International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania

COMBINED SYSTEMS FOR SEISMIC PROTECTION OF BUILDINGS

Moussa Leblouba1

ABSTRACT

Since the contemporary technology entered into design of buildings the base isolation has become a new reliable procedure for retrofitting of old buildings and designing of new constructions. The elastomeric rubber bearing, the lead rubber bearing, the high dumping rubber bearing and the sliding system, are the widely used in recent years. Many projects use one type of base isolator, but others use more than one base isolator device (mixed system). This paper is intended to give an insight on the seismic performance of seismically isolated buildings using a combination of base isolation devices. The paper also intends to answer the questions: what is the performance expected from the use of more than one device? If the combination of different systems gives a good level of seismic performance, so which is the better combination to achieve the best performance? Narrow

1. INTRODUCTION

To date, it is obvious that the base isolation system seismically protects buildings from minim damages, for structural elements as well as for non-structural elements and buildings’ contents. Conceptually, isolation reduces response of the superstructure by decoupling the building from the ground (FEMA 274, 1997). Typical performance of isolation system is felt in the reduction of forces transmitted to the superstructure, by lengthening the period of the system (structure-base isolation), which in fact reduces the interstory drifts, therefore, minimizes the contents damages before reducing structural elements’ damages (if not avoids them directly!) (Moussa, 2007). Generally, two categories of isolation system exist and widely used. The first category includes the family of elastomeric bearings, in which we find the high damping rubber bearing system (HDRB), the lead rubber bearing system (LRBs) and other systems. The second category includes the family of sliding bearings, in which we found the friction pendulum system (FPS) and sliding bearing system without recentering (SI). Several buildings were constructed or retrofitted using one type of isolation systems; some of them are the followings:

- Foothill Communities Law and Justice Centre (New, 1985, California) isolated using 98 HDRB.

- Stanford Linear Accelerator Centre Mark II Detector (Retrofit, 1987, California) isolated using LRBs.

- San Francisco International Airport Terminal (New, 1998, California) isolated using 272 FPS.

However, other buildings were isolated at their base using a mixed-system or a combination of different isolation devices, the following are selection of some applications of this system:

- Evans and Sutherland Building (New, 1988, Utah) isolated using a combination of 40 LRB and 58 NRB.

- Mackay School of Mines (Retrofit, 1993, Nevada) isolated using a combination of HDRB and Sliding bearings.

- Cathedral of Our Lady of the Angeles (2002, California) isolated using a combination of HDRB and Sliding bearings.

1 PhD Student -Technical University of Civil Engineering, Bucharest, mlablouba@gmail.com

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In this paper, we will give an insight on the seismic performance of the three mostly used isolation devices (LRBs, HDRB and FPS), after, a proposal of some combinations of these devices to isolate a simple structure in order to investigate the seismic performance of every combination and get some conclusions.

2. PERFORMANCE OF VARIOUS SYSTEMS OF BASE ISOLATION

Three-story reinforced concrete building was isolated at its base using three types of isolation systems from the two categories discussed above. First we used the LRBs as the only device (let’s call it BI-LRBs), then we used the HDRB (BI-HDRB) and finally the FPS (BI-FPS). Fig.2.1 depicts the fixed base building (FB) and the isolation devices designed for it using the UBC-97 (UBC, 1997) and IBC2000 requirements. As a first remark it is evident from the geometric characteristic (size) of isolators that the BI-LRBs will cost more than the other buildings, and this must be taken into account. A nonlinear time history analysis using ETABS package (Computers and Structures, 2003) assuming the El Centro 1940 record was carried out for every structure; table 2.1 summarizes the fundamental period and modal participating mass ratios of both structures, from this table it is clear that the fundamental period is lengthened in the base isolated buildings (more than three times), so the isolation system provides a high flexibility to the structure. In addition the first mode is dominant in the base isolated building (> 94%), in which the latter behaves essentially as a rigid body, and the other modes contribute little in the global response of the structure.

Figure 2.1 (a) general view of the building being isolated and location of isolators

Figure 2.1 (b) Different isolation devices used and their characteristics

A B

C 3m

3m

h

e

e

d

s

r

HDRB

L

r

s

d

h

e

e

LRBs

d

R

h

FPS

Isolator Location e (cm) h (cm) d (cm) s

(mm) r

(mm) R (m)

A 30 1 4 - B 40 1 5 - HDRB

C

2,5 20

50 2 1 - A 50 2 2 - B 50 3 2 - LRBs

C

2,5 48

60 4 2 - FPS ABC - 20 20 - - 1,5

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Table 2.1 fundamental period of both structures

Structure 1st period (sec.)

Modal participation (%)

FB BI-LRBs BI-HDRB BI-FPS

0,453 1,671 1,657 1,373

87,51 94,67 94,67 94,49

The flexibility provided by the isolation systems is most important feature and results in the avoidance of the resonance and the decrease of accelerations of the structure at different levels (Moussa, 2007). It should be noted that the FPS lengthens little the fundamental period when comparing with the isolators of the first category, this can be demonstrated by the radius of curvature (R) which is the only parameter that affects the period (Zayas et al.,1989), but this has not a big problem, the essential is that the first period was lengthened and this didn’t place the structure in more dominant earthquake region. Fig.2.2 shows the acceleration history at the top of the buildings, the acceleration was reduced in the base isolated buildings by ~2 times for BI-LRB and BI-HDRB than in the pinned base building, but by about 3~times the acceleration at the top was reduced in the BI-FPS. In addition, as shown in fig.2.3, the top and the base of base isolated buildings move horizontally with a close magnitude while comparing with the lateral movement of the top of FB. Thus, there is a significant reduction in interstory drift, which is very important in insuring the safety of the building components and contents. Also, and as shown in fig.2.4 the reduction in base shear is significant by isolating the building, which is important to limit the dramatically damage of the building. However, this is a property of the isolation system, which doesn’t permit the total transmission of the seismic energy to the superstructure (most deformation occurs at the isolation level (Petros, 2000), and this result in the reduction of the superstructure deformation, and the other important feature is that in the superstructure the distribution of shear forces will be close for each floor, contrary in the fixed base building.

Figure 2.2 Acceleration at the top of both structures…

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Figure 2.2 Acceleration at the top of both structures

Figure 2.3 Displacement at the top and the base of both structures

FB

BI-LRBs

BI-HDRB

BI-FPS

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It should be noted that for the BI-FPS the floors move horizontally with a magnitude greater than in the case of the other buildings, also and for the same building the reduction in base shear was very important while comparing with the other two base isolated buildings (BI-LRB and BI-HDRB), which is beneficial when accounting the total cost of the isolation system. Now examining the response of the base isolated buildings when subjected to any of the following earthquake components: the El Centro 1940 270° component, the Loma Prieta 1989 270° component, the Pacoima Dam 1971 196° component, the Parkfield 1966 40° component and the Taft 1952 69° component. The El Centro and Taft are typical California earthquake records, one representing a long duration record and the other a short duration signal with dominant frequencies in the 1Hz to 5Hz range. The Parkfield record is a short duration signal with considerable low-frequency energy in the region below 1Hz. The Pacoima Dam record has a high-frequency pulse in the middle of the signal that produces a very high acceleration (Kelly, 1984).

Figure 2.4 Base Shear in both structures

Summary of the displacement response obtained from the series of analysis is presented for each isolated building in table 2.2. Clearly the Pacoima Dam 1971 controls the maximum response of the BI-LRBs and BI-HDRB while the El Centro 1940 controls the maximum response of the BI-FPS, the responses to the other components are much less.

Table 2.2 Summary of maximum displacement results obtained by nonlinear THA

Structure BI-LRB BI-HDRB BI-FPS

Ux(cm) 6,23 7,16 18,62 El Centro

1940 Time(sec.)

5,44 3,08 8,56

Ux(cm) 9,13 10,78 8,85 Loma Prieta

1989 Time(sec.)

12,96 12,96 12,60

Ux(cm) 16,52 19,84 14,76 Pacoima Dam 1971

Time(sec.)

3,84 3,88 8,48

Ux(cm) 1,89 3,11 1,98 Parkfield

1966 Time(sec.)

8,76 8,76 2,16

Ux(cm) 2,33 4,38 11,31 Taft 1952 Time(sec.

) 10,08 3,96 43,84

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3. PERFORMANCE OF VARIOUS COMBINATIONS OF DIFFERENT BASE ISOLATION

SYSTEMS

After investigating the response of each isolation system when mounted separately; in this section, the same building (FB) was reused and isolated at its base using six different combinations of isolation systems as depicted in fig.3.1 and a nonlinear time history analysis was carried out for each combination assuming always the El Centro 1940.

Figure 3.1 The different combinations considered

It was found that the acceleration at the top for all combinations proposed is approximately the same (fig.3.2), but we have always the reduction in the magnitude comparing with the case when the building is pinned at its base, the reduction in acceleration for all the combinations is about two times. As a first remark, these combinations didn’t change significantly the acceleration while comparing the case when the isolators were mounted separately (see section 2). Fig.3.3 shows the displacement histories for the combinations considered, displacement’s magnitude at top and base of Comb1 and Comb11 are approximately the same, also, in Comb1 the base has displacements greater than the top (remain close) but the inversion of the combination (we used here the HDRB and LRBs) inversed the things; the top had displacements greater than the base in Comb11. For combinations Comb2 and Comb22 where we used FPS with HDRB, the displacements were increased in Comb22 by report to Comb2, in Comb1 the displacements at the top are up to 8 cm, but at the top of Comb22 the displacements were increased up to ~15 cm. In addition, stories of Comb22 moves very closely which is not the case in Comb2, same remarks for the 5th and 6th combinations.

Figure 3.2 Acceleration at the top of both structures

Comb1 Comb2 Comb3

Comb1 Comb2 Comb3

HDRB LRBs FPS

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Figure 3.3 Displacement at the top and the base of both combinations considered…

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Figure 3.3 Displacement at the top and the base of both combinations considered The base shear as expected was reduced by isolating the base in all combinations (see fig.3.4), but for combinations where we used the FPS (Comb2, Comb22, Comb3, Comb33) the reduction in base shear is affected by the FPS and this reduction is significant for Comb22 and Comb33 while comparing with Comb2 and Comb3, respectively, this can be demonstrated by the change in the number and location of the FPS. However, it is clear that the FPS reduces the base shear significantly when is used as a base isolator separately or combined with another isolator devices.

Figure 3.4 Base shear of both combinations considered Summary of maximum displacements obtained from the time history analysis of each combination assuming the earthquake components used in section 2 is presented in table 3.1. from this table it is clear that the Pacoima Dam 1971 controls the maximum response of the all combinations, and the El Centro 1940 controls also the maximum response of the

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Comb22 and Comb33, the responses to the other components are much less. The El Centro 1940 controls also the Comb22 and Comb33 because the existence and the location of the FPS, as expected from the investigation done in section 2 in which was shown that the El Centro 1940 controls the BI-FPS.

Table 3.1 Summary of maximum displacement results obtained by nonlinear THA

Structure Comb1 Comb2 Comb3 Comb11 Comb22 Comb33

Ux(cm) 6,19 8,06 7,24 6,77 14,45 10,46 El Centro Time(sec.

) 5,44 5,56 5,52 3,04 4,96 4,96

Ux(cm) 9,63 10,03 8,34 10,03 9,78 10,14 Loma Prieta Time(sec.

) 12,96 13,00 13,00 13,00 12,56 12,56

Ux(cm) 17,52 18,85 15,63 19,66 15,12 14,58 Pacoima

Dam Time(sec.

) 3,84 3,92 3,88 3,88 3,28 3,28

Ux(cm) 2,12 2,50 2,17 2,54 3,26 2,49 Parkfield Time(sec.

) 9,60 7,68 9,64 7,68 13,66 7,28

Ux(cm) 2,57 3,90 2,62 3,19 3,69 4,14 Taft Time(sec.

) 6,84 9,88 6,88 3,96 4,04 4,20

4. CONCLUSION AND REMARKS

Three different isolation systems were investigated when mounted separately and when mounted in combination. From this research, it was demonstrated that the use of base isolation is an advantageous technique to diminish significant damages in structural elements and contents by avoiding the total transmission of the ground motion into the superstructure. Isolate the structure at its base lengthens the first period, hence provides high flexibility to this latter and shifts the structure from the dominance and severe region of ground motion, the lengthening of period was about 3 times. The FPS provides less flexibility while comparing with the use of the other isolation systems of the first category (elastomeric bearings). However, while the isolation systems of the first category reduce the acceleration by about 2 times, the FPS reduces it very significantly (by about 3 times). Same remarks for base shear. The use of FPS makes the structure moves horizontally with a magnitude much greater than in the case when using the LRBs and HDRB, this feature of the FPS can be demonstrated by the contact surface between the isolators and the superstructure. The fact that the period is independent of the structure mass is another property of the FPS which can have advantages in controlling the response of a building. The desired structure period can be selected by simply choosing the radius of curvature of the concave surface. The period does not change if the structure weight changes or is different than assumed. One of the potential advantages offered by the FPS approach, compared to other available systems, is the cost of installation (Zayas et al., 1989). It was shown that the LRBs and HDRB provide approximately the same degree of isolation to the structure, also, the LRBs after researchers provides more damping to the structure which is advantageous, in addition to the high degree of nonlinearity presents the LRBs, it

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seems that when considering the total cost of the building, isolate the building using the HDRB appears efficient to diminish the cost to have approximately the same features that can be provided by the use of the LRBs. Combining isolators did not affect the acceleration; the isolation system mounted separately reduces the acceleration with the same amount while combining it with another isolation device. Mixing the FPS with LRBs or HDRB decreases significantly the base shear, and increases the displacement. From this study, we conclude that the use of FPS as a unique isolator is a good idea when the total cost is considered as an important thing. However, combining the FPS with a rubber-based isolator provides a good seismic isolation to the structure, diminishes the total cost. In addition, the number and the location of the FPS at the base of a structure when is combined with a rubber-based isolator affect the response of the structure. F. Braga et al. in their paper (Braga et al., 2001) mentioned that “The main feature of the mixed isolation technique is the decoupling of the stiffness and damping, this is impossible when using HDRB only. Friction interfaces can provide reliable wind restraints, energy dissipation and control of displacements together with vertical loads support, while rubber can give restoring effects as well as carry vertical loads” also “Rubber bearing are not able to reach a good compromise between required horizontal stiffness and vertical stability when, as in the case of low rised buildings”.

AKNOWLEDGEMENTS

Special thanks addressed to my advisor Prof. Dr. Ing. DAN Lungu for helping me to prepare this paper.

REFERENCES

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California F. Braga, M. Laterza, R. Gigliotti, 2001. Seismic isolation using slide and rubber bearings:

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International Conference of Building Officials, 1997, Earthquake Regulations for Seismic-Isolated Structures, Uniform Building Code, Appendix 16, Whittier, CA

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Leblouba, M., 2007. Effects of some parameters on the seismic performance of base isolated buildings, 6th International Conference of PhD students, P95-102

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Victor Zayas, Stanley Low, Luis bozo, Stephen Mahin, 1989. Feasibility and performance studies on improving the earthquake resistance of new and existing buildings using the friction pendulum system, Report No. UCB/EERC-89/09, Earthquake Engineering Research Center

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