Two faces of One World Trade Center, and its originally chamfered base

The figure of One WTC, when it’s straight to you, that’s the figure of the old World Trade Center, two hundred by 1368 feet. And then as it changes, it rotates into a more obelisk shape, where it is sloping on the sides, and has that sort of memorial aspect to it. On the left the obelisk in Washington DC, built to commemorate George Washington, the first American president. The simple, iconic form of One WTC is indeed reminiscent of that of the tapering monument. Standing 554 feet, the Egyptian obelisk was at the time the world’s tallest man-made structure. Not until 12 years after the collapse of the North and South Twin Towers, once the tallest buildings in the world, a new skyscraper was completed on the same site: One World Trade Center, with a rooftop just as high as the North Tower’s rooftop: 1,368ft (417m) and with identical square footprint: 200x200ft. The top floor below roof is counted as 104th; the Twin Towers counted 110 floors. As the tower rises from the square base, it tapers to the top, thereby transforming the square into eight extremely tall isosceles triangles in elevation (four that point up and four that point down, alternating), culminating in a smaller square at the top, half the size of the base square. Halfway the shrinking floors, at the 60th floor, the tower forms a perfect, eight-sided shape: the octagon. A geometrical three-dimensional object underlies the shape of One WTC and is called a “square antiprism”. The perspective drawing shows a uniform square antiprism consisting of two horizontal squares: the caps or base faces, connected to each other by eight equilateral triangles, four in upright position and another four in upside down direction. This shape is for instance relatively common in chemistry where eight (groups of) atoms are arranged around a central atom, defining the eight corners (or vertices) of a square antiprism. Another example is this triangular antiprism, disclosed only by triangles. It is also called an octahedron, a three-dimensional solid figure with eight equilateral triangular side faces and six vertices. There are of course an infinite set antiprisms, on the left a pentagonal antiprism with five-sided base faces (pentagons) joined by a ring of ten equilateral triangles and a hexagonal antiprism with six-sided base faces (hexagons) and twelve equilateral triangles. Antiprisms can be generated from prisms. For our square antiprism this is the square prism, in other words: the cube. The parallel, congruent base faces of prisms, like a cube, have the same rotational orientation. The square antiprism is a distorted cube with one of the bases twisted 45° relative to the other base. Antiprisms are similar to prisms except the bases are twisted relative to each other, and the side faces are triangles, rather than quadrilaterals. On the left antiprisms with above their original prisms. The far left triangular antiprism is for instance generated from the triangular prism above by a twist of 60 degrees. On the left a realistic object with the shape of a square antiprism. The two elongated boxes with a square footprint might be compared with the Twin Towers, although not depicted in correct proportions. By rotating the roof surface a quarter, an elongated or prolate square antiprism is created. With its outward sloping edges not an ideal shape for a skyscraper. The shape of One World Trade Center is based on this oblong antiprism, however, contrary to a true antiprism, the top square has half the area of the bottom one. Evidently the One WTC has considerably less volume than the original Twin Towers. The twisted orientation of the top square makes that, seen from the front, the tower has a box-like appearance with vertical in stead of oblique edges. To elaborate a little more on this phenomenon, on the left a top view of the square antiprism is shown. The dash lines indicate the bottom square, twisted relative to the yellow top square. The top square of the One WTC is formed by connecting the centers of the sides of the bottom square, indeed comprising half the area and indeed twisted relative to the bottom square. The perspective drawing of the side view of the square antiprism shows how the smaller top square is generated from the bottom square. On the left a front view of the antiprism, and again the generation of the top square. The appearance of the resulting quasi square antiprism, in principal the shape of One WTC, differs greatly. The front view shows straight vertical outlines and the side view tapering outlines. Now follow some further comments on the shape of One World Trade Center, in particular regarding the shape of the building’s base. The 185-foot steel and concrete base was added to the design in 2005 by architect David Childs. It approximates a cube, and had in the original design symmetrical outward sloping surfaces at the corners. This cutaway appearance is called a chamfer. This ground floor plan shows the chamfering of the corners. These corners slope gradually as they rise, at an angle of 3.8 degrees. It was said that this design was intended as a subtle homage to the cutaway corners of the original Twin Towers. The sloping corners are also included in this constructed image after the original design of the tower. On top, the originally designed 408ft tall spire, comprising nearly a quarter of the building’s height. This spire consisted of a communications mast sheeted in a tapering fiberglass and steel radome protecting the antenna surfaces from weather and conceal electronic equipment from public view. Although criticized by the tower’s architect (SOM), the radome was not constructed, saving $20 million, but leaving the antenna, ringed by circular maintenance platforms, in sight. Nevertheless, the International Height Committee declared that the mast on top of the building is a spire since it is a permanent part of the building’s architecture, making the One WTC the tallest building in the US. However, it has not the “tallest roof” of the country. On the basis of this criterion, with a roof line at 442m (1,450ft), the Sears/Willis Building in Chicago is the tallest. But not for long, as it seems. In this never ending height competition in skyscraper enterprise, the Central Park or Nordstrom Tower seems to become the next title holder. But quickly back to the One WTC. This construction drawing of the tower’s base shows that the chamfered corners were encompassed in the structural steel frame and concrete exterior wall. Behind this wall is an expansive, 20 meter-high (65ft) public lobby, topped by a series of mechanical floors. Above this base (beginning on the nominal 20th floor) are sixty-nine office floors. This is the base under construction, showing the steel frame with sloping columns at the corners. Close behind the massive exterior columns we see the timber framework for a concrete inner wall to which we’ll return in a second. Following a 2005 order of the New York City Police Department, save for the entrances, between the perimeter columns a windowless, 15 meter-high (49ft) concrete wall was placed. Here we see temporary timber formwork into which the concrete is poured, on top protrude steel reinforcing bars (rebar). However, the two large north and south entrances are not unprotected, as the inner concrete wall with the oblong openings on the right shows. This is the wall of which we have seen the formwork a moment ago. This photograph, taken before the exterior walls were built, gives a better view of the more than two feet thick inner wall, slotted to allow daylight to penetrate. Interestingly, behind this wall, part of the tower’s extremely strong central core visible. This core is enclosed by three feet thick reinforced concrete all the way up the tower. On the ground floor plan the location of the two inner walls behind the two entrances is indicated by red bars. A view of the inner wall from a distance. This photograph is taken after the tower’s completion. On the left the entrance doors, in the middle the inner wall, and on the right the lobby with employees and desks. The natural light entering through the entrance opening is dimmed by the inner wall. A bright architect’s vision of the lobby… This recent picture gives an impression of the soaring height of the lobby, which is reminiscent of the lobbies of both Twin Towers. But there was a price to be paid for the concrete outer and inner walls at the base. Contrary to the One WTC, the Twin Towers had lobbies which were not hampered by the relative absence of natural light and which gave views to the outside. But in the days of their construction, terrorist attacks in New York City were just unthinkable. This picture shows the lobby of the North Tower. On the left, seen from a different view point, the lobby under construction. The windowless, blast-resistant concrete walls also created an aesthetic problem: how to avoid that the tower’s base looks like a gigantic fortress? Originally, the plan was to drape the base with clear prismatic glass panels and welded aluminum screens to create, in the words of the architect, “a dynamic, shimmering glass surface.” In small scale, prismatic glass is more commonly used for lamps and shop windows. However, the plan proved unworkable. The glass was difficult to manufacture at that scale. In trials, the refinishing required for the prismatic effect left the glass brittle and prone to shatter. A unique alternative was conceived consisting of angled glass fins protruding from steel panels. From a distance the vertical glass fins look like this. This is the entrance to the public observation deck at the west side of the building, smaller than the north and south entrances. Sets pairs of frosted glass fins with perforated aluminum back panels, are attached to stainless steel latticework at varying angles to create a series of v-shaped wing effects along the building surface. A luminous glass curtain wall, starting at the nominal 20th floor, raises up to the top of the building. This extraordinary glass cladding might have been the reason behind a redesigning of the shape of the tower’s base. Late in the construction process the chamfered corners were squared off, and covered with the vertical glass fins. A remarkable and probably painful step, since the sloping corners, which were from the start embedded in the complex steel structure of the base, are now completely concealed behind the glass panels. This alteration in design received a mixed reception. At each corner of the building, a stainless steel column with horizontal beams connected to the steel frame of the base made it possible to create rectangle façades. The extension column and beams seen from a different perspective. A first glass panel seems to be installed. The glazing of the wall on the left is nearly finished. Notice the extension column and beams at the right corner. Finally, the cladding of the base is completed. The main entrance resides in a rectangle façade, And LED-lights behind the panels illuminate the base at night, creating a soft exterior glow. Yes, in a city that never sleeps…

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