Comparison of seismic behavior of steel moment frames equipped with linear and nonlinear viscosity dampers under near-fault earthquakes

The displacement-based design method in structural design codes, in which displacement is considered a criterion for evaluating the structure, has been accepted due to the dependence of failure on displacement more than forces, overcoming the inherent shortcomings of force-based design methods. On the other hand, different levels and states can be defined concerning structures' performance. In the functional state of immediate occupancy, the relative lateral displacement due to cracking or plastic behavior does not remain in the structure. The structural members' stiffness and strength do not change fundamentally, and microscopic cracks are created in the structural members and façade. In the second case, i.e., life safety, some stiffness, and strength will be lost in all classes, relative deformation due to plastic behavior in the structure will be observed, and the risk of loss of life will be slightly higher. At the performance level, the collapse threshold remains in the members of structures of low stiffness and strength to withstand lateral loads, but the columns and load-bearing walls maintain their function, the relative deformations are high, and the structure is likely to collapse due to aftershocks. Due to decreased energy caused by earthquakes, dampers with passive control systems are installed in certain parts of the structure and absorb a relatively large part of the energy entering the structure under different mechanisms, and as a result, the structure does not suffer severe damage. After the Northridge and Kobe earthquakes, extensive laboratory studies were conducted to strengthen and increase the joints' ductility, and many modified joints were proposed. Extensive applied structural studies have been conducted on dampers' seismic behavior as one of the advanced tools of passive structural control systems due to earthquake energy dissipation. This work compares the effects of linear and nonlinear viscous dampers on the seismic behavior of 3, 9, and 20-stories steel moment frames, and the structural seismic responses are discussed. In linear dampers, the axial force is obtained by multiplying the damping coefficient ratio at the relative speed of the damper's two ends, and in the nonlinear state, the relative speed of the damper is between 0.2 and 1, in which 0.25 is used in the current study. The results showed that viscous dampers' implementation generally reduced the responses and decreased the structural damage during the earthquake. Absolute displacement of structural models in nonlinear dampers compared to linear ones has decreased with an increasing number of stories, but for maximum relative displacement with the increasing number of stories, nonlinear dampers had a more negligible effect. Due to the hysteresis extracted, nonlinear dampers in short and intermediate structures have more energy loss, which is significantly reduced in high damping structures. The plastic hinge distribution for these dampers has eliminated the failure LS at the life safety performance level. The base shear of 3, 9, and 20 story frames with a nonlinear damper is significantly reduced compared to a linear damper and shows the nonlinear damper's positive effect, especially on high-rise structural frame models.