
From that height, the Empire State seems to tower over the second tallest completed building in New York, the Bank of America Tower.
Yet, in many record books, the two skyscrapers are separated by just 50 feet.
That's because the tall, thin mast on top of the Bank of America building isn't an antenna, but a decorative spire.
Unlike antennas, record-keepers like spires. It's a tradition that harkens back to a time when the tallest buildings in many European cities were cathedrals. Groups like the Council on Tall Buildings, and Emporis, a building data provider in Germany, both count spires when measuring the total height of a building, even if that spire happens to look exactly like an antenna.
This quirk in the record books has benefited buildings like Chicago's recently opened Trump International Hotel and Tower. It is routinely listed as being between 119 to 139 feet taller than the Empire State Building, thanks to the antenna-like mast that sits on its roof, even though the average person, looking at the two buildings side by side, would probably judge the New York skyscraper to be taller.
The same factors apply to measuring the height of One World Trade Center.
Designs call for the tower's roof to stand at 1,368 feet - the same height as the north tower of the original World Trade Center. The building's roof will be topped with a 408-foot, cable-stayed mast, making the total height of the structure a symbolic 1,776 feet.
So is that needle an antenna or a spire?
"Not sure," wrote Steve Coleman, a spokesman for the Port Authority of New York and New Jersey, which owns the building.
The needle will, indeed, function as a broadcast antenna. It is described on the Port Authority's website as an antenna. On the other hand, the structure will have more meat to it than your average antenna, with external cladding encasing the broadcast mast.
Without that spire, One World Trade Center would still be smaller than the Willis Tower in Chicago, formerly known as the Sears Tower, which tops out at 1,451 feet (not including its own antennas).
Debate over which of those buildings can truly claim to be the tallest in the U.S. has been raging for years on Internet message boards frequented by skyscraper enthusiasts.
As for the Council on Tall Buildings, it is leaning toward giving One World Trade the benefit of the doubt.
"This is something we have discussed with the architect," Hollister said. "As we understand it, the needle is an architectural spire which happens to enclose an antenna. We would thus count it as part of the architectural height."
But, he noted, the organization has also chosen to sidestep these types of disputes, somewhat, by recognizing three types of height records: tallest occupied floor, architectural top, and height to the tip.
Hollister also pointed out that, technically speaking, One World Trade Center isn't a record-holder in any category yet, as it is still unfinished.
"A project is not considered a building until it is topped out, fully clad, and open for business or at least occupiable," he said.
The debate doesn't quite end there.
Neither of the Willis Tower nor One World Trade are as high as the CN Tower, in Toronto, which stands at 1,815 feet. That structure, however, isn't considered a building at all by most record-keepers, because it is predominantly a television broadcast antenna and observation platform with very little interior space. The tallest manmade structure in the Western Hemisphere will continue to be the 2,063-foot-tall KVLY-TV antenna in Blanchard, N.D.
As for the world's tallest building, the undisputed champion is the Burj Khalifa, in Dubai, which opened in 2010 and reaches 2,717 feet.
Not counting about 5 feet of aircraft lights and other equipment perched on top, of course.
Some information from ABC News.
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