The height is a number with a reason

One World Trade Center measures 541 m, which is 1775 ft, and that second figure came first. The design team worked backward from 1776 and shaped the spire until the tape agreed. Few tall buildings have a target height set by a calendar year.

It is the tallest building in the United States and in the Western Hemisphere, well ahead of Central Park Tower and the Willis Tower in Chicago. Measured against what stood on the site before, the roof at 1,368 ft repeats the height of the original North Tower, so the tower only passes the Twin Towers once the spire is counted.

The floor count depends on which number you want. The building is marketed as 104 stories, but the Council on Vertical Urbanism counts 94 floors above ground, because the upper numbers skip and some levels hold machinery rather than offices. The highest occupied floor sits at 1,268 ft, more than 500 ft below the top of the mast.

Who designed it, and when it opened

The original towers were destroyed in the September 11, 2001 attacks. Rebuilding started with a competition for the site, won in February 2003 by Daniel Libeskind, whose plan fixed the memorial footprints, the street grid and a tall tower on the northwest corner. The tower itself went to David Childs of Skidmore, Owings & Merrill a few months later, and the design that got built is his rather than Libeskind's.

Construction began in April 2006. Steel topped out in 2012, the spire went up in May 2013, and the first tenant moved in on November 3, 2014. The cost reached about 3.9 billion dollars, which made it one of the most expensive office buildings ever put up. Most of that went below the lobby and into the structure, not the finishes.

A square that turns into another square

The plan is the clearest thing about the design. The base is a 200 ft square, the same footprint as each original tower. From the 20th floor up, the four corners are cut back so the elevation reads as eight tall isosceles triangles. Halfway up, where the triangles meet, the plan is a regular octagon. At the parapet it is a square again, rotated 45 degrees from the one at the bottom.

That twist is why the tower changes character as you walk around it. From some streets it looks like a slab, from others a spike, and the chamfered faces catch light at different angles through the day.

The bottom 57 m are a windowless concrete pedestal, and getting its cladding right took two attempts. The first plan called for about 2,000 panes of prismatic glass; the panels broke in testing and were too costly to produce at that scale, so they were dropped. The pedestal wears angled glass fins on stainless steel panels instead, lit from behind at night.

Built to take a hit

The structure is a concrete core carrying the loads, wrapped by a steel perimeter, and the core walls are roughly a meter of reinforced concrete. Stairwells, elevator shafts, risers and sprinkler lines all run inside that concrete rather than behind drywall, which was the point: the original towers lost their egress routes early. The stairwells are extra wide and pressurized, and one set is reserved for firefighters so crews climbing up do not meet people coming down. The whole base is blast-resistant by design.

One World Observatory

The observatory occupies floors 100 to 102 and opened on May 29, 2015. Visitors reach it in the Sky Pod elevators, which climb more than 1,200 ft in under a minute while the cab walls play a time-lapse of Manhattan growing up around the site. The deck gives a 360 degree view over the harbor, Brooklyn and New Jersey, with a glass floor panel looking straight down at the street and a bar and restaurants on the same levels. Tickets are timed.

The spire, and why the record is argued about

The top 124 m are a mast, and whether they count decides the building's rank. The Council on Vertical Urbanism ruled in November 2013 that the mast is architectural, which confirmed the 1775 ft figure and put the tower ahead of the Willis Tower. The ruling is still contested, because the mast was designed with a radome cladding that was cut from the project for cost and maintenance. Critics argue that an uncladded mast is an antenna, and antennas do not count. The council kept its decision.

The same rule has settled records before. The Petronas Twin Towers took the world title from the Willis Tower in 1998 on spires alone, which the record-by-year page follows through the whole list.

The rest of the site

The tower is one building on a sixteen-acre site. Two square pools sit in the footprints of the original towers, each ringed by bronze parapets carrying the names of the people who died, and the National September 11 Memorial and Museum runs underneath the plaza. Three more office towers, the Oculus transit hall and a performing arts center share the block. One World Trade Center holds offices, with Condé Nast as the anchor tenant, and New York's other towers give it plenty of company.