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DOI: 10.23968/2500-0055-2024-9-2-57-64
impact of lead rubber bearing base isolation systems on building structures designed as per eurocode 8
Mohammed Tamahloult1, Mouloud Ouanani2, Boualem Tiliouine1*
laboratory of Earthquake Engineering and Structural Dynamics, Department of Civil Engineering,
Ecole Nationale Polytechnique, Algeria
2Department of Civil Engineering, University of Djelfa, Algeria
Corresponding author's e-mail: boualem.tiliouine@g.enp.edu.dz
Abstract
Introduction: Seismic base isolation has been classified as a structural protection system designed to minimize the seismic forces transferred to a structure during an earthquake. This can be achieved through the use of various devices, such as elastomeric bearings, sliding bearings, and hybrid systems. purpose of the study: The study aims to evaluate the impact of using lead rubber bearings (LRB) as a base isolation system in building structures. Methods: In order to achieve this, nonlinear dynamic analyses of a seven-story building with and without an isolation device at its base were performed using the Fast Nonlinear Analysis (FNA) algorithm. The building was designed according to Eurocode 8 (EC8) criteria and then subjected to analysis using data from two previous earthquake events. Results: It is concluded that the bilinear behavior assumption made in the design stage according to EC8 is appropriate. Additionally, implementing an isolation system with LRBs at the building foundation can significantly enhance building performance by reducing floor accelerations, inter-story drifts, and base shear responses. Furthermore, it is demonstrated that isolating a building at its base with LRBs effectively reduces internal forces due to both gravity and seismic loads.
Keywords: base isolation; LRB system; 3D nonlinear earthquake response analysis; Eurocode 8; bilinear hysteresis.
Introduction
Seismic isolation is an excellent method for passive protection of a building structure. It enhances structural performance and reduces potential earthquake damage by lengthening the fundamental period of vibration and increasing energy dissipation (Naeim and Kelly, 1999). Seismic base isolation systems, including laminated rubber bearings, friction pendulum systems, and Teflon-steel friction bearings, have been utilized to reduce the transmission of seismic forces from the ground to a building structure.
Each of these systems has its own characteristics and advantages. Laminated rubber bearings are known for their durability, cost-effectiveness, and optimal control of their characteristics (Jain et al., 2004).
A laminated rubber bearing consists of alternating thin layers of rubber and steel, giving it the ability to support heavy weights due to its stiffness in the vertical direction. At the same time, it is horizontally flexible, allowing superstructures to move similarly to the motion of a rigid body during an earthquake (Koo et al., 1999). A lead rubber bearing (LRB) is a specific type of the laminated rubber bearing that includes a lead core in its structure, providing high initial rigidity and high damping, with equivalent damping varying from 15 to 35 % (Attanasi et al., 2009). Buildings equipped with LRBs demonstrated excellent performance during past earthquakes
(1994 — Northridge; 1995 — Kobe), confirming the effectiveness of LRBs as suitable base isolators (Asher et al., 1997).
Several mathematical models have been used to characterize the hysteresis behavior of various types of bearings. The idealized hysteresis behavior of bearings has been the subject of extensive studies. Among the various models proposed, the bilinear model is widely used in both research and design practice (e.g. Amanollah et al., 2023). Its simplicity allows for an accurate characterization of the mechanical properties of bearings, making it suitable for both elastomeric-type and sliding-type bearings (Cheng et al., 2008).
According to Mayes (Mayes and Naeim, 2001), any design process must ensure that (i) the bearings will safely withstand the maximum gravity service loads for the lifetime of the structure and (ii) provide period shift and hysteretic damping during one or more design earthquakes.
The current generation of building codes has progressed in two significant ways. Firstly, they provide guidelines for incorporating energy dissipation mechanisms, taking into consideration both the lateral strength method and the type of structural material used. Secondly, these updated codes have expanded their scope to include additional considerations, such as geotechnical aspects. Furthermore, these new regulations incorporate a semi-probabilistic approach to assess safety, in line with the principles defined
For citations: Tamahloult, M., Ouanani, M., Tiliouine, B. (2024). Impact of lead rubber bearing base isolation systems on building structures designed as per Eurocode 8. Architecture and Engineering, No 2 (9), pp. 57-64. DOI: 10.23968/2500-0055-2024-9-2-57-64.
in EN 1990 (Elghazouli, 2009). Eurocode 8 (European Committee for Standardization, 2004) includes a dedicated chapter on the seismic isolation of buildings and bridges. In that chapter, the calculation of maximum isolator displacement is carried out in the preliminary design phase using the Equivalent Linear Force (ELF) method (Cavdar and Ozdemir, 2022). In the same context of designing building structures with base isolation, the Chinese code GB50011-2010 (Ministry of Housing and Urban-Rural Development of the People's Republic of China, 2010) recommends a distinct design approach. This approach ensures that the isolation system and superstructure are designed independently, and introduces the concept of horizontal seismic isolation coefficients (Hu et al., 2023).
In this study, we aim to demonstrate the impact of seismic base isolators on building structures during earthquakes. The analyses were carried out on a multi-story building model with a base isolation system with LRBs incorporated at the base, as well as on the same model with a fixed base, both designed according to EC8. The earthquake data included ground motion records from the 1940 El Centro and the 1996 Kobe earthquakes.
Subject, models, and methods
Building model
The subject model is a seven-story frame building with dimensions of 15*8 m2. The beam sections are 40^30 cm2, and the column sections are 50*50 cm2. Each story has a height of 3 m, as shown in Fig. 1a and Fig. 1b. The building is isolated with LRBs placed under each column between the foundation and superstructure, and attached to a 10 cm rigid base slab. The total weight of the building is 14.066 kN. The fundamental period of the building is 0.61 s, and the modal damping ratio is expected to remain constant at 5 % for each mode. The building structure is intended to be located in a highly seismic zone, resting on a soil profile categorized as stiff soil profile
type C. The system isolator to be used is a lead rubber bearing (LRB) as shown in Fig. 2. Two LRB profiles are designed for the building because the gravity load transferred to the corner bearings is less than that transferred to the inner and side bearings.
The bearings are labeled as (A) for the columns at the corners and (B) for those on the sides and inside (Fig. 1b). The force deformation behavior of the isolators (LRBs) in this study is modeled as a nonlinear hysteretic loop directly idealized by the bilinear model (AASHTO, 2010; Kelly, 1997; Mori et al., 1998) as indicated in Fig. 3.
Seismic displacement criteria as per EC8
First, we define the design response spectrum of each isolator (LRBA, LRBB) in accordance with the seismic requirements specified by Eurocode 8, Type 1 spectrum (Fig. 4). This applies to areas with high seismicity and near-field earthquakes, relative to a reference peak ground acceleration (PGA) of agR = 0.4 g. The importance factor for the building gI = 1; soil type — C, spectral parameters from EC8 (Table 3.2) are as follows:
T(s) is the linear SDOF system's vibration period and Se(T) is the elastic response spectrum; the lower and upper limits of the period of the constant spectral acceleration branch are TB = 0.2 s and TC = 0.6 s, respectively. Soil factor S = 1.15. Damping correction factor n = 0.7.
The desired effective period (Tff and effective damping (^J of the isolation system are assumed to be Teff = 2 s and ^ = 0.137, respectively. Following a gravity load analysis, we determine that the vertical reaction is as follows: Ra = 989 kN for the corner columns and Rb = 1.461 kN for the side and inside columns. Subsequently, the effective stiffness of each rubber isolation bearing is defined as follows:
Keff =
gTf
(1)
where: R is the vertical reaction (Ra, Rb).
Fig. 1: a) 3D frame building, b) plan view of the structural model
Fig. 2. Lead rubber bearing (LRB) (Takenaka Corp., Japan)
For: LRBA keff = 989X4= 994 kN/m;
9.81x 2
LRBB keff =
1461x 4 xn
= 1.468 kN/m.
9.81x T
The total effective stiffness of the isolation system can be calculated as follows:
Z keff = 4 X kfff + 8 X kbeff =
= 4 x 994 + 8 x 1468 = 15.720 kN/m.
The design level damping ratio of the isolation system can be calculated as follows:
vA ¡^A srB j^B LeffKeff +LeffKeff
4 KeAf+8KBf
= 0.137,
(2)
where and if are damping ratios of individual bearings of 0.10 and 0.15, respectively.
The design displacement ddc of the isolation system along the main horizontal direction is calculated as per EC8 (10.9.3) using the following expression:
WSe (Teff eff ) ddc =-If/ = 0.228 m,
g S K
(3)
eff
where:
Se (Teff,*eff ) = (2 sec, 13.7 %) = 0.25 g;
W is the total weight of the building, i.e., 14.066 kN.
The total eccentricity does not exceed 7.5 % of the length of the superstructure transverse to the horizontal direction, as specified in EC8 (Chapter 10).
e — e
tot, y
= 0.05 L = 0.4 m < 0.075 L = 0.6 m (the condition is met).
L is the dimension of the building perpendicular to the direction of the seismic action (EC8, 4.3.2).
The total design displacement ddb, including torsional effects, can be calculated for each direction by multiplying the design displacement ddc by given factor 5i.
For the action in the x direction:
0.4
S xi =1 +
"tot, y
'y
yi = 1 +
36.60
8 = 1.07 m, (4)
where:
$ = YXX Kyi + y? Kxi)/Z Kxi = 39.18 m2, (5)
where y is the horizontal direction transverse to the x direction under consideration; (x, y) are the coordinates of the isolator unit i relative to the effective stiffness center (Fig. 5); e = etot = 0.05*8 = = 0.4 m is the total eccentricity in the y direction; ry is the torsional radius of the isolation system in the y direction.
Total design displacement of the isolator unit: ddb = § xixddc = 1.07 x 0.228 = 0.24 m.
Bilinear hysteretic behavior of the isolator
The isolation system may be modeled with bilinear hysteretic behavior, taking into account the conditions required by EC8 (EC8 S10.9.2). The bilinear model of the isolator is essentially described by three parameters: elastic stiffness (K1), post-yield stiffness (K2), and characteristic strength (Q). These three parameters are calculated using the convergence procedure as described below (Datta, 2010):
1. Energy dissipation per cycle, or WD, can be estimated for very small post-yield stiffness as follows:
Force, F+
Displacement, D
WD (shaded area = 2 KegnÇeg D2)
1 E D h \ \ —- c i h Vt ' \ B
Tis)'
Fig. 3. Idealized force-deflection curve
Fig. 4. EC8 Type 1 spectrum
T
1
sl
Isolator unit
Keff ddc
ddb
(Xi = -4 , y = -
b--1
c.m: center of mass c.s: center of stiffness
Fig. 5. Dimensions of the plan to calculate the total eccentricity
WD = 2nKeffddb2 and WD = 4Q(ddb -Dy). (6)
WD is also measured by the area bounded by the force-deflection curve loop (Fig. 3).
2. Neglecting the yield displacement Dy, the first approximation for the short-term yield force Q is as follows:
Q - 2 Keffddb zeff ■
(7)
3. K1 and K2 are the pre- and post-yield stiffness (K1 = 10 K)
Q
K2 = Keff -
4. D can be estimated as follows:
Dy =
9K
(8)
(9)
5. Adjusting the first estimate of Q for D using the convergence procedure, we obtain the fo^owing:
Q = (10)
4 (ddb - Dy)
The properties of the isolators (LRBA and LRBB), designed according to EC8, after the convergence procedure are given in Table 1 and Fig. 6.
In addition, EC8 requires that the effective stiffness of the isolation system is not less than 50 % of the effective stiffness at a displacement of 0.2ddc (EC8 S10.9.2).
f (02ddb )= °2ddbFy + 6 (" °2ddb ]; (12)
dy
I Keff (0.2ddb) = 29928 kN/m;
I Keff = 15720.00 kN/m > 50 %;
IKeff (0.2ddb) = 14.964 kN/m (the condition required by EC8 is met).
Seismic inputs and numerical analyses
The numerical analysis investigates the performance of nonlinear time history for both fixed and base-isolated building structures under 3D seismic excitations of the 1940 6.9 MW El-Centro earthquake (PGA = 0.281 g) and the 1995 6.9 MW Kobe earthquake (PGA = 0.834 g), classified
as far-field and near-field earthquakes, respectively (Gudainiyan and Gupta, 2023; Tamahloult and Tiliouine, 2023). The major components of each earthquake, as shown in Fig. 7, are applied in the longitudinal X direction of the building. The Nonlinear finite element software SAP2000v.14 (SAP, 2000) is used to obtain the dynamic responses at discrete time intervals. The solutions to the motion equations were obtained using the Fast Nonlinear Analysis method (Wilson, 2002). The isolators were modeled using LINK elements.
Seismic performance evaluation
The seismic criteria for evaluating the performance of the base-isolated building include the following parameters:
1) Peak base displacement (P1):
(P) = max, (Idb\^
where db is relative displacement with respect to the ground.
2) Story drift (P2): the ratio between the interstory displacement (top floor displacement d7 and base floor displacement db) and the height of the building H, defined as follows:
(P ) = max, (I(d7 -db)|/ H),
where d7 is relative displacement of the top floor (7th floor).
3) Maximum base shear (P3):
() = max, (IVb I).
400
350
300
250
200
u! 150
100
50
-LRB -A -LRB-B
^ / "eff
0.05
0,1
0.15
0,2
0.25
-¡db
0,3
Displacement (m}
Fig. 6. Bilinear curves for the isolators LRBA and LRBB
Table 1. Isolator characteristics
Isolator characteristics lrba lrbb
Characteristic strength, Q (kN) 38.27 86.08
Post-yield stiffness, K2 (kN/m) 834.55 1109
Pre-yield stiffness, (K1 = 10 K2) 8345.5 11090
Yield displacement, Dy (m) 0.0051 0.0086
Yield force, FY = K1 Dy (kN) 42.52 95.64
-1
Kobe 1995 K000.AT2
40
El Centro 1940 ELC180.AT2
30 40 50
Time (s)
Time (s)
Fig. 7. Acceleration time history of longitudinal components recorded during the following earthquakes: a) Kobe, b) El-Centro
where Vb is maximum shear at the base of the building.
4) Top floor acceleration (P4):
(p ) = max, (|aq |),
where a7 is total acceleration of the top floor.
5) Internal forces (bending moment values) (P5). Results and discussion Table 2 summarizes the numerical results obtained from time history analyses of seismic performance for both fixed and base-isolated structures, with comments presented in the following subsections. For the sake of brevity, we only present results in the X direction (similar conclusions are found for the results in the Y direction). The findings illustrate that, in contrast to the ductility-based approach aimed at reducing earthquake damage, seismic base isolation effectively reduces maximum values of seismic inter-story drift, floor acceleration, base shear, and internal forces simultaneously, thus enhancing the structural performance of the building. Base displacement response The peak displacement at the base is a very important parameter in the case of base-isolated buildings, which must not exceed the predicted maximum total design displacement calculated according to EC8. The values of peak displacement in the principal direction X were found to be P1 = 2.6 cm and P1 = 15 cm for the El Centro earthquake and the Kobe earthquake, respectively. It should be noted that the peak base displacement in the case of the Kobe earthquake increased drastically, reaching up to 62 % of the total displacement. The hysteresis curves for the force displacement of LRBB bearings are presented in Fig. 8a and Fig. 8b for the El-Centro
and Kobe earthquakes, respectively. It is evident that the bilinear behavior assumption according to EC8 is compatible with the force-deformation curves of the seismic isolator obtained from the time history analyses.
Inter-story drift displacement response
The drift ratio in the base-isolated structure shows a minor reduction in the case of the El Centro earthquake but a significant reduction of about 53 % during the Kobe earthquake, as shown in Table 2. However, in the case of the fixed-base building, the drift ratio calculated for the Kobe earthquake is equal to 0.8 %, which is very close to the EC8 requirement limit (0.005/v = 1 %). This result illustrates the effectiveness of the LRB isolation in reducing the drift displacement of the structure and suggests that the superstructure behaves similarly to a rigid body when placed above the isolation system.
Base shear response
The results of the base shear time history demonstrate a significant reduction due to the incorporation of an isolation system. For example, in the case of the El Centro earthquake, the peak base shear values for the base-isolated building and its fixed base are 1.089 kN and 1.290 kN, respectively. In the case of the Kobe earthquake, the base shear values are 2.504 kN and 3.433 kN, respectively, as shown in Fig. 9.
Absolute acceleration response
The comparison of maximum top floor accelerations between fixed-base and isolated-base structures is presented in Table 2. In the X direction, the maximum top floor acceleration decreased from P4 = 7.77 m/s2 to P4 = 5.25 m/s2 for the El Centro earthquake
Table 2. Seismic performance of fixed and base-isolated buildings
Seismic performance evaluation El-Centro (pGA = 0.281 g) KOBE (pGA = 0.834 g)
Fixed base Base-isolated Fixed base Base-isolated
Base displacement (P1) (cm) - 2.30 - 14.9
Roof drift (P2) 0.0031 0.0032 0.0085 0.0066
Base shear (P3) (kN) 1290 1089 3433 2504
Top floor acceleration (P4) (m2/s) 7.77 5.25 20.23 10.03
LINK LRBTYPE B
LINK LRB TYPE B
DISPLACEMENT (m) DISPLACEMENT (m)
Fig. 8. Force-deformation curves of the seismic isolator for 3D input acceleration ground motion components recorded during the El
Centro (a) and Kobe (b) earthquakes
(a reduction of 32 %) and from P4 = 20.23 to P4 = 10.03 m/s2 for the Kobe earthquake (a reduction of 50 %). This decrease in absolute acceleration for the base-isolated structure demonstrates the effectiveness of the isolation system.
Internal forces (bending moment values)
Table 3 presents the calculated values of the maximum bending moments in the fixed-base and base-isolated building structures for both the El-Centro and Kobe earthquakes. It has been observed that for the base-isolated building, there is a significant reduction in bending moment values compared to those of the fixed-base building, as shown in Table 3, for both load cases, the El-Centro and Kobe earthquakes. For example, at the base level, the maximum bending moments decrease from 302 to 222 kN-m and from 829 to 631 kN-m for the El Centro and Kobe earthquakes, respectively. Furthermore, at the top level, a significant reduction of approximately 50 % can be observed. For example, the maximum bending moments decrease from 42 kN-m to 28 kN-m for the El Centro earthquake and from 113 kN-m to 53 kN-m for the Kobe earthquake.
These results once again demonstrate the success of LRB bearings in controlling internal forces under both gravity and seismic loads. As a result, it may be interesting to consider the possibility of resizing the cross-sectional dimensions of structural elements, especially columns and beams (all beams 35*30 cm2, all columns 40*40 cm2), within the base-isolated building. This adjustment has the potential to enhance structural efficiency and yield cost savings.
Conclusion
The design of base isolation systems is well defined in EC8 for building structures. The design displacement of an isolator unit is calculated using a formula defined in EC8, depending on the spectral acceleration (type 1 spectrum). This formula includes several parameters such as the reference peak ground acceleration of each seismic zone, the soil factor S, the behavior factor, the importance factor of buildings, effective fundamental period, and effective damping. The dynamic response behavior of a multi-story building structure isolated using an LRB system was evaluated. Seismic response parameters for structures with fixed and isolated
Fig. 9. Comparison of base shear between fixed-base and base-isolated buildings for the El Centro (a) and Kobe (b) earthquakes
Table 3. Maximum bending moment values
Maximum bending moment values El-Centro (pGA = 0.281 g) KOBE (pGA = 0.834 g)
Fixed base Base-isolated Fixed base Base-isolated
1st story Column 302 222 829 631
Beam 144 189 397 433
2ndstory Column 224 205 618 444
Beam 178 155 488 321
3rd story Column 190 170 523 347
Beam 174 120 476 268
4th story Column 189 127 511 295
Beam 149 89 408 212
5th story Column 165 96 449 254
Beam 115 67 308 157
6th story Column 127 81 340 189
Beam 74 47 198 96
7th story Column 82 46 219 118
Beam 42 28 113 53
bases were evaluated in accordance with the provisions outlined in EC8. The output results clearly demonstrate the effectiveness of the isolator system in significantly and simultaneously reducing seismic responses, including floor accelerations, inter-story drifts, and base shear. In addition, it was observed that the isolation system with LRB bearings reduces
internal forces caused by both gravity and seismic loads. As a result, it may be useful to consider the possibility of resizing the cross-sectional dimensions of structural elements, especially columns and beams, within the base-isolated building. This adjustment has the potential to enhance structural efficiency and yield cost savings.
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влияние систем сейсмоизоляции на основе свинцово-резиновых опор на строительные конструкции, спроектированные в соответствии с еврокодом 8
Мохаммед Тамалулт1, Мулуд Уанани 2, Буалем Тилиуин1*
1Лаборатория сейсмостойкого строительства и динамики сооружений, факультет гражданского строительства
Национальная политехническая школа, Алжир
2Факультет гражданского строительства Университет Джельфы, Алжир
*Е-таП: Ьоиа1етЛШошпе@д.епр^и^
Аннотация
Введение: Сейсмоизоляция представляет собой систему защиты сооружений, минимизирующую воздействие сейсмических сил на сооружение во время землетрясения. Этого можно достичь с помощью различных устройств, таких как эластомерные опоры, скользящие опоры и гибридные системы. Цель исследования: оценить влияние свинцово-резиновых опор, используемых в строительных конструкциях в качестве системы сейсмоизоляции. Методы: для достижения указанной цели, с помощью алгоритма Fast Nonlinear Analysis (FNA) был проведен нелинейный динамический анализ семиэтажного здания с изолирующим устройством в основании и без него. Здание спроектировано в соответствии с критериями Еврокода 8 (EC8), а затем подвергнуто анализу с использованием данных о двух произошедших землетрясениях. Результаты: сделан вывод о том, что допущение о билинейном поведении, сделанное на этапе проектирования в соответствии с EC8, является обоснованным. Кроме того, применение системы сейсмоизоляции фундамента здания с использованием свинцово-резиновых опор может значительно улучшить эксплуатационные характеристики здания за счет уменьшения ускорений перекрытий, межэтажных перекосов и горизонтальной сейсмической реакции. Кроме того, показано, что сейсмоизоляция здания с помощью свинцово-резиновых опор эффективно уменьшает внутренние силы, возникающие как от гравитационных, так и от сейсмических нагрузок.
Ключевые слова: сейсмоизоляция; система свинцово-резиновых опор; трехмерный анализ нелинейной реакции на землетрясение; Еврокод 8; билинейный гистерезис.
