I last visited the Twin Towers in August of 2001. I was in the middle of my Architecture undergraduate degree, and I went into the city with my Dad on a weekend morning, to look at buildings and take some photos.

A week or so later, I headed back to Colorado for my fall semester, and I vividly remember watching 9/11 happening; in my roommates room, on a tiny TV with rabbit ears, with no cell phones.

We will always remember 9/11, but this is a retrospective of what made the Twin Towers remarkable buildings, before their destruction on September 11th.

History and Site Plan:

The World Trade Center was built between 1966 and 1975, and it officially opened in 1972. When they opened the WTC, it was 100 feet taller than the Empire State Building, making it (briefly) the tallest building in the world. It was almost immediately supplanted by the Sears Tower in 1974, by about 100 feet. A short-lived “tallest building” designation.

The Twin Towers were always a strange pair of buildings, for the simple but crucial fact that (as the name suggests), there were 2 of them.

Each building was a massive square-plan tower that stood parallel to each other, but not directly next to each other, aligned along the west edge of the plaza that they helped define.

Since they were not directly next to each other, but diagonal to each other, the Twin Towers looked different from every side and changed as you moved around them. Sometimes you could clearly see the 2 towers, and sometimes they would look like one large block, or even just a single tower from certain angles. As you looked up from the plaza at these super tall towers, they always seemed to be tilting towards each other in strange ways.

Each tower was notable for its engineering and Architecture, but it was the relationship between the 2 towers that made the buildings powerful.

When a 17 year old Philippe Petit saw an image in a French newspaper of the towers-to-be, and dreamed of walking on a tightrope between the towers, it was because of their “magnificent uniqueness” that inspired his dream.

The Architect:

The Architect of the Twin Towers was Minoru Yamasaki. His work attempted to blend the Modernist beliefs of the 1900’s with a ‘humanist’ softness by including more traditional historical and global architectural influences.

In the Twin Towers for instance, the close spacing of the columns along the exterior walls made the towers appear solid from far away, but also created a series of 22 inch wide windows, which Yamasaki believed would be more comfortable for the inhabitants at the extreme heights. The exterior of the building wasn’t a “window wall”, but columns with windows between them that was breaking down these massive towers into a scale that an inhabitant could relate to.

Yamasaki was attempting to build “dignified” buildings for the city and the inhabitants, but as is the case with many modernist and minimalist buildings, the intention of the designers and the reception of the public isn’t always the same.

For instance, Yamasaki also designed the Pruit-Igoe apartment complex in St. Louis, which was 3,000 units. It suffered from a slashed budget and an increased density, which resulted in a series of imposing concrete slab blocks. The complex was planned as 2 racially divided sections, but after the Supreme Court’s Brown vs. Board of Education ruling that separate was not equal, it opened as one massive facility. By the late 1960’s, it was known for the poverty and crime in the complex, and is credited with forcing policymakers to reconsider funding these public housing developments generally. It was demolished in the 1970’s.

Yamasaki has a strange history within Architecture, because of the fact that Pruit-Igoe and the Twin Towers had such public and tragic ends. Other Yamasaki projects however, seem to blend modernism and his humanist sensitivity more effectively, so his career isn’t a one-sided story.

Other more well-received projects include his IBM tower in Seattle and his work at Wayne State, where he combined a series of university buildings with reflecting pools in a way that has influences from Classical, Islamic, and Japanese traditional Architecture.

The Structure:

To build a structure as tall as the Twin Towers, the simple necessity to get people that high in a reasonable amount of time, meant that the elevator systems needed to be reinvented. The Otis elevator company developed a system of both local and express elevators, because otherwise, at the base of the building, half of the floor plan would have had to be elevators to move that many people efficiently. To make this work, they added sky lobbies at the 44th and 78th floors, to effectively halve the number of elevator shafts that would have been required.

At the Twin Towers, Yamasaki worked very closely with the structural engineers, and they created a “hollow tube” structure that allowed for open floors between the core and the exterior. They used trusses with concrete floor slabs to span between the core and the perimeter columns, but otherwise, there was no concrete or masonry to protect the structure. Instead, different fire-resistant plaster and other fire-resistant materials were used to protect the steel structure from fire.

The exterior columns were spaced very closely, and were constructed as multi-floor sections that were lifted into place on site. This created a grid structure that did not have diagonal members, and worked as a Vierendeel truss (a “truss“ with no diagonal members) to resist structural loads. This structural exterior wall created the outside of the hollow tube.

The structure has been studied extensively both before and after it’s demise by MIT and FEMA among others, but it was a remarkable and unique structure that was generally agreed to have been robust and redundant. To test this back in the 1960’s and 70’s, large scale models were constructed and tested at 3 different wind tunnels in Colorado, Canada, and the UK.

Pre-2001 Lease Rights:

In 1998, the Port Authority of New York and New Jersey decided to privatize the WTC by leasing the buildings to a private company to manage. They thought that this process would take months, but they put it up for lease in 2000, and awarded a 99 year lease to Silverstein Properties in July 2001.

After 9/11, when Silverstein’s insurance disputes ended up in court, a central issue was if these insurance policies were even in effect at the time of the attack, since many of them were still being finalized.

These insurance disputes bounced around the courts for over 5 years before being settled.

My Takeaways:

There’s more to learn from the twin Towers, before their infamous end.

Things like the innovation required to figure out and build this engineering feat. The conflicted history of the Architect involved, and things we can do differently in the future.

The scale and impact on the city that the Twin Towers projected was unique. In large part because there were 2 of them, and because of the way that they loomed over Manhattan.

They had a sublime feel as they stood over the city. Their scale was hard to comprehend, and they were almost always visible, but they also changed from every side.

I’m not sure we’ll see another set of twin buildings like these again, but we can learn from their entire life, as well as the end of it.

-Chris

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