Evolution of the design of the ATLAS barrel toroid structure

The ATLAS barrel toroid magnet provides the magnetic field needed for the ATLAS muon spectrometer. The barrel toroid structure holds the eight superconducting coils evenly positioned around the beam axis with an outer diameter of 20m. It supports not only the coils but also the barrel muon chambers, the services and the access for the ATLAS experiment. The structure withstands about 1400tons of weight and strong magnetic forces. The physics performance of the muon spectrometer, and the fact that many design aspects of the toroid and of other related sub-systems of the ATLAS detector are deeply intertwined, impose stringent requirements on the design of the barrel toroid structure. The evolution of the design is presented in the paper