REDUCING THE LATERAL DISPLACEMENT OF LEAD RUBBER BEARING ISOLATORS UNDER THE NEAR FIELD EARTHQUAKES BY CROSSWISE DISSIPATERS CONNECTED TO RIGID SUPPORT STRUCTURE

The purpose of base isolation is to absorb earthquake energy, prolong the life of the structure, and enable the structure to be similar to a rigid body. However, since resonance can occur due to the closeness of the period of structures to the long period and large velocity pulses of the near field earthquakes, the stability of these buildings greatly reduces, and with the large displacement above isolation level, sometimes, tendency of overturning is created in isolators leading to their destruction. The main objective of this study is to significantly reduce the lateral displacement of base isolation subjected to near field earthquakes. In this research, seismic response calculation has been carried out for five steel moment frame structure with the 3, 5, 8, 11, and 14 stories in two states of with and without stiff core structure and energy dissipaters. The analyses has been done under fourteen scaled records of seven near-source and seven far-source earthquakes. It has been shown that the lateral displacement of base isolation system can be reduced by 87% for low-rise buildings, and 77% for high-rise buildings.


INTRODUCTION
Based on the official records, many buildings collapse due to major earthquakes, and more importantly, due to the lack of having appropriate earthquake-resistant designs. Mortality rates due to earthquakes can be significantly restrained if buildings are equipped with suitable earthquakeresistant designs which mainly rely on the ductile behavior of the design. When efforts are made to protect the integrity of the building by boosting the stiffness, unluckily, the acceleration of floors can rise successively leading to the destruction of the building. Seismic isolation has arisen as an alternate earthquake-resistant design method. This method has increasingly become popular since it offers to protect both structural and non-structural mechanisms and the contents of a building. Hence, seismic isolation attempts to reduce floor accelerations to focus on limits while keeping base displacements below a reasonable range. This is achieved by pendulum system isolated buildings under specific far-field motions increases. Other studies [11,[13][14] demonstrated that floor acceleration could enhance if seismically isolated structures were exposed to near field earthquakes with a pulse period close to their solution period. They also suggested additional base displacements in case of near field earthquake probably result in the pounding of the seismically isolated building provided that the isolation system displacement surpasses the seismic gap left around it. Moreover, it was observed that substantial increases in floor acceleration are caused by such a pounding [15]. Likewise, Taflanidis and Jia [16] conducted the most dangerous base-isolated structures supplied unit lead rubber bearing through a proposed simulation-based framework. They demonstrated that the seismic risk test in their work. The study revealed that when the deformation of bearings exceeds the isolation gap, the seismic risk amplifies. It was recently revealed by Mazza and Vulcano [17] and Mazza et al. [18] that the isolators might even take tensile loads when vertical components of near-field earthquakes with high peak values are in question with a possible failure of isolation system caused by high seismic displacement demands. Alhan and Sahin [19] investigated the role of isolator characteristics in reducing the floor accelerations of seismically isolated buildings with flexible superstructures under near-field earthquakes. They found that higher isolation damping would decrease floor accelerations up to the creation point, but further increases in isolation damping may cause higher floor accelerations.
In another study, Mazza and Vulcano [20] proved that supplemental damping is crucial to control the base displacements of seismically isolated buildings. However, it may not guarantee better performance in terms of structural and non-structural damage subjected to near-field earthquakes. In addition, for relatively short pulse periods, some undesirable results are created. Nigdeli et al. [21] offered a harmony study optimization metrology for seismically isolated buildings subjected to both near-field and far-field earthquakes to optimize isolation system parameters such as isolation period and damping ratio. Alhan and Davas [22] stated that "Benchmark buildings with base isolation systems of different isolation periods and characteristic force ratios are subjected to synthetically developed near-field earthquake records at different fault-distances with different velocity pulse periods, and their seismic performances are reported." In their study, for seismically isolated buildings, protecting vibration-sensitive equipment in operating conditions in case of large magnitude pulse-like near field earthquakes with very long pulse periods is a very challenging task. Furthermore, the ratio of the isolation period to the pulse period has a huge impact on the peak base displacement demands and the peak floor acceleration demands when subjected to long and short pulse periods, respectively. It is reported that the mentioned effect becomes even more tangible for shorter fault-distances and smaller characteristic force ratios.
Responses of base isolation systems subjected to near-field ground motions are one of the well-researched areas. However, researches have indicated that base isolation systems utilized in near-field earthquakes have experienced substantial lateral displacement. Moreover, a large number of structures equipped with base isolation have overturned. Consequently, in this study, a new method is adopted to limit the lateral displacement of base isolation in all types of structures under near-field earthquakes, add more stability to the structure, and reduce the destructive effects of this phenomenon. Furthermore, a support structure with viscous dampers is employed to reduce the effects of the resonance due to near-field earthquakes.

Design of structures
As it is shown in Figure 1

Rigid support structure
The rigid support structure is a square column situated in the middle of the base structure. The stiffness of this column in all types of structures is the same as the stiffness of the braces in one direction of the same structure. This column is designed in the center of the base, and it is rigidly connected to the foundation. In addition, the structures are connected to the column in the roof by horizontally crosswise viscous dampers, as is shown in Figure 2. It is notable that all of the rigid support structures are designed in PERFORM 3D.

Base isolation system
The base of lead rubber bearing (LRB) isolators is designed based on International Building Code (IBC) [23]. The initial design of the Base isolation is by force-displacement method, in which effective stiffness ( ) is required. As a result, the target period can be determined using is the total weight of the superstructure. Moreover, Equation (2) is used to estimate the displacement of the design. In Equation (2), is the damping factor, and 1 is the spectral pseudo acceleration obtained from the design spectrum. The parameters selected to define the utilized isolators, lead rubber bearing, in PERFORM 3D are demonstrated in Figure 3 and Table 1.

The fluid viscous damper system
According to the Iranian manual for structural damping systems in the design and retrofitting of buildings [24], the force in the viscous damper is calculated using Equation 3.
In Equation 3, 0 is the damping factor, ̇ is the relative velocity between the two ends of the damper, is the numerical power of damper velocity, and is the sign function. Moreover, the parameters selected to define the utilized fluid viscous damper in PERFORM 3D are similar to LRBs, which are indicated in Figure 4 and Tables 2-6.

Equation of motion
In 1999, Naeim F and Kelly JM [25] evaluated the relative displacement ( ) of each degree of freedom with respect to the ground. The equation is as follows: Where, is a vector that couples each degree of freedom to the ground motion. When this structural model is superimposed on a base isolation system with the base mass , stiffness , and damping , Equation 4 becomes: Mv + Cv + Kv = -Mr(u + υ) g b Where v is the displacement relative to the base slab, and ̈ is the relative displacement of the base slab to the ground. Now, the overall equation of motion for the combined building and the base slab is: In Equation 6, is the displacement relative to the fluid viscous dampers, which can be rewritten in the following form: Equation 7 identifies as the total mass m of the building. Therefore, + is the total mass carried by the isolation system. Equation 7 can be written in matrix form as follows: Where:

Ground motion information
For time-history analyses by PERFORM 3D, the ground motions should be scaled such that the average value of the 5 percent damped response spectra for the suite of motions is not less than the design response spectrum of the site for periods ranging from 0.2T to 1.5T. T is the fundamental period of structure in the fundamental mode for the direction of the response being analyzed (standard no. 2800) [26].

Discussion and results
When the time history analyses, the acceleration response, the velocity, and displacement of the classes for all of earthquakes has been discovered, it is observed that the changes in the output responses of all earthquakes are very close to each other. Therefore, only the Northridge earthquake responses are presented. As shown in Figures 5 -15 and Table 9, the response of the structures is extracted under near-field scaled Northridge (North. NF) and far-field scaled Northridge (North. FF) records. Hence, the reduced amounts in most displacements of the upper level of base isolation are observed according to the following Figures and tables:    Table 10, the response of these structures is extracted similar to the previous section, (North. NF) and (North. FF). Furthermore, reduced amounts in most base shears of the upper level of base isolation are observed in the following figures and tables: Fig. 16   The results demonstrate that the structures with viscous dampers have four main advantages. The first considerable effect of support structures with viscous dampers in the nearfield earthquakes is reducing the displacement of the upper level of base isolation and decreasing the base shear in all of the structures, especially in shorter buildings. Similarly, the same effect of the mentioned structure is observed in the far-field earthquakes.
As shown in Figures 27 -37 and Table 11, the velocity of stories in the structures is extracted as (North. NF) and (North. FF) records, and reduced amounts in the velocity of the stories were observed through the following figures and tables:  The second significant feature of support structures with viscous damper in near-field earthquakes is reducing the story velocity in all structures, especially in tall buildings. Evidently, the same effects are observed when the structure is subjected to far-field earthquakes. As shown in Figures 38-48 and Table 12, the response of these structures is extracted as (North. NF) and (North. FF) records, and reduced amounts in the acceleration of the stories could be observed in the following figures and tables: Fig. 38 -Stories   The third vital strength of support structures with viscous damper in near-field earthquakes is reducing the stories' acceleration in all of structures, especially in short structures. The mentioned effects are also evident in all types of structure subjected to far-field earthquakes.
The fourth important advantage of the support structures with viscous dampers is changing the first modal shape of the structure from shear to torsional. This important effect plays a significant role in reducing the effect of modal mass on the first action of the structure. Despite early long pulses in near-field earthquakes records, the risk of structural collapse in base isolation is greatly reduced.

CONCLUSIONS
The base isolation is usually designed to reduce the destructive effects of an earthquake, prolong the effective life of a building, and help the structures to be similar to a rigid body. Although using a base isolation system could improve the mentioned features, in case of a near-field earthquake, it may cause a significant reduction in the structures' stability leading to the building overturn and destruction. The previous studies have demonstrated that the structure with a base isolation design usually experiences critical lateral displacement leading to the structure's overturn. Consequently, one of the main concerns in the utilization of base isolations is the inconsistent performance of the design when subjected to near-field earthquakes. Therefore, if the lateral displacement is handled properly and its amplitude is decreased enough to be safe for the structure and its residents, the application of such a system is fully justified.
As a remedy to the mentioned problem, in this study, a safe method for reducing the lateral displacement of base isolation under near-field earthquakes was proposed. For all the models in the study, a support structure is designed which is connected crosswise with a viscous damper to the roof, and it is rigidly connected to the foundation of the building. Analyses of the results revealed that the implementation of the proposed method has five main advantages which are as follows: • The lateral displacement of the base isolation in all types of structures subjected to near-field earthquakes is significantly reduced. • The base shear was proved to substantially decrease when subjected to near-field ground motions. • The acceleration of the stories is observed to experience a major decrease as a result of the support structure in all types of buildings when subjected to near-field earthquakes. • The velocity of the stories in the case of a near-field earthquake is remarkably dropped in all types of buildings. • The first modal shape of the structures has shifted from shear to torsional.
These advantages demonstrate that most of the destructive effects of the resonance, which could be caused by near-field earthquakes, leading to the overturn of the buildings are eliminated. Hence, the method justifies the application of the lead rubber bearing isolation systems. Moreover, this method could help to save the structural and non-structural properties of the building, and more importantly, this method could reduce the mortality rates caused by near-field earthquakes.