Article Title
Article Title

Replacing Monuments

by Josh Zeisel

As most people in this country know it has been ten years since September 11, 2001. The Pentagon has been rebuilt to its previous glory and the trees and grass have (probably) grown back in Pennsylvania, but what about New York? What’s going on downtown?

The area is one of the busiest places during the day and one of the quietest places after banking hours have past. As the ten years have passed much progress has been made into the rebuilding of the area, with many elements of  well on their way toward completion. The museum is almost ready for opening and the waterfalls on the footprints of the original Twin Towers are flowing. Since this is the science and engineering section of The Inclusive, we should look into the engineering of the new structures. While interesting to study these new structures in a strictly engineering sense, the architects also utilized some astronomy and human physiology to enchance their designs.

First, let’s take a look at the original engineering of the Twin Towers. The Twins were, of course, identical except for the spire at the top of the north building. They were built with a crucial structural element called a Vierendeel Truss.Vierendeel trusses differ from “normal” trusses in that they used rectangular forms instead of the more readily used triangular structures. Look at any common span bridge and you’re sure to find a truss somewhere, and most likely you’ll find that the beams that connect the points in the truss form triangles. It is assumed, in the analysis of a traditional truss, that there are zero total "moments" at the connection points, but not so in the Vierendeel truss. A torque is to a moment as a push or a pull is to a force. The Vierendeel Truss needs to be stronger to withstand the moments that it feels under just gravity alone.

Think of the Vierendeel truss as a frame or a ladder. The towers utilized a large amount of these frames to be able to withstand the large moments and other forces exerted on the buildings. The benefit of using such a design is that most of the support lies on the outer columns of the building, leaving vast spaces for offices and other uses inside the outer walls. This also explains why the towers fell in on themselves rather than toppling over as many other buildings would. When the inner columns failed, the outer walls were still able to provide support until they eventually were pulled down towards the middle of the buildings. The Twins contained 59 support columns on each side of the building. At 208 feet per side, that’s only about 3.5 feet of window space.

The new One World Trade Center, formerly known as the Freedom Tower, is constructed in a more traditional manner. The supporting columns are not on the outside of the building, but are inset and there are less of them. This allows for a glass curtain wall to be constructed on the outside, made of high strength, low iron glass that has aspects of being bomb proof. This specialty glass provides enough support so that the use of steel mullions, the supports typically seen between panes of glass, are no longer needed. This allows for a strong outer wall that allows increased light to each floor rather than the Twins that had steel columns in between each window.

There are other safety aspects included in the new design.  There is a private, pressurized, extra-wide stairwell solely for the use by the fire department along with public stairwells implementing the same features. The first 200 feet are built with reinforced concrete made to withstand a bomb blast. The concrete core runs up the length of the tower protecting sprinkler systems, power systems, and elevators. 

The tower rises and is chamfered, forming a prismatic shape with an octagonal cross-section that ends at the 82nd floor creating a rectangular cross-section.  Reminiscent of the original 1 World Trade Center the new building will feature a spire, one that ends at an elevation of 1,776 feet, representing the year that we as a group of proud, strong people declared our freedom to the British and the world. The elevation will make 1 World Trade Center the tallest building in the United States, 46 feet taller than the Willis Tower, formerly the Sears Tower, in Chicago. Take that Chicago! Oh, and also you terrorists out there.

What makes downtown Manhattan such a great area is its accessibility. From my loft in SOHO, I have access to a multitude of subway lines, with many just a five-minute walk way. It doesn’t change down at the end of West Broadway where the World Trade Center is getting its own Grand Central Station. The WTC Transit Hub will be an access point that will allow passengers to gain access to both the east and west sides of Manhattan, a short hop across the East River to Brooklyn, and on the PATH to New Jersey, specifically Newark. 

The ground level sits atop a floor constructed of enormous steel ribs that are connected with a central spine, 24 in total.  Just the sight of the structure makes you think that it can hold up anything. The above-ground structure features a roof that has rib like structures shooting up diagonally out of the ground. The most interesting fact about the above ground portion is that the Hub is angled with respect to the street. The purpose of the angle only comes around once a year.  At 10:28 AM on September 11, the sun shines directly down the center of the roof. Since the roof is made of the steel ribs and glass, a shadow forms perfectly down the center of the station with bright light on either side, giving tribute to the exact time when the second tower collapsed.

As mentioned before, the ground level of the transportation hub and part of the memorial area around the Twin Tower footprints sits on a very bulky roof. The ribs are enormous, bigger than necessary, in fact.  The loads are interesting to think about. There will be snow, rain, and foot traffic, of course.  Yet these ribs are massive. It can’t be stressed enough how big they are.  In 2001, the PATH train and subways were cut off when the Towers fell. The protection provided by the roof was not enough to bear the loads they withstood that day. This new roof seems to take this sort of load into account. It’s built like a rib cage with a sternum right in the middle, protecting the heart of the city in a way that the older design could not have anticipated.

The final aspect of the rebuilt area is the Memorial Museum and its main attraction, the waterfalls that lie on the footprints of the Twin Towers. They are the largest man-made waterfalls in the world and serve as a fitting way to represent the Towers that, at completion, were the tallest structures on the earth. Around the perimeter of the waterfalls are panes of brass. Each person who perished in that specific tower, including those firemen, policemen, and medical personnel who risked their lives to save the lives of others, are machined into the brass. A high-pressure water jet, rather than a milling machine, was used to machine the names leaving a smooth finish on the brass.

All of the names of those who died on Tuesday September 11, 2001 were read at the Memorial Museum in front of the waterfalls, the same names enshrined in the brass. It’s a tribute and ceremony that will never get old because all of those people died for living their lives in a way they believed was right. For those of us who still see the old skyline when we close our eyes, the new structures will serve as a reminder to what happened ten years ago. For those who will view the new buildings as the norm in the skyline the structures will serve as a symbol of technological, economic, and human growth, pride, and freedom.

(Photo courtesy of Nicholas Berardi)

Josh Zeisel is a professional mechanical engineer and graduate of Boston University. His favorite meal is a chicken parm sub and an orange soda. On clear sunny days you might look up and find him flying something. Strike up a conversation with Josh at  josh.zeisel[at]theinclusive.net.