Back in May I wrote optimistically that the World's tallest prefab, Sky City, is breaking ground in June. While strictly speaking the ground was broken, that was about it; the construction was halted for what will probably be a couple of months, for more approvals. In the meantime, Broad Sustainable Buildings has built a slice of the building to test and refine the design.
The slice of the building appears to be floors 165 through 173, where the building reduces to a square form for its final 32 floors. It is built with a different structural system than the other BSB structures, which had columns sitting on stubs built into the floors, and diagonal bracing to stiffen the structure and give it lateral stability; That system was only good for about 30 stories.
Now the prefab floors are independent of the structure, which is made from a steel frame that is bolted together. There is a price to be paid for this in terms of weight; where the older design took about 80 kilograms of steel per square meter of area, the new structure is 120 kg/m2. However that is still less than conventional construction and now the sky is the limit.
There are advantages to eliminating the diagonal bracing, including much bigger windows.
One of the key features of Sky City is the "columnless square" elements found every three floors; this is to be used for public functions such as sports, theatres, and vertical farms.
These spaces are in the middle of the building, which is unfortunate; they could use natural light. The square at the top of the building, with windows on three sides, is spectacular; the lower ones are less impressive.
A network of interior ramps act as "sky streets", you can walk from top to bottom, all six miles of it. This would be a serious hike; I found the ramps to be a little steep by North American standards. But there are great views overlooking the squares.
The latest iteration of the exterior cladding is the slickest yet from BSB, all glass with automatic venetian blinds between the panes. I do wonder how they will be maintained; there doesn't seem to be any way to open the units to get at the blind mechanism.
Like Broad's other buildings, the windows do not open; given the quality of the outdoor air and the effectiveness of the Broad air conditioning and filtering, this is logical. However sometimes I wish there was a little less logic, a little more natural light and freedom to choose to open a window.
One thing I admire about Broad Sustainable Buildings is that they don't just build a computer model, but they have built a full nine-storey slice of the project to work out the details. This is a good thing; in some places it is a little rough around the edges. If they are going to build it 24 times as high they have to get it right.
Many are skeptical about whether the world needs supertall buildings like this. In an interview, Chairman Zhang Yue claimed that it will conserve energy, use less land and reduce the need for car ownership. Having seen how closely most fifty or sixty storey buildings are packed together in China, a supertall building with a little more space around it begins to look very attractive. Having seen how most buildings in China are being built, this is a huge improvement in quality, efficiency and livability.
Whether it should be a hundred or two hundred storeys is another discussion.