The Burj Khalifa's superstructure is supported by a huge reinforced concrete raft. Bored reinforced concrete piles support this raft in turn. The raft was built in four different pours and has a thickness of 3.7m. The concrete raft grade is C50, which is self-consolidating concrete. It is reported to be the heaviest piece of concrete ever poured. The ends of the raft rest on cylindrical caissons that contain elevators that take passengers up to the top floor. The elevator shafts are 9 meters deep, and each one weighs about 200 tons. They use counterbalance weight systems with gas springs to reduce the load on the motor. The total weight of both shafts is estimated to be 2,700 tons! On the 22nd floor there is an observation deck called the Top of the World. On clear days you can see all the way to the Middle East from here. The building was designed by Fadi Abu Diab who also designed the Nile River bridge in Cairo. He chose to use self-supporting structures for the main tower because they are easy to build and less likely to collapse in an earthquake. However, he could have used conventional construction techniques too. The fact that this innovative design was chosen shows how confident Abu Diab was in the strength of modern concrete.