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Interview with David Childs (cont'd)

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PR: How did you approach the design process?

DC: Again the difficulties were split into the emotional and the practical. Freedom Tower will be a symbol of the entire project, as well as marking the memorial, and it occupies a very important piece of New York City property: the sky.

We really wanted our design to be grounded in something that was very real, not just in sculptural sketches. We explored the infrastructural challenges because the proper solution would have to be compelling, not just beautiful. The design does have great sculptural implications, and we fully understand the iconic importance of the tower, but it also has to be a highly efficient building. The discourse about Freedom Tower has often been limited to the symbolic, formal and aesthetic aspects but we recognize that if this building doesn’t function well, if people don’t want to work and visit there, then we will have failed as architects.

We wanted the building to emanate from the geometry of New York. We took the distorted rectangular street block, which changes to a parallelogram because of the angle of the Hudson River at the site of the Tower, and used that shape to inspire the footprint and twist of the building, as well as the exterior ‘diagrid.’ The exterior of the building is made up of what essentially amounts to many triangles.

The top of the building will be an extremely innovative structure that will reflect at its highest sections New York’s great landmark, the Brooklyn Bridge, with its twisting cables.

PR: Safety is a huge concern downtown now. How are you dealing with that?

DC: The Freedom Tower’s twisted taper makes it much stronger. Tapered buildings, like the John Hancock Tower in Chicago, are stronger than straight up-and-down rectangles. The form of the tower always seemed very natural to me but it wasn’t until after I’d designed it that I realized that there is an aspect of bio-mimicry to the building. Trees taper, and this is part of what makes them very strong structurally.

We’ve also used a diagrid system, which has an inherent structural strength, to form the perimeter of the building. The skin is a super brace, a network that stretches across the diagonal. It forms a very strong fabric around the whole building. Imagine chain mail, a system of intricate loops and links that work in tandem, so that if one, or even two or three are ever broken, the surrounding ones transfer the stresses away from the site of failure.

The core is different from what has been customarily built in New York City. Most office buildings have had concrete cores with a steel frame, which is efficient, strong, and allows the flexibility of long floor spans. But, up until now, New York City cores have generally been constructed with a steel frame coated with gypsum board. In the case of Freedom Tower and 7 WTC, the steel and concrete unions agreed to work together to design composite buildings with very thick concrete cores, Among the advantages of this system are increased space for protected shafts and twice as much storage space for emergency water for the sprinkler systems.

We have reinforced areas of the building that are closer to the ground – really, there is an extremely long list of safety innovations that I could cite about the Tower. But in the end, I think the most important thing for the public to know about this building is that its form is inherently extremely strong and that our team – SOM and all of its consultants – are spending every effort thinking about what we can do to make this building as safe as possible.

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