Bill Baker, structural engineer extraordinaire at SOM, gave a great lecture at Northwestern earlier this week about Burj Khalifa. Given that the crowd was mostly engineers, he was able to ramp it up a bit and give a few technical details that I hadn’t heard before.
The most interesting point he raised was that of all the differences between the Burj and supertall skyscrapers of even a few years ago (concrete instead of steel, e.g.) the most important one was that the program for the Burj–mostly residential, with some boutique office spaces at the top for fly-in meetings–allowed much smaller floorplates than, say, Taipei 101. Apartments don’t need acres of contiguous, flexible space, they need as much access to daylight and views as the economics of the tower can stand. As a result, even though the Burj is nearly twice the height of Sears (828 meters), it only has about 2/3 the floor space (3m sq. ft. as opposed to 4.4m.) Particularly near the top, the building is basically a concrete mast with a narrow ring of rentable space around it. This allowed SOM to design the core as a fairly simple beam against wind resistance, since the structure is almost all shear walls and therefore can’t rack like a frame. So the numbers are big, but they’re not complicated–Baker said that you could do the basic wind calcs for the structure by hand.
OK, the cool stuff? Concrete that was super fluid enough that the “slump test” was more of a “puddle test.” The well-described vortex shedding shape that prevents harmonic resonance by ‘designing the wind’ (Baker said this was like a kid on a swing trying to kick 24 times a cycle instead of one, which is a great description that I’m going to steal). Concern that entrained water in the 3.7m thick concrete mat would actually boil because of the endothermic reaction in curing concrete. And–this was the biggest surprise for me–the tower is so tall that it benefits from adiabatic cooling. Because the air pressure at the top is noticeably lower than that at the base, the air temperature is dramatically cooler. So the air handlers always take their air from the top of a tower zone and feed down. At this height they can also harvest water from onshore breezes, picking up something like 14 olympic sized swimming pools a year. No one would call the tower ecologically optimal, but this strategy makes a good point about microclimates and building height.