New structural seismic protection for highrise building structures
Azer A. Kasimzade^{1} , Obaidullah Abrar^{2} , Gencay Atmaca^{3} , Mehmet Kuruoglu^{4}
^{1, 2}Department of Civil Engineering, Ondokuz Mayis University, Samsun, 55100, Turkey
^{3}Provincial Directorate of Disaster and Emergency, Samsun, 55200, Turkey
^{4}Department of Civil Engineering, Dokuz Eylul University, Izmir, 35160, Turkey
^{1}Corresponding author
Journal of Vibroengineering, Vol. 22, Issue 4, 2020, p. 831848.
https://doi.org/10.21595/jve.2019.20741
Received 21 April 2019; received in revised form 6 November 2019; accepted 13 November 2019; published 30 June 2020
JVE Conferences
Presented Structural Seismic Isolation Method (SSIM) aims to provide high safety for Highly Reliable Structures (HRS) against strong earthquakes including nearfault and longperiod ground motions. The examined structure is converted to Structural Seismic Isolation System (SSIS) by the SSIM method which exhibited inverse pendulum behaviour. For this purpose, structure foot base and foundation contact surfaces have been designed as any curved surfaces (spherical, elliptical, etc.) depending on the earthquakesoilsuperstructure parameters and this contact surfaces have been separated by elastomeric (lead core rubber or laminated rubber bearings) seismic isolation devices. It would allow the structure foot base to turn around gyration centre through rubber bearing contact and maintains similar behaviour to the superstructure. SSIS system provides the possibility of keeping the naturalperiod of the structure in a larger interval, which is greater than the predominantperiod of the majority of possible earthquakes (including nearfault pulse) using currently existing conventional elastomeric isolators with up to 4 second period. Thus, the structure can sustain its serviceability after strong and longperiod earthquakes. In this study SSIS system’s performance is presented for highrise building structures, for this aim, the finite element model of the building (Bg) structure with SSIS system (SSISBg) has been prepared and the nonlinear dynamic analysis of the model has been conducted using strong and longperiod ground motions. Results indicate that the base and top accelerations, base shear and base moment responses of the SSISBg structure is 23.21 %, 75.47 % and 85.74 % in average lower than the Conventional Application Method of Seismic Base Isolation Devices for Building (CAMSBIDBg) structures respectively and it is not prone to resonant vibrations under longperiod earthquakes related with the excessive deformation in the isolation layers in case of using CAMSBIDBg structures. It should be noted that in this study with the presented SSIM method and SSIS system, it is aimed to protect only the Highly Reliable Structures(HRS) from the effects of strong and longperiod ground motions and these structures (HRS) are classified as follows: 1) Nuclear Containment Structures; 2) Highrise buildings that contain information, operating systems, sensitive instruments, communication systems, routing systems, bank operating systems, databases, management systems and other similar facilities that are linked to the security and economy of a country; 3) Highrise hospitals etc.
 Base shear and base moment responses of the SSISBg are 75.47% and 85.74% on average lower than CAMSBIDBg (Conventional Application Method of Seismic Base Isolation Devices for Building) structure
 The significantly lower response of the SSISBg (Structural Seismic Isolation System for Building) structures allows making it even lighter
 In terms of total useful area of the buildings mainly the foundation part of the SSISBg is much more efficient than CAMSBIDBg and FBBg (Fixed Base for Building) structures
 SSISBg system allows the efficient use of seismic protection devices (LRB, LCRB, etc.) which have a predominant period (up to 4 seconds) in the resonance ranges for the longperiod earthquakes
 The usage of the foundation contact curved surfaces (spherical, elliptical, etc.) depending on the earthquakesoilsuperstructure parameters, it is another striking dimension of the research subject
Keywords: aseismic base isolation, SSIM method and SSIS system, earthquake resistant structures, nonlinear FEM, longperiod earthquake.
1. Introduction
Nearfault ground motions (e. g. 1978 Tabas, 1995 Kobe, 1999 ChiChi) caused severe damages to buildings and bridges and resulted in serious causalities [1, 2]. The pulse period of nearfault earthquakes ranges between 1.47 secs for the earthquakes with a magnitude of 6 to 7.6 [3]. The pulse period ranges for all of the nearfault earthquakes records used in this study are between 0.512.9 secs. Characteristics of strong earthquake ground motion in the period range from 1 to 15 seconds was classified [4], thus, it can generate longperiod ground motions which can be harmful to longperiod structures [4, 5]. On the other hand, farfield longperiod earthquakes (e. g. 1985 Mexico City, 2010 Darfield, 2011 Tohoku) can cause the same amount of damage for highrise structures as well [6, 7]. Usage of seismic isolation systems for the purpose of protection of structures against strong earthquakes became widespread and considered as an effective method for seismic protection of structures [8, 9]. Various types of seismic isolation methods such as elastomeric, friction pendulum etc. have been developed and implemented in practice. It is obvious that seismic isolation provides better safety for structures during strong earthquakes. In short, as a result of using the Conventional Application Method of the Seismic Base Isolation Devices for Building (CAMSBIDBg) structures [8, 9], superstructure’s dominant period will be shifted to the seismic base isolation device’s dominant period, which is the about 24 seconds in currently available devices. Therefore, the accelerations in the superstructure are significantly reduced compared to the earthquake acceleration. Several reports indicate the vulnerability of baseisolated structures against nearfault pulse and longperiod earthquakes (with a dominant period of more than 24 seconds). For instance, during the 2011 Tohoku, Japan earthquake seismic isolation level of buildings suffered serious damages due to large displacements [10, 11]. In other words, in the CAMSBIDBg structures, the resonance of the upper structure is inevitable under longperiod (more than 24 second) earthquake effects and this leads to structural damage and halts its service.
As seen from the references protection of the structures from the effects of strong and longperiod ground motions has not been solved by the conventional application method of the seismic base isolation devices (CAMSBID) method [8, 9] and the reasons (A, B) are summarized below:
A) The vibration period of Lead Rubber Bearing (LRB) and Lead Core Rubber Bearing (LCRB) seismic base insulation devices are currently limited between the range of 24 seconds. The mentioned types of seismic base isolation devices are added to the structures by the CAMSBID method and their dominant period is shifted to the range of 24 seconds, thereby mitigating the earthquake effect. However, these structures remain unprotected by resonance in the event of the effects of longperiod (more than 4 seconds) components of ground motion.
B) Because the Friction Pendulum Bearing (FPB) type seismic isolator carries the building load to the small area pendulum bearing on the friction surface of the device and it causes following three disadvantages: This device cannot work immediately by creating a permanent deformation on the friction surface of the device during the waiting period until the devastating earthquake occurs (1). In structures with high gravitation loads, the risk of tearing of the plate under the pendulum support is very high (2). The vibration period of FPB seismic base isolation devices is currently in the range of 26 seconds in case of ideal zero friction (3). Due to these known shortcomings in the implementation of these devices, the effective usage of the FPB devices is limited.
According to the literature, there are several studies on enhancing the performance of seismic baseisolated structures against nearfault and farsource longperiod earthquakes. Sliding seismic isolation with controllable stiffness method was proposed and studied by Lu et al. [12]. Hosseini et al. carried research on the feasibility of the application of orthogonal pairs of rollers on concave beds as a seismic isolation system for the protection of midrise structures [13]. Ismail (2015) proposed a new seismic isolation device with a selfstopping mechanism to dissipate the harmful effect of nearfault earthquakes [14]. Another study suggests using of wave generator in the boundary of structure to reduce the amplitude of ground vibrations [15]. Seismic protection of mediumrise RC (reinforced concrete) structure using elastomeric and sliding seismic isolators against the harmful effect of the nearfault earthquake was studied by Fabio et al. [16]. These studies [1216] include solutions for specific situations and are difficult to generalize like conventional seismic isolation (CAMSBID) methods [8, 9].
Kasimzade et al. [1721] proposed and developed new Structural Seismic Isolation Method (SSIM) for protection of structures against strong and longperiod ground motions and aims to eliminate the limitation and vulnerability of the conventional elastomeric (lead rubber or laminated rubber bearing) baseisolated structures for same excitations [22]. On the bases of this method, the present structure is converted to a Structural Seismic Isolation System (SSIS) by the SSIM method, which exhibited inverse pendulum behaviour. SSIS system provides the possibility of keeping the naturalperiod of the structure in a larger interval, which is greater than the predominantperiod of the majority of possible earthquakes (including nearfault pulse) using currently existing conventional elastomeric isolators with up to 4 second period and it is not caused resonant vibrations under longperiod earthquakes.
The related patents “Antiearthquakes structure insulating kinetic energy of earthquake from buildings” [23], “Earthquakeproof object support device” [24], “Rockingtype seismic isolation base for protecting structure against earthquake [25], “Friction Pendulum Bearing” [26], ”Anti Seismic Support” [27], “An EarthquakeProof Building” [28], ”The doublespherical friction single pendulum support saddle” [29], “Spheroidal joint for column support in Tuned Mass Damper System” [30], ”Assembly Type BuildingFoundation And Construction” [31], “Base Isolation Supporting Device” [32], “Circleegg thinshell frame shaketype earthquakewaveresistant architectural structure” [33], “Ovum circle shell frame shaking type ripples building structure of antiseismicing” [34] and ”Sliding Bearing For Supporting Civil Or Structural Engineering Works” [35] on the subject of the seismic protection of the structures were examined and it was found that the solution of the mentioned subject with SSIM method was a first and original and a patent application was made on the subject [21].
In this study, the dynamic performance of SSIM method application for highrise steel SSIS system building structure (SSISBg) was presented in comparison with the conventional application method of the seismic base isolation devices for building structures (CAMSBIDBg) and fixed base building structures (FBBg) using finite element simulation.
1.1. Fundamentals and advantages of SSIM method
As mentioned, Structural Seismic Isolation Method (SSIM) aims to provide high safety for Highly Reliable Structures (HRS) against strong earthquakes including nearfault and longperiod ground motions. On the bases of this method, the present structure is converted to a Structural Seismic Isolation System (SSIS) by the SSIM method, which exhibited inverse pendulum behaviour. For this purpose, structure foot base (Fig. 1(a), part 2) and foundation contact surfaces (Fig. 1(a), part 4) can be designed as any curved surfaces (spherical, elliptical, etc) depending on the earthquakesoilsuperstructure parameters and this contact surfaces are separated by elastomeric (lead core rubber or laminated rubber bearings) seismic isolation devices (Fig. 1(a), part 3). It would allow the structure foot base to turn around gyration centre through rubber bearing contact and maintains similar behaviour to the superstructure (Fig. 1(a), part 1). SSIS system provides the possibility of keeping the naturalperiod of the structure in a larger interval, which is greater than the predominantperiod of the majority of possible earthquakes (including nearfault pulse) using currently existing conventional elastomeric isolators with up to 4 second period. In the case of using the CAMSBIDBg structure (Fig. 1(c)), superstructure’s vibration dominant period will be approximately equal to the elastomeric isolator’s dominant period (24 second). Therefore, CAMSBIDBg structure is prone to resonant vibrations under longperiod earthquakes related to the excessive deformation in the isolation layers. Consequently, CAMSBIDBg structures are vulnerable under nearfault and longperiod ground motions. Due to this problem, the conventional application of the currently available seismic base isolation elastomeric devices is limited. In addition, as the SSIS system provides the opportunity of controlled rotation to the superstructure, less bending moment and shear force will be formed in the superstructure base, in comparison with CAMSBIDBg structures.
Major advantages of SSIS system obtained by the SSIM method over previously studied methods [1216] and CAMSBID method [8, 9] can be noted as follows:
– SSIS system is applicable for highly reliable structures including highrise buildings while the majority of mentioned studies are focused on lowrise or mediumrise structures.
– Some of the mentioned seismic isolation methods require extra energy to function which may be inconvenient in some cases. On the other hand, the SSIS system is a totally passive seismic isolation which does not require any extra energy for functioning.
– The usage of friction pendulum isolator for the purpose of seismic isolation could be problematic due to stickslip and nonuniform pressure distribution of the pendulum on steel plate, while the SSIS system uses elastomeric isolators.
– SSIS system is more reliable than CAMSBIDBg systems under the effect of longperiod and nearfault earthquakes due to the fact that the period of seismic isolators used in CAMSBIDBg structure is between 24 seconds. While SSIS system’s period is much higher thanks to turnaround gyration centre and it does not cause resonant vibrations.
Fig. 1. Schematic illustration of a) the SSIS system obtained by SSIM method and b) completed SSISBg structure and c) CAMSBIDBg structure: 1 – superstructure, 2 – curved surface superstructure foot base, 3 – elastomeric seismic isolation devices, 4 – foundation contact curved surface, 2b – plane surface CAMSBIDBg superstructure foot base, 4b – Foundation contact plane surface of CAMSBIDBg structure [21]
The governing equation and the mathematical model of the SSIS system with the spherical structure foot base and foundation contact surfaces (Fig. 2) have been presented as following [19]:
With:
where $\left[m\right]$, $\left[c\right]$, $\left[k\right]$ are mass, Rayleigh damping and stiffness matrix of the superstructure respectively and it is composed by FEM [35], $u$ is the relative displacement vector of the deformed states of the superstructure. $\dot{u}$ and $\ddot{u}$ are the velocity and acceleration vector respectively. ${F}_{\ddot{u}g}$ stands for seismic force:
with:
where, $\phi $ represents the absolute rigid structure’s rotation angle around the gyration centre, ${\ddot{u}}_{g}$ is the ground motion excitation; ${F}_{c0}={c}_{b}{\rho}_{2}\dot{\phi}$ is total damping and ${F}_{kb}$ total stiffness forces of seismic isolator deployed in SSIS system which possess ${c}_{b}$ (total dampingcoefficient) and spherical radius (${\rho}_{2}$). ${F}_{c,eq}={c}_{d}{\rho}_{2}\dot{\phi}$ represents the sum of external dampers’ equal damping force which contains the ${c}_{d}$ (damping coefficient). $\phi $ is the solution of the Eq. (2):
where ${h}_{i}$ ($i=1,\mathrm{}n$) is the $z$ distance of the superstructure’s $i$th mass ${m}_{i}$ from the gyration centre, ${m}_{0j}$, ${h}_{0j}$, ($j=1,\mathrm{}m$) are similar parameters for the underground part of the SSIS system.
Fig. 2. General working mechanism schema of the SSISBg system
The lateral displacement of superstructure’s base is indicated by (${u}_{b}$) which correlates to the contact surface of the foundation. ${u}_{y}$ (yield displacement), ${k}_{b}$ (total stiffness) of the isolators, $\alpha $ (post to preyielding stiffness ratio commonly taken as; $\alpha =$ 0.1). The rations ${d}_{r}={u}_{yb2}/{u}_{y}=$ 9.98 and ${f}_{r}={F}_{yb2}/{F}_{y}=$ 2.01 respectively as described in Fig. 3. The parameter $\dot{Z}$ refers to dimensionless hysteresis displacement component satisfies the nonlinear firstorder [37, 38] differential Eq. (5), ${F}_{y}$ and $Q$ refer to yield and characteristic strengths of the seismic isolator respectively. By defining the ${F}_{0}$ parameter as presented in Eq. (12) with regard to the total weight of the structure $W$ the yield strength of the seismic isolator can be normalized:
In some references normalized stiffness has been expressed as follows:
where ${m}_{i}$, ${m}_{b}$, and ${m}_{t}$ are the mass of the storey, base slab and total mass of the building respectively.
Fig. 3. Illustration of the hysteresis loop of the LCRB isolator and its geometric rations [19]
In Eq. (5) $\beta $, $\alpha $, $n$ and $\gamma $ are dimensionless parameters and affects the shape of the hysteresis loop, the value of these parameters are predicted through experiments. Here, the value of above mentioned parameters are taken as: $n=2$; $\alpha =1$ and $(\beta +\gamma )/a=1$. The model of Eq. (5) decreases to a viscoplasticity model, in Eq. (5) ${u}_{y}$ refers to yield displacement. The base and top absolute displacement $({u}_{b,abs},{u}_{n,abs})$ and acceleration $({\ddot{u}}_{b,abs},{\ddot{u}}_{n,abs})$ behaviour of SSISBg structure are described via Eq. (16):
${u}_{b,abs}=\left{u}_{g}+{u}_{b}\right,{\ddot{u}}_{b,abs}=\left{\ddot{u}}_{g}+{\ddot{u}}_{b}\right,$
were, ${u}_{b}$, ${u}_{n}$ and ${\ddot{u}}_{b}$, ${u}_{0n}$_{}are the base and top relative displacement and acceleration; ${u}_{0n}$, ${\ddot{u}}_{0n}$ are the top relative displacement and acceleration of the SSISBg structure as a rigid body.
Presizing and assessment of SSISBg structure have been conducted by MATLAB and Simulink programming tools using presented governing equations by Kasimzade et al. [19] in the following section.
1.2. Assessment of the SSISBg structure according to the basic demands of the SSIM method
A numerical verification of the SSISBg structure is presented with an example of 26 storey steel framed structure. For comparability of the SSISBg, FBBg and CAMSBIDBg structures, its storey height, column axes ranges were accepted same respectively with an equal total mass of 3.04894E+7 kg as presented in Table 3. Presizing SSISBg (26storey, 104 m), CAMSBIDBg (24storey, 96 m) and FBBg (24storey, 96 m) steel superstructures are designed so that the maximum story angle is lower than 1/200. The steel grade SN490 (with 357.0 MPa yield strength) for superstructure and reinforced concrete for the base part is used as material for members (Table 1). Total floor load (per meter square) containing the dead load of the columns and beams is 7840.0 N/m^{2}. Storey height, column axes ranges are accepted 4 m, 8 m respectively. Presizing results for beams and columns were presented in Table 2. The floor mass distribution for FBBg, CAMSBIDBg and SSISBg structures are presented in Table 3.
The total superstructure mass (${M}_{totalsuperstore}$) of CAMSBIDBg and FBBg is presented according to the total mass of SSISBg as following:
$\left(3.17750E+70.12858E+7\right)kg=3.04894E+7kg.$
Table 1. Material properties of steel for superstructure and reinforced concrete for the base part of SSISBg
Material properties

Steel

reinforced concrete

Elasticity modulus [N/m^{2}]

2.05E+11

3.80E+10

Density [kg/m^{3}]

7860

2400

Poisson’s ratio

0.3

0.2

Table 2. The dimension of the storey column and beams
Stories

Column (boxsection)

Beam (Isection)


Width $x$ breadth [m]

Thickness [m]

$H$* [m]

$W$ [m]

$FT$ [m]

$WT$ [m]


110th

0.8×0.8

0.02

0.8

0.3

0.03

0.015

1120th

0.65×0.65

0.016

0.8

0.3

0.03

0.015

2126th

0.47×0.47

0.012

0.8

0.3

0.03

0.015

*$H$ (Height), $W$ (Flange width), $FT$ (Flange thickness and $WT$ (Web thickness)

Table 3. The distribution of the floor masses for SSISBg, CAMSBIDBg and FBBg structures
SSISBg

CAMSBIDBg

FBBg


Underground part

–6th floor (isol. lay.)

1.7926E+06

Isolation layer

1.28580E+6

1st floor

1.33118E+6

–5th floor

6.3729E+05

1st floor

1.33118E+6

2nd floor

1.33118E+6


–4th floor

8.5559E+05

2nd floor

1.33118E+6

3rd floor

1.33118E+6


–3rd floor

9.9259E+05

3rd floor

1.33118E+6

4th floor

1.33118E+6


–2nd floor

1.0798E+06

4th floor

1.33118E+6

5th floor

1.33118E+6


–1st floor

1.1288E+06

5th floor

1.33118E+6

6th floor

1.33118E+6


Superstructure

13rd floor

1.33827E+06

67th floor

1.33118E+6

7th floor

1.33118E+6

4th floor

1.31392E+06

8th floor

1.30683E+6

8th floor

1.30683E+6


513th floor

1.28957E+06

918th floor

1.28248E+6

918th floor

1.28248E+6


14th floor

1.28410E+06

19th floor

1.27701E+6

19th floor

1.27701E+6


1519th floor

1.27863E+06

2023th floor

1.27154E+6

2023th floor

1.27154E+6


Top floor

6.76301E+5

Top floor

6.76342E+5

Top floor

6.76342E+5


Total superstructure

2.53450E+7

Total superstructure

3.04894E+7

Total structure

3.04894E+7


Total structure

3.17750E+7

Total structure

3.17752E+7


* all of the floor masses are presented with [kg] units

Assuming that the predominant period of the earthquakes in the area where FBBg, CAMSBIDBg and SSISBg structures will be built is about 11 s. The required total elastomeric isolator horizontal stiffness for the first approximation for the SSIS system is described in accordance with Eq. (2), in case of free vibration as ${k}_{b}=$ 8.2455E+7 N/m. Other parameters of the elastomeric isolator such as period, damping coefficient and damping ratio were defined as ${T}_{b}=$ 4 s, ${c}_{b}=\mathrm{}$1.5182E+7 Ns/m^{2}, ${\xi}_{b}=$ 0.15 respectively.
Based on above SSISBg structure’s parameters and using governing equations for SSIS system from the previous section, SSISBg structure’s performance was preliminary assessed to Kobe 1995 Earthquake $X$direction acceleration excitation and base acceleration response were presented in Fig. 4.
As seen, the acceleration in the SSISBg structure’s base significantly (about four times) was reduced. Based on preliminary design parameters and assessments results presented in this section, detailed finite element modelling of the SSISBg structure comparing with FBBg, CAMSBIDBg structures were presented in the following section.
Fig. 4. Base acceleration responses of SSISBg structure (red) in $X$direction under the effect of the Kobe earthquake (cyan)
2. Finite element structural model of steel building with SSIS system
Finite element modelling of the steel building with the SSIS system has been prepared in the LSDYNA software [39], tetrahedron solid, beam and isolator link finite elements were used (Fig. 5). The material properties, section properties, and mass distribution of the finite elements are in accordance with Tables 1, 2, 3 respectively. Isolator links (discrete beam isolator) finite element’s parameters definition, presizing and finalization design were exhibited in the following sections. The isolator links are modelled based on bidirectional coupled plasticity theory, the hysteretic behaviour was proposed by (Wen 1976) [25].
2.1. Preliminary design of the seismic elastomeric isolators
The preliminary dimension and analytical parameters of the seismic isolators are calculated based on ASCE 716 [40] and ASCE 4113 [41] codes. Yield force (${F}_{v}$), yield displacement (${u}_{y}$), damping ratio and the vertical stiffness (${K}_{v}$) are the necessary analytical parameters for finite element modelling of the seismic isolators. Minimum horizontal stiffness and the design displacement of the isolator are calculated using Eq. (17) and (18) respectively:
where $W$ stands for the total weight on a single bearing, ${T}_{D}$ for design period (here ${T}_{D}=$ 4 s), ${B}_{D}$ for damping coefficient, $g$ for gravity, ${S}_{D1}$ for spectral coefficient. The crosssection area of rubber (${A}_{r}$) and postyielding stiffness are calculated using Eq. (19) and (20) respectively:
where yield displacement (${u}_{y}$) is 0.050.1 times of total rubber thickness (${R}_{T}$) based on experimental data; ${f}_{L}$ is a factor which commonly taken 1.5. The characteristic strength ($Q$) of the elastomeric can be calculated using Eq. (21). Then the yield force (${F}_{y}$) of the bearing can be calculated by Eq. (22):
Fig. 5. a) The position of deployed seismic isolator in SSISBg structure, b) the overall view and illustration of the finite element model of 26 storey structure SSISBg equipped with SSIS system, and c) the general floor plan including the position of isolators of the structure
a)
b)
c)
Finally, the vertical stiffness (${K}_{v}$) of the elastomeric bearing is calculated via Eq. (23):
where ${E}_{c}$, $G$ and $K$ is the compression of rubbersteel composite, shear and bulk modulus of rubber respectively. The value of $K$ and $G$ differs based on the type of rubber, the value of $K$ can vary between (1000 to 2500 MPa) and $G$ between (0.45 to 1 MPa). $S$ represents the hysteresis loop shape factor of the seismic isolator and the value of $S$ should range between 12 and 20. Based on the presented equation the parameters of the LCRB for SSISBg and CAMSBIDBg structures are calculated and presented in Table 4.
The final design of the elastomeric isolators parameters is implemented based on the compering first iteration results ${K}_{h,total}=$ 53×1.56E+6 = 82.68E+6 N/m with the required total horizontal stiffness of the isolators ${k}_{b}=$ 82.455E+6 N/m obtained from Eq. (2). Then it can be confirmed in every iteration by the assessment of the hysteresis loop of the elastomeric isolator.
Table 4. Properties of seismic LCRB isolators for SSISBg and CAMSBIDBg structures
Parameters

SSISBg

CAMSBIDBg

${K}_{v}$ (Vertical stiffness) [N/m]

2.2800E+09

2.8330E+09

${F}_{y}$ (Yield force) [N]

4.550E+05

6.520E+05

${K}_{h}$(Horizontal stiffness) [N/m]

1.560E+06

2.230E+06

Damping ratio [%]

1.50E01

1.50E01

${u}_{y}$ (Yield displacement) [m]

4.50E02

4.50E02

$\varphi $ (Diameter) [m]

9.70E01

9.70E01

${R}_{T}$ (Rubber thickness) [m]

5.00E01

5.00E01

Number of isolators

53

37

3. Numerical study
Nonlinear dynamic analysis of the presented finite element model (Fig. 5) has been analysed using a total of five strong and longperiod earthquakes, general characteristics of these earthquakes are presented in Table 5 and Fig. 6. Timehistory data of the ground motions are obtained from PEER Berkeley Strong Ground Motion database [42]. The spectrum presented in Fig. 6 is used in the preliminary design of isolator in Section 2.1.
Fig. 6. The response spectra of the ground motions in $X$ and $Y$ direction respectively
Table 5. Ground motion characteristics
Earthquake

Year

Station

PGAX [g]

PGAY [g]

Type

Duzce

1999

Bolu

0.739

0.805

Near fault

Kobe

1995

KJMA

0.833

0.628

Near fault

Elmayor

2010

Chihuahua

0.248

0.196

Far fault (longperiod)

ChiChi

1999

CHY028

0.636

0.760

Near fault

Darfield

2010

Cathedral College

0.194

0.233

Far fault (longperiod)

The dynamic analyses of SSISBg, CAMSBIDBg and FBBg structures are conducted using LSDYNA explicit solver [39], total CPU time for each analysis is approximately 10 hours and 40 minutes. Total of 12 CPU cores with 18 GBs of RAM is used during analyses. Base, top, base shear and base moment responses SSISBg, CAMSBIDBg and FBBg structures under effect of 1995 Kobe earthquake is provided in Fig. 711.
Fig. 7. Base acceleration responses of SSISBg and CAMSBIDBg structures in $X$ and $Y$ directions
Fig. 8. Top storey acceleration responses of SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
Fig. 9. Top storey displacement responses of SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
Fig. 10. Base shear response of SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
3.1. Results and discussion
As presented in Figs. 78 the base and top acceleration response SSISBg is tangibly lower than CAMSBIDBg and FBBg structure in both $X$ and $Y$ directions. On the other hand, the top storey displacements of SSISBg and CAMSBIDBg are lower than FBBg structure and similar to each other as shown in Fig. 9.
While the acceleration response of SSISBg is considerably lower than the CAMSBIDBg and FBBg structures, the reduction of the base shear and base moment response of SSISBg is significantly lower than CAMSBIDBg and FBBg structures as well. It is achieved due to turn around the gyration centre of the SSISBg structure. The base shear and base moment responses of SSISBg, CAMSBIDBg and FBBg structures due to the effect of the 1995 Kobe earthquake are presented in Fig. 10 and Fig. 11.
Fig. 11. Base moment response of SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions respectively
Fig. 12. Comparison of the hysteresis loops of the central seismic isolator of a), b) SSISBg, c) d) CAMSBIDBg structures
a)
b)
c)
d)
All peak top acceleration, top displacement (Table 6), base shear and base moment responses (Table 7), base acceleration, base displacement (Table 8) of the SSISBg, CAMSBIDBg and FBBg structures are presented as follows.
The SSISBg, CAMSBIDBg and FBBg structures were analysed under the effect of strong and longperiod earthquakes listed in Table 5. The base acceleration responses of the SSISBg structure is 22.94 % and 24.47 % lower (on average) than the CAMSBIDBg structure in $X$ and $Y$ directions respectively as shown in Fig 13. Similarly, there are 19.21 % and 26.22 % difference between the top storey acceleration response of SSISBg and CAMSBIDBg structures as presented in Fig. 14. The response of FBBg structure clearly indicates to extreme vulnerably of similar structures under effect of longperiod earthquakes.
Table 6. Top level acceleration and displacement peak responses of SSISBg, CAMSBIDBg and FBBg structures
Ground
motions

$X$direction

$Y$direction


Top storey acc. [m/s^{2}]

Top storey disp. [m]

Top storey acc. [m/s^{2}]

Top storey disp. [m]


SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg


Duzce

3.63

4.28

7.68

0.563

0.625

0.721

3.51

4.64

6.59

0.238

0.231

0.245

Kobe

4.621

6.25

12

0.31

0.212

0.414

3.11

4.25

7.89

0.28

0.27

0.351

Elmayor

2.05

2.19

2.48

0.683

0.703

1.06

2.15

3.1

2.34

0.634

0.8032

1.027

ChiChi

3.47

4.56

6.89

321

0.278

0.343

3.9

5.69

8.06

0.474

0.538

0.752

Darfield

2.71

3.12

3.7

0.845

0.895

1.01

2.04

2.26

2.85

0.302

0.352

0.475

Table 7. Base shear and base moment peak responses of SSISBg, CAMSBIDBg and FBBg structures
Ground
motions

$X$direction

$Y$direction


Base shear [N] × 10^{5}

Base mom. [N.m] × 10^{5}

Base shear [N] × 10^{5}

Base mom. [N.m] × 10^{5}


SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg

SSISBg

CAMSBIDBg

FBBg


Duzce

1.87

6.67

10.0

5.31

19.3

31.5

2.36

5.54

9.11

4.18

28.9

36.6

Kobe

2.1

7.86

15.21

4.56

24.87

27.5

2.28

7.96

12.25

3.53

30

33.3

Elmayor

1.24

7.55

13.19

2.68

29.6

52.2

1.014

7.99

14.13

2.26

28.49

47.7

ChiChi

2.67

6.15

8.22

4.41

32.6

47.6

1.97

8.89

13.2

6.00

21.14

35.3

Darfield

1.178

7.81

10.3

2.03

19.6

25.1

0.918

5.34

6.76

2.56

29.3

40.38

Table 8. Base level acceleration and displacement responses of SSISBg, CAMSBIDBg
Ground
motions

$X$direction

$Y$direction


Base acc. [m/s^{2}]

Base disp. [m]

Base acc. [m/s^{2}]

Base disp. [m]


SSISBg

CAMSBIDBg

SSISBg

CAMSBIDBg

SSISBg

CAMSBIDBg

SSISBg

CAMSBIDBg


Duzce

2.57

2.65

0.255

0.262

3.28

3.69

0.101

0.113

Kobe

2.651

3.95

0.125

0.152

2.45

3.551

0.151

0.153

Elmayor

1.49

1.58

0.39

0.405

1.46

1.47

0.316

0.422

ChiChi

2.52

3.3

0.151

0.148

2.81

4.56

0.146

0.159

Darfield

1.46

2.41

0.271

0.335

1.3

1.69

0.143

0.15

While the acceleration response of SSISBg structure is considerably lower compared to CAMSBIDBg and FBBg structures, there is a tremendous difference between base shear and base moment responses of the SSISBg, CAMSBIDBg and FBBg structures. As presented in Fig. 15 the base shear response of SSISBg structure is 74.86 % and 76.08 % lower (on average) than CAMSBIDBg structure in $X$ and $Y$ directions respectively. On the other hand, the even higher differences are observed between the base moment response of SSISBg and CAMSBIDBg structures which is 84.92 % and 86.55 % in $X$ and $Y$ directions respectively (Fig. 16).
Fig. 13. Peak base acceleration response of SSISBg, CAMSBIDBg structures in $X$ and $Y$ directions
Fig. 14. Peak top storey acceleration responses of the SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
Fig. 15. Peak base shear response of the SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
Fig. 16. Peak base moment responses of the SSISBg, CAMSBIDBg and FBBg structures in $X$ and $Y$ directions
4. Conclusions
The SSIS system obtained by the SSIM method for the seismic isolation of building structures (SSISBg) has shown the following performances compared with the conventional application method of seismic base isolation devices for building (CAMSBIDBg) structures and fixed base building structures (FBBg):
– Due to the reason that the SSIS system provides the opportunity of controlled rotation to the superstructure, less bending moment and shear forces were formed in the SSISBg superstructure base, in comparison with CAMSBIDBg structures. Mainly the base and top accelerations, base shear and base moment responses of the SSISBg structure is 23.21 %, 75.47 % and 85.74 % on average lower than CAMSBIDBg structure respectively.
– The SSISBg structure is not prone to resonant vibrations under longperiod earthquakes related to the excessive deformation in the isolation layers.
– The significantly lower response of the SSISBg structure compering with CAMSBIDBg and FBBg structures allows to make it even lighter (in presented study approximately same total mass for SSISBg, CAMSBIDBg, FBBg structures was used only for comparability of the responses).
– Generally, in CAMSBIDBg and FBBg highrise buildings, approximately 1/3.1 –1/13.62 of the height of the superstructures mostly is considered a pile foundation. Hence, a considerable part of the structure cannot be used effectively. On the other hand, in CAMSBIDBg system besides the feasibility of the usage of the underground part, all the mentioned system will be included in a curved surface foundation. Thus, in terms of total useful area of the buildings, the SSISBg is much more efficient than CAMSBIDBg and FBBg structures.
– SSIS system allows the efficient use of today's manufactured seismic protection devices (LRB, LCRB, etc.). As presented, structures with the conventional application of the seismic base isolation devices for Building (CAMSBIDBg) are vulnerable under longperiod earthquake excitation.
The feasibility of the usage of the SSIM method is not limited to highrise buildings, it could be used as a seismic protection system for other important structures such as nuclear power plants, offshore oil platform, highrise hospitals etc.
The feasibility of the usage structure foot base and foundation contact curved surfaces’ types (spherical, elliptical, etc.) depending on the earthquakesoilsuperstructure parameters, it is the subject of the futures research and is being studied.
Summarise, structural seismic protection SSIS system obtained by the presented SSIM method for highly reliable structures are very productive and attractive for applications and it closes certain known shortcomings of the conventional application of the seismic base isolation devices.
Acknowledgements
This research was supported by the Scientific and Technological Research Council of Turkey (TUBITAK). The support is gratefully acknowledged.
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