Performance-based Plastic Design Of Earthquake Resistant Steel Structures: Concentrically Braced Frames, Tall Moment Frames, Plate Shear Wall Frames
Bayat, Mohammad Reza
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It is well known that structures designed by current codes experience large inelastic deformations during major earthquakes. However, current seismic design practice in the U.S. is based on elastic structural behavior and accounts for inelastic behavior only in an indirect manner through certain modification factors such as R, I, and Cd. Under moderate to severe earthquakes, inelastic activity, including severe yielding and buckling of structural members can be unevenly distributed in the structure which may result in global collapse or costly repair work. Recently, a new design method has been developed and referred to as Performance-Based Plastic Design (PBPD). This method directly accounts for inelastic behavior by using pre-selected target drift and yield mechanism as key performance limit states.In this research work, the application of PBPD is successfully extended to design of mid-rise to tall steel Concentrically Braced Frames (CBF) with increased confidence level against collapse and also to tall steel Moment Frames (MF). The PBPD procedure for design of Steel Plate Shear Walls (SPSW) is also developed.The PBPD method is extended to design of mid-rise to tall CBF structures by proposing several key modifications in the calculation of design base shear. These include: consideration of column axial deformations in estimation of yield and target drifts, lateral force distribution to prevent large story drifts at upper stories due to higher mode effects, and target drift by proposed C2-factor to account for pinched hysteretic behavior. Moreover, different methods are suggested to enhance the confidence level of mid- to high-rise CBF structures against collapse. These methods include: increase in design base shear by using slightly larger C2-factor for mid- to high-rise frames, using Split-X configuration for braces, and increasing the minimum required fracture life, Nf. Application of PBPD method in design of tall MF structures is successfully carried out. Modifications for design of tall MF systems, primarily on design of columns, are proposed to achieve this goal. The current PBPD procedure for design of columns in steel MF structures works well for low-rise frames, but results in overdesigned sections for mid- to high-rise frames. It is shown that by applying the proposed modifications in design of tall MF, excellent seismic performance under pushover as well as time-history analyses can be achieved.The PBPD procedure for design of SPSW, an emerging lateral load resisting system, is developed. This procedure uses target drift and yield mechanism as key performance limit states. The pinched hysteretic behavior of SPSW is directly accounted for in this method by using the proposed C2-factor method. By applying this method in the design of a 4-story SPSW frame, it was shown that the proposed PBPD procedure works very well for design of these systems. The performance criteria of target drifts and yield mechanisms were successfully met for the PBPD designs. In addition, with the proposed PBPD procedure, multiple level design based on appropriate target drifts for each hazard level, can be easily implemented. In general, the PBPD designed frames showed improved performance compared to the code designed SPSW frame, especially under MCE ground motions.