1 Introduction

Aseismic isolation is a well-accepted effective method for protecting structures against earthquakes. It is generally used in the foundation level and is known as base isolation. This strategy of structural design is based on reducing the demand instead of increasing the capacity. Increasing the capacity can sometimes be uneconomical or impractical and may lead to situations in which the structure itself is undamaged but the contents are damaged or destroyed and the occupants injured. Aseismic base isolation reduces the seismic demand by reducing the fundamental frequency of the structure and also provides an amount of damping. If this technology is used properly, the seismic performance of the structure will be improved (Tavakoli et al. 2014). Base isolation of a structure can result in the reduction of inter-story drifts and floor accelerations, which are considered as the performance measures in most of the design codes such as IBC (2012) and ASCE 41-13 (2013). The first application of aseismic base isolation refers to the ancient Iran (Sepahbodnia 2006; Botis and Harbich 2012; Bek et al. 2013). It continues to get considerable attention, particularly after the recent strong earthquakes such as the Kobe earthquake in 1995. Base isolation technology is used in many countries (Warn and Ryan 2012). To date, different isolation systems (ISs) have been developed (Martelli et al. 2014; Narjabadifam 2015; Falborski and Jankowski 2017). Isolated buildings have performed well in the previous earthquakes (Du and Han 2014). Seismic performances, however, depend on the characteristics of both structures and earthquakes (Kelly 1999). Several studies have been carried out about the roles of the properties of superstructures and ISs on the performances of base-isolated structures. Jangid (2002) studied these performances through a parametric study and concluded that structural parameters significantly influence the effect of isolation. Jain and Thakkar (2004) investigated the effect of structural stiffness and showed that increasing stiffness in superstructure increases the effectiveness of isolation. Jalali and Narjabadifam (2006) presented a study on the effects of additional mass and damping on the seismic performances of isolated buildings and indicated that the performances can be improved by the modification of dynamic properties of superstructures. In a study by Providakis (2009), the effects of supplemental damping on laminated rubber bearings (LRBs) and friction pendulum systems (FPSs) were investigated and it was shown that additional damping is the main parameter affecting the seismic response of isolated buildings located in near-fault regions. It had already been revealed by Kelly (1999) and Hall (1999) that isolated buildings subjected to near-fault ground motions are struggling with large displacements and the solution for this problem is to use damping to mitigate displacements. Sharbatdar et al. (2011) have also studied the seismic performances of structures isolated with FPS and high-damping laminated rubber bearing (HRB), showing that large displacement and velocity pulses of near-fault motions severely affect the performances of base isolation. The references reviewed above are not the only references related to the subject, and some other remarkable works have also been reported by Fan et al. (1990), Kulkarni and Jangid (2003), Hong and Kim (2004), Matsagar and Jangid (2004), Rabiei (2008), Sharma and Jangid (2009), Abrishambaf and Ozay (2010), Ounis and Ounis (2013), Chun and Hur (2015), Tolani and Sharma (2016), Folic and Stanojev (2016), and Bhandari et al. (2017). In practice, however, it is still required to investigate the effects of inherent characteristics of both superstructure and IS to reveal the practical effectiveness of ISs, remaining as an important engineering question. This is intrinsically different than the investigations of the effects of additional mass and damping or stiffening the superstructure. A comprehensive study to investigate the effects of inherent structural characteristics on seismic performances of ISs, in this regard, has been carried out within a postgraduate research program to find the answer to the above-discussed question. Next sections report on the methodology and outcomes of this research.

2 The Methodology of Research

For the investigation of effects of inherent structural characteristics on seismic performances of ISs, a detailed parametric study is required. Numerical analyses for such a study must be nonlinear, due to the nonlinearities of isolators, and the structures must be properly designed. Both the design and the analysis procedures must be verified before to be sure about the accuracy of results. The reference work for the purpose of verification is the work reported by Tavakoli et al. (2014). They have studied the responses of the base-fixed and isolated building frames and reported the base shears of a base-fixed and isolated 4-story reinforced concrete building frame, as shown in Fig. 1. The same has been carried out in this research, and the results have been compared with the results reported by Tavakoli et al. (2014). Base shears are compared in Table 1.

Fig. 1
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The building frame studied by Tavakoli et al. (2014)

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Table 1 Base shears obtained for the building frame studied by Tavakoli et al. (2014) based on the design and analysis method of this research, compared to those reported in the reference work (Tavakoli et al. 2014)
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As can be seen, the base shears obtained are close to those reported by Tavakoli et al. (2014). So, the design and the analysis procedures used in this research are verified.

Flowchart of the research conducted as the basis for this paper is represented in Fig. 2, with the details given for the structural characteristics varying inherently in the practical ranges. This research investigates the effects of inherent structural characteristics through 1176 nonlinear time history analyses (NTHAs) on 84 structural models (as described in Tables 3 and 4) subjected to 14 near- and far-field ground motions. The superstructures are 3-, 7-, and 11-story steel and reinforced concrete moment-resisting (MR) and braced frame buildings (12 cases) with different inherent structural characteristics in terms of mass, stiffness, and damping, while the superstructure damping will not be studied due to its negligible effect discussed already in the literature (e.g., Jangid 2002; Jalali and Narjabadifam 2006). Seven isolation strategies are designed based on three design displacements (Dd) and two coefficients of friction (µ, representing the lubricated and nonlubricated sliding surfaces in practice) using HRB and FPS.

Fig. 2
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Flowchart of the research

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3 Superstructures

The superstructures are two-dimensional models of typical 3-, 7-, and 11-story buildings on soil type III in a region with very high level of relative seismic hazard of Standard No. 2800 (2015), designed according to ACI 318-11 (2011) and AISC 341-10 (2010) using ETABS (2016).

The seismic performances of these superstructures are studied for three earthquake-resistant structural systems resulting in 36 structural models described in Table 2.

Table 2 The structural models
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4 Isolation Systems

The isolators considered for this research are HRB and FPS, as the most practical ISs. The mechanical behavior of HRB is shown in Fig. 3. These isolators provide damping around 20% according to AGOM (2017), FIP (2017), Maurer (2017), DIS (2017), and OILES (2017). The Isolator1 nonlinear link element (rubber isolator) of ETABS is used to model the HRBs with the design details given in Table 3. Figure 4 shows the force–displacement behavior for FPS isolator. Frictional parameters (rate parameter, coefficient of friction at slow and fast velocities) are calculated based on Dolce et al. (2005). FPSs are modeled by Isolator2 nonlinear link element (friction isolator) of ETABS with the design details given in Table 4.

Fig. 3
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The HRB isolator and its mechanical behavior (AGOM 2017)

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Table 3 The mechanical properties of HRBs in this research
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Fig. 4
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The FPS isolator and its mechanical behavior (OILES 2017)

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Table 4 The mechanical properties of FPSs in this research
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5 Ground Motions

The structures are subjected to seven near-field and seven far-field ground motion records. The records are selected and downloaded from the ground motion database of PEER (2017). The events are the same for both the near- and far-field records. The stations are selected as the nearest stations to the origin in the cases of near-field records, and the largest distances are selected for the far-field records.

Table 5 shows the details for the near-field ground motion records, regarding the names of the stations that the ground motions have been recorded, magnitude of the main event, closest distance to the rupture, and the peak ground acceleration in the record. Far-field ground motion records are similarly shown in Table 6 for their technical details, the same as those given above for the near-field records.

Table 5 Near-field ground motions used in the NTHAs
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Table 6 Far-field ground motions used in the NTHAs
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All the ground motions are scaled to the design spectrum of the structures, using SeismoMatch (2016). The design spectrum is obtained based on the specific requirements of the Iranian guideline for design and practice of base ISs in buildings (2010) known as guideline No. 523 of office of deputy for strategic supervision of the bureau of technical execution system of the vice presidency for strategic planning and supervision and the Iranian guidelines for design of seismic base-isolated buildings (2016) considered in addition to the Iranian code of practice for seismic-resistant design of buildings (2015) known as the yellow book or Standard No 2800. The matching algorithm is the default well-known wavelets algorithm of the SeismoMatch, proposed by Hancock et al. (2006). Matching is carried out for the period range 0.05–2.05 s based on the period range of the structural models varied between 0.1 and 2 s, as reported in Tables 7 and 8.

Table 7 Base shears in near-field ground motion records
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Table 8 Base shears in far-field ground motion records
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Figure 5 shows the near-field spectra obtained from SeismoMatch for the near-field ground motion records of Table 5 scaled to match the design spectrum within the period range of the structural models.

Fig. 5
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Near-field spectra scaled to design spectrum

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Figure 6, similarly, shows the scaled far-field spectra obtained from SeismoMatch for the far-field ground motion records of Table 6.

Fig. 6
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Far-field spectra scaled to the design spectrum

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All the 14 scaled ground motion records are used in both nonlinear (for the base-isolated building frames) and linear (for the fixed-base building frames) time history analyses carried out by ETABS, when the results are presented and discussed in the next section.

6 Results and Discussion

Seismic performances of the aseismically base-isolated building frames are studied in terms of base shears, story accelerations, and story displacements obtained from 1176 NTHAs on the structural models introduced in Tables 2, 3, and 4 subjected to the ground motion records of Tables 5 and 6 scaled to the design spectrum as shown in Figs. 5 and 6. These performance criteria are also studied through 168 additional linear time history analyses for the traditional fixed-base buildings (see the descriptions for 3TMS, 3TBS, 3TMC, 3TBC, 7TMS, 7TBS, 7TMC, 7TBC, 11TMS, 11TBS, 11TMC, and 11TBC in Table 2) subjected to the same records in order to provide the opportunity of comparing the seismic performances of the base-isolated buildings to those of the traditional fixed-base buildings.

Table 7 summarizes the base shears obtained from the analyses carried out on the isolated and the fixed-base buildings subjected to the near-field records. The base shears are reported on average over the all isolation strategies (three cases for HRB and four cases for FPS) designed with the details given in Tables 3 and 4, respectively, for isolation with HRB and FPS. The same are reported in Table 8 for the buildings subjected to the far-field records. The data provided in Tables 7 and 8 are used to generate the diagrams of Figs. 7 and 8 in order to reflect the sensitivities of the ISs to the inherent structural characteristics, regarding base shear as the main performance criteria indicating the energy input during ground motion.

Fig. 7
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the base shear, in the near-field ground motions

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Fig. 8
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the base shear, in the far-field ground motions

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Detailed effects of the inherent structural characteristics on base shears are represented by the trend lines logarithmically fitted on the analytical data in Figs. 9, 10, and 11. As it was already mentioned in Sect. 2, superstructure mass and stiffness and isolation damping are considered as the variables. The isolation stiffness is not discussed because it is basically controlled through the design for the target displacement. The effect of the superstructure damping is also neglected compared to the effect of the damping in IS.

Fig. 9
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The effects of the inherent superstructure mass on the reduction of base shear through aseismic isolation

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Fig. 10
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The effects of the inherent superstructure stiffness on the reduction of base shear through aseismic isolation

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Fig. 11
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The effects of the inherent structural damping on the reduction of base shear through aseismic isolation

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As far as the discussion regarding other performance criteria (story acceleration and story displacement) is considered, for the purpose of brevity the sensitivity diagrams and the trend-line curves reflecting the effects of pre-defined inherent structural characteristics on the seismic performance are presented without reporting the numerical data given in tables. Figures 12 and 13 reflect the sensitivities of the ISs (in terms of reducing the story accelerations) to the inherent structural characteristics. Figure 12 presents the sensitivities of the ISs in near-field ground motions, and Fig. 13 reflects the situation in far-field ground motions. Detailed effects of the inherent structural characteristics on story accelerations are also represented in Figs. 14, 15, and 16. Similarly, Figs. 17 and 18 reflect the sensitivities of the ISs (in terms of reducing the story displacements) to the inherent structural characteristics. Detailed effects of the inherent structural characteristics on story accelerations are also represented in Figs. 19, 20, and 21.

Fig. 12
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the story acceleration, in the near-field ground motions

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Fig. 13
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the story acceleration, in the far-field ground motions

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Fig. 14
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The effects of the inherent superstructure mass on the story acceleration control through aseismic isolation

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Fig. 15
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The effects of the inherent superstructure stiffness on the story acceleration control through aseismic isolation

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Fig. 16
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The effects of the inherent structural damping on the story acceleration control through aseismic isolation

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Fig. 17
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the story displacement, in the near-field ground motions

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Fig. 18
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The sensitivities of the ISs to the inherent structural characteristics, in terms of reducing the story displacement, in the far-field ground motions

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Fig. 19
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The effects of the inherent superstructure mass on the story displacement control through aseismic isolation

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Fig. 20
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The effects of the inherent superstructure stiffness on the story displacement control through aseismic isolation

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Fig. 21
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The effects of the inherent structural damping on the story displacement control through aseismic isolation

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Comparing the dashed lines passing separately over the average responses of FPS and HRB in Figs. 7 and 8, it is clear that FPS is less sensitive to the inherent structural characteristics, in terms of reducing the base shear. It can also be obviously concluded from the average response curves on the histograms of Figs. 7 and 8 that FPS is almost always more effective than HRB, when the base shear is considered, in both the near- and far-field ground motions. The average 65% reduction of base shear in the lighter structures compared to 85% reduction of base shear in the heavier structures in Fig. 9 indicates that aseismic base isolation is more effective in heavier structures. As shown in Fig. 10, base isolation further reduces the base shear if the superstructure is stiffer. A minimum amount of damping is useful for the reduction of base shear through aseismic base isolation, as it is shown in Fig. 11 for FPS. The higher sensitivity of HRB to the inherent structural characteristics is more evident based on the higher zigzaggedness of the dashed line passing over its average responses compared to those of FPS in Figs. 12 and 13. It is again clear that FPS better controls the story accelerations compared to HRB, in both the near- and far-field ground motions. Story accelerations in the structure mounted on FPS are almost 20% less than those controlled by HRB. Based on the data presented in Fig. 14, the effectiveness of HRB in reducing the story acceleration reduces obviously by the inherent increase in the superstructure mass, while the effect is lighter for the effectiveness of FPS in controlling the story acceleration. The effect of the stiffness in terms of controlling the story accelerations is like its effect on base shear. Damping is always useful for the reduction of story accelerations.

Both FPS and HRB are almost similarly sensitive to the inherent structural characteristics, in terms of controlling the story displacements (see Figs. 17 and 18). The sensitivities are less than those in terms of the base shears and story accelerations. HRB is, however, more effective than FPS in terms of reducing the story displacements, always, in both the near- and far-field ground motions. All the effects of the inherent structural mass, stiffness, and damping on the control of story displacements via base isolation are less important compared to the effects of those on the control of story accelerations and base shears (compare Figs. 19, 20, 21 to Figs. 14, 15, 16 and Figs. 9, 10, 11, 12, 13, 14, 15, 16). It is, however, remarkable that inherent increase in the isolation damping will increase the story displacements, as it can be concluded from Fig. 21. It is expectable because the damping generally adds an amount of stiffness to the IS. Superstructure stiffness increases the effectiveness of isolation in terms of reducing the story displacements a little. As far as the effects of the inherent structural mass are considered, Fig. 19 shows that the effectiveness of base isolation in terms of controlling the story displacements reduces for the heavier structures.

7 Conclusions

The outcomes of an extensive parametric study investigating the effects of the inherent structural characteristics on the performances of aseismic isolation were reported. It was discussed that this study is different than the investigations of the effects of additional mass and damping or stiffening the superstructure, which are aimed at evaluation of the performance enhancement. The purpose of this study is to understand the effects of the inherent structural characteristics to reveal the practical effectiveness of ISs (isolation systems), which is also different than the study of the aging effects that result in some deteriorations through the increases in post-yield stiffnesses and characteristic strengths (McVitty and Constantinou 2015).

Mass, stiffness, and damping (the fundamental dynamic characteristics) were varied through the variation of the materials, the structural systems of the superstructures, heights of the superstructures, types of the ISs, and the design parameters of the ISs to practically capture the effects of the inherent structural characteristics. The materials used in the superstructures were steel and concrete, as in the everyday practice of construction, leading to the consideration of the two common types of buildings (steel-framed and reinforced concrete buildings). Structural systems considered were braced frames (X-bracing for steel-framed and shear walls for reinforced concrete buildings) and moment-resisting frames, as the two practical systems. The heights were varied based on the numbers of the stories of the buildings designed with 3, 7, and 11 stories. All the buildings were designed according to the provisions of the Iranian code of practice for seismic-resistant design of buildings known as the yellow book or Standard No 2800 (2015) for soil type III in a region with very high relative risk of seismic hazard. The ISs were chosen to be HRB (high-damping laminated rubber bearing) or FPS (friction pendulum system), as the most famous currently used practical ISs. Damping and coefficient of friction, varied in the practical ranges, were, respectively, selected as the main design parameters of ISs. The seismic performance criteria were base shear (as the criterion for the energy input) and story acceleration and displacement (as the serviceability criteria). The methodology was described, and the results were discussed including the sensitivities of the ISs to the inherent structural characteristics together with the effectiveness of the ISs in terms of reducing the performance criteria. The conclusions are summarized as follows:

  • The inherent structural mass has a positive effect on the reduction of energy input through aseismic base isolation. This means that aseismic base isolation is more effective for the structures with larger mass. The story accelerations and story displacements, however, will poorly be controlled in the heavier superstructures.

Based on the results, it can also be concluded that additional mass will help the aseismic base isolation in energy input reduction. This is in accordance with the results reported already by Jalali and Narjabadifam (2006) through the investigation of the effects of additional mass, stiffness, and damping on the performances of buildings base-isolated using lead-plug laminated rubber bearings.

  • The inherent stiffness of superstructure is useful for the improvement of the performances of aseismic base isolation, regarding all the performance criteria including the energy input and the mitigation of the responses. In the other words, aseismic base isolation performs better with the stiffer superstructures.

The stiffening of the superstructure will also useful in aseismic base isolation, as it was already indicated by Jain and Thakkar (2004) investigating the effects of stiffening on the performances of aseismic base isolation.

  • The damping provided inherently by the IS further reduces the story accelerations, while it has a reverse effect on the energy input and story displacements. Damping is, however, required for the mitigation of the large isolation displacements in near-field ground motions.

It should be added that better performances are expected by the modern damping mechanisms like the hysteretic damping provided by austenitic shape memory alloys, as it has been demonstrated by Cardone et al. (2011), regarding also the outcomes of the research by Kelly (1999) and its discussion by Hall (1999).

  • The seismic performances of base isolation by FPS are less sensitive to the inherent structural characteristics, when compared to HRB.

  • The effectiveness of FPS in reducing the energy input is more than HRB in both near- and far-field ground motions. FPS is also able to better control the story accelerations. Story displacements are, at the same time, better controlled by HRB. It should, however, be noted that while the design displacements of FPS and HRB are taken to be the same in this study, the levels of energy dissipation capabilities should be compared with more details, which was not the scope of this paper, but suggested for further investigations.