Improving the Seismic Performance of Diagrid Steel Structures using Friction Mass Damper

As mankind pursues construction into new heights, tall building structures become more efficient. Several lateral and gravity load resisting systems have been developed for tall buildings. One of them is the steel diagrid system which is relatively unexplored in the western United States. The diagrid structural system consists of diagonal exterior steel members that carry gravity loads as well as lateral loads. The main difference between conventional steel exterior-braced frame trusses (X, K, V, and Chevron type braces) is that in the diagrid structural system almost all vertical columns are eliminated. This system is possible due to the evolution in technologies for design and construction of tall buildings. The first major example in the use of this structural system in the United States is the Hearst Tower in New York City. Unfortunately, this technology evolution does not arrive without drawbacks. One possible reason for the little use of diagrid systems in earthquake prone regions is the lack of guidelines and application examples illustrating the design and analysis of these structures. Furthermore, most new structures designed using this system are lighter and more flexible than conventional tall building structures and thus can experience large displacement demands, especially under seismic loading. In this work, the design and analysis of a diagrid system is performed. A prototype building with 72 stories is used as an example. To mitigate the possible large displacement demands that these structures may undergo under seismic events, friction mass dampers are provided to the building. Using a nonlinear finite element model of the designed structure, the effectiveness of the friction mass damper system is studied. The mass damper system consists of an additional concrete / steel wall placed in between the the building core -- consisting of elevator cores, stairwells, and other vertical communication for utilities -- and the exterior diagrid system. This mass damper is connected to the structure using friction isolators which is chosen due to its ability to accept large deformations. Parametric studies are carried out in order to optimize the mass damper design in improving the seismic performance of the building. Optimization of the seismic performance is assessed in terms of minimization of displacements, base shear, floor accelerations, and demand to capacity ratios of the steel members of the diagrid system. The end result provides designers...