s would provide the main resistance to lateral seismic forces. These columns absorb both dead and live load compression plus vertical post-tensioning forces. They were also designed to maintain zero tension if subjected to lateral movements by earthquake. The trusses are 9 ft deep, spaced 20 ft on center and have a clear span of 65 ft (diagram 2). He made use of the 9 ft high resultant space as service area, allowing pipe chases to be dropped to the 65x 245 ft floor below with more latitude than before. The vertical end of the trusses alternate with the columns to form the integral, vertical support of the building. The post-tensioned stress steel bars were coated with asphalt paint and inserted in metal conduit to prevent bonding with the concrete. Three of these bars are draped through the bottom chord of each of the 18 ft wide trusses, acting like elastic 'tendons' that will elongate if an earthquake should occur. In this way, the building has twice the amount of controlled ductility of a steel structure of comparable size. This structural system is as unique as Kahn's composition of positive (i.e. laboratory towers) and negative (i.e. courtyard) elements in this project.Kahn consulted Luis Barragan about the design of the courtyard. Barragan told Kahn, 'Don't put one leaf nor plant, not one flower, not dirt. Absolutely nothing... a plaza... will unite the two buildings, and at the end you will see the line of the sea.' He also said that the courtyard would act as a facade that rises to the sky and unite the two buildings as if everything had been hollowed out. The simplicity of this idea supports Kahn's believe in 'timeless' architecture which was high above whim and fancy. Concrete was chosen as the material for the exterior facade of the towers, the Living and Meeting places, and slate was chosen for the courtyard to further emphasize the simplicity of the design.In determining the mix to be used in the concrete, Kahn...
