Buildings and more precisely high buildings are very sensitive to earthquake ground motions. Since the 1960–1970’s, the study of structural behavior subjected to seismic motion has been developed and continues to evolve, after strong earthquakes that struck different urban regions around the world caused buildings destruction and loss of lives. These buildings should be protected against seismic motions in order to make their structures safer. Many researchers use the principle of energy dissipation by adding damping devices and other systems to the structure as protective systems [1–4]. These seismic protective devices have been developed to improve the seismic response of building structures against seismic vibrations. The base isolation technique is one of the most used seismic protection techniques in earthquake areas. This isolation technique consists of the separation concept between the structure and its foundation, where the reduction of interaction between the ground and the structure is expressed by the term ‘‘isolation’’, the term ‘‘base’’ means the foundation of the structure [5]. This tool is used to control the passive vibrations of the structure, and it reduces the damage in the structural and non-structural parts of the building subjected to an earthquake [6–8]. It is based on a separation system between the base and the structure which is made by fixing the isolators between the building and the foundation [9]. This provides flexibility and allows for energy dissipation which leads to reduce the seismic movement effect. The main difference between the conventional design approach and the seismic isolation concept is that the first one is based on the increasing of structure resistance (strengthening), whereas the second reduces the dynamic load produced by the seismic waves at the base of the structure [10].

Furthermore, the traditional design approach increases the stiffness of the building, i.e., increases the stiffness of the structural components. However, the non-structural components may be subjected to significant damages during a major earthquake. To reduce the inter-storey drifts, the storey accelerations and the storey displacements, the concept of base isolation is increasingly being adopted [11]. This acceleration reduction protects the non-structural elements, while the reduction in displacements should protect the structural and the non-structural elements during the earthquake with minor damages. Base isolators can be placed at the interface base-superstructure to reduce or filter out the forces transmitted from the ground to the whole structure, or at a specific level to isolate the above storeys. They adjust the dynamic response of the building so that the soil can move below the structures without transferring these seismic loads into the superstructures. In an ideal isolation system when the building achieves this flexibility, they will be a total separation. The increase of the seismic performance of the structure and its content arises from the dissipation of a part of the energy created by the extreme earthquake effect on the structure. The building can survive after an earthquake using the conventional methods, but it may not remain operational [12,13]. However, base isolation technique maintains the building functionality during an earthquake [14].

The application of isolators is well known and familiar in the literature, but there is no proper research for a tall irregular building using real floor plans. This study presents the seismic analysis for a high-rise building located in Beirut (Lebanon), which is irregular in shape using software package ETABS. It is the first implementation of this isolation technology (Lead Rubber Bearing) in this country. Hence, a non-linear time history analysis will be performed to understand the effect of seismic loading on the structural response throughout the loading period. In order to demonstrate the effectiveness of using LRB as an isolator system instead of a conventional shear wall system, a comparative analysis of the response results (such as the fundamental time period, storeys acceleration, storeys displacement, base shear force and inter-storey drift ratio) is accomplished for two structural models. In the following, Section 2 describes the type of isolator chosen for this work and its advantages. Section 3 describes the developed structural models with their properties. Section 4 presents the design of the isolators and their properties calculation. In Section 5, the results of the modeling for the two structural systems and the effect of isolator system on the dynamic behavior of the building are presented. Finally, Section 6 concludes the main findings of this work and opens a brief discussion of its possible extensions.

The development of isolation techniques has been progressed to provide more flexibility and damping to the structure during the seismic attack. Nowadays, lead rubber bearing (LRB) is one of the most used isolator types among all other categories. The first use of LRB was in New Zealand (1970’s) giving a new concept to the design of base isolated structures [15,16]. This technique has been extensively used in Japan and the United States [17], then China joined the group until recent days where the using of LRB has become much popular. From a construction point of view, the addition of a lead-plug which is press-fit into a central hole in the bearing is the difference between lead-rubber bearings and low-damping natural rubber bearings. Hence, this lead-plug deforms plastically under shear stresses, so the energy dissipation capabilities increase in comparison with the low-damping natural rubber bearing [18]. Moreover, LRB is an elastomeric bearing which is made by rubber layers with steel plates, cover plates and a lead core at the center.

Fig. 1 shows the composition of a typical LRB isolator [5]. The advantage of LRB is that it can combine the function of isolation and recentering in one element. Which gives structural support, horizontal flexibility and recentering force to the isolation system [16]. There are many isolation systems that provide sufficient isolation and self-centering too, like low damping rubber bearings and friction pendulum. The yielding of the lead core provides an energy dissipation which allows to achieve an equivalent viscous damping coefficient up to 30%, thereby reducing the horizontal isolator displacement effectively [19,20]. A high vertical stiffness can be obtained by these thin layers of rubber reinforced with steel plates in addition to damping. These characteristics permit the LRB system to carry a significant vertical load owing to its high vertical stiffness and to move laterally with relative lower stiffness than other isolation systems [21]. Therefore, one device can support a structure member vertically, providing a horizontal displacement and increasing the damping in the system to a desired value [6]. Several researchers [22–24] studied the behavior of base isolated building (LRB) under the action of seismic forces, and they compared the results with fixed base building. From these studies, they concluded a reduction in the lateral displacement and acceleration at the top storey. They also concluded that the base shear and the storey drifts are lower for base isolated buildings compared to fixed base buildings.

Dynamic Isolation System [25] is one of the leader companies in the manufacturing of lead rubber bearings. In this study, different types of lead rubber bearings have been examined based on their properties provided by the manufacturer’s website. The method of selection and calculation is detailed in the sections below. The hysteresis loop of the LRB isolator can be modelled as a bilinear curve as shown in Fig. 2, which provided the elastic stiffness (K_e) and the post-yield stiffness (K_d) for the material. The force-displacement relationship of a typical LRB is non-linear [26]. This curve is also given by the manufacturer’s website, depending on the material properties and the mechanical behavior of its LRB.

In the present study, a 168.2 m height (44-storey, 3 basements, and a roof) high rise RC building has been modeled using software package ETABS. In order to proceed with the study two structural models has been designed. The first model is based on a shear wall system (1 m thickness) with fixed support at the base, as shown in Fig. 3a. While in the second model, the shear wall system is replaced with a frame resistance system (Fig. 3b), with the incorporation of the isolation system (LRB) near the base of the columns and while conserving the 3 core walls for the building. The building is subjected to a probability of 10% of exceedance seismic hazards in 50 years, corresponding to the design based earthquake (DBE) [27]. The response modification factor for the fixed base model (Model 1) was taken as the value of 4.5 consistent with shear wall system case according to UBC97 [28], and 8.5 for the moment frame system (Model 2) [29]. This factor should be modified to the value of 2 (R1 = 2) in Model 2, according to IBC isolation section [27] after the incorporation of the LRB isolators. The design of structural members is performed in accordance with ACI-318-14 [30]. LRB isolators are modeled on ETABS as explicit isolator elements with chosen properties.

Fig. 4 shows the two structural systems described above. In Fig. 4a, it has been noticed the positions of the shear walls of the Model 1. Whereas, Fig. 4b demonstrates the replacement of these shear walls by columns, with LRB isolators near the base of these columns. The shape irregularity of the studied high rise building is clearly demonstrated by the floor surface areas, presented in Tab. 1. The ground accelerations generate normally the seismic loads in a structure. A dynamic non-linear time history analysis has been performed on two building models. Such type of analysis allows to understand the dynamic performance of the structure under a real earthquake strike. The main objective of this study is to understand the effect of a base isolator, on the roof storey displacement, roof storey acceleration, base shear reaction and the inter-storey drift ratio of the structures. This analysis has been performed using the two seismic waves for El Centro array 6 earthquake [31], in both X and Y directions (the two perpendicular directions of the ground floor surface plan), as we can observe in Fig. 5. The ground motions were scaled using the gravity acceleration g = 9.81 m/s². The ground motion processes during 39.1 s with 0.436 g as the peak ground acceleration for the two structural models. The soil acceleration vs. time record was available at a time step of 0.01 s. Fig. 6 shows the spectral accelerations for El Centro array 6 seismic waves in function of period domain. It has been noticed the maximum values of spectral accelerations occur at low period values i.e., at high frequencies. Buildings with a low natural period will be subject to high excitations under these seismic waves. There are several different versions of the El Centro record in existence [32,33]. The El-Centro ground motion is one of the earliest recorded and most widely used near-field ground motions. It does not contain pulses in the acceleration, velocity, or displacement histories. The peak values of ground acceleration, velocity, and displacement (PGA, PGV and PGD) for El Centro ground motion are available in the work of Malhotra et al. [34].

A static design of the high-rise building has been performed on ETABS, in order to calculate the vertical load acting on each isolator. This was the first step of LRB design. After that, the LRB isolators was divided into 4 types regarding the range of load acting on them, as shown in Fig. 4b. Two main factors which control the design of the LRB isolator system and affect its elastic forces are: the total effective stiffness and the maximum displacement of the bearing. These two parameters should be minimized in order to have an economical design. Using the design equations of IBC code-section 1623 for isolator design [27,35] and the data provided by the manufacturer [25], all design criteria of LRB have been calculated.

LRB isolators can be designed to carry different values of displacement by changing their diameter and their design parameters. The main parameters calculated for the four isolator types are: the effective stiffness K_eff (the isolator force divided by the displacement), the effective damping (25%), the isolator diameter (D_i), the lead diameter (D_L), the isolator Height (H), and finally the number of layers (n). The elastic stiffness K_e and the yielding stiffness K_d (Fig. 2) are specific for each type and size of LRB isolator, regarding its Hysteretic behavior curve and represents the input parameter for the calculation of K_eff. The design limit of rubber shear strain is 250%, and the rubber shear modulus is G = 0.7 MPa. The maximum horizontal displacement D_max is the control parameter of each LRB isolator. After the time history analysis, the displacement calculated in each isolator should be less than the maximum displacement property provided in the manufacturer data, in order to have a safe design and avoid isolator failure. The design properties of the 4 isolator types are shown in Tab. 2. However, isolator stiffness was the same in both longitudinal and traversal directions of the building.

The results of the non-linear time history analysis have been performed for the two structural models, for fixed base model (shear wall system) and for isolated model, in terms of the time history functions of storey responses (accelerations, displacements, base shear forces) and also storey responses in function of each level. These functions give better insight into the response behavior of the structure at each time step of the analysis. The following sub-sections detail these results.

In this study, a non-linear structural response analysis of high rise building in an active seismic region was performed by the incorporation of the innovative LRB isolators. The seismic analysis compared two structural models of the same high rise building. After providing LRB isolator system, the fundamental period of the isolated structure is increased by approximately 3 times compared with the fundamental period of the fixed base structure. We can also conclude by the comparison of the two models that:

The reduction in the energy dissipation, i.e., acceleration, at the roof storey is 55.24%.

Under seismic loading the response performance of a conventional fixed base structure is greatly improved by the incorporation of the LRB isolation system. The efficiency of this system depends on the characteristics of the seismic earthquake, its designed properties, and on the structural criteria. Therefore, it is important to perform a detailed analysis to make an efficient design of the rubber bearings according to the vibration data. The main goal of this isolation system is the gained flexibility in the structure and the reduced damages in the vibration movements of the structure. This study also proved also the feasibility of this system in high rise buildings. It should be mentioned that in this study, only one earthquake record has been used to find the optimum seismic response of the building. However, more generalized solutions can be achieved using additional records.