USING ANALYTICAL MODELING FOR VERIFYING EXPERIMENTAL TESTING OF LEADRUBBER BEARING ISOLATION SYSTEM FOR THE PROTECTION OF NONSTRUCTURAL SYSTEMS AND COMPINENTS IN MULTISTORY BUILDINGS August 22, 2014 Paquilla Jones Home University: Howard University REU Site: University at Buffalo Project PI: Dr. Claudia Marin-Artieda Graduate Students mentors: Diego Buitrago and Amdedebrhan

Abstract It is known that base isolation implemented on structures help reduce seismic effects on structures by dissipating energy and reducing the acceleration experienced by structures, the same principles being applied in this project for local isolation of equipment within essential facilities. This document is a summary of studies on leadrubber elastomeric bearing base isolation system for the protection of equipment within multistory buildings using lead-rubber elastomeric bearings. The lead-rubber elastomeric bearing is studied to test the effectiveness of the system to protect equipment from damaging seismic effects within a structure. The objective of this NEESREU project is to analyze the performance of the lead-rubber elastomeric bearing in protecting equipment locally, determine the performance of the system under various input acceleration, and to create an analytical model of an experimental test that verifies the results of the experiment. In experimental and numerical modeling of system results showed a reduction of acceleration in the horizontal direction. However the lead-rubber elastomeric bearings were not effective in reducing the acceleration in the vertical direction.

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Contents 1. Introduction: ............................................................................................................................ 1 1a. NEES-REU Project Scope: ................................................................................................... 1 1b. Objectives of NEES-REU Project ........................................................................................ 2 2. Literature Review .................................................................................................................... 2 2a. Introduction: What is Base Isolation? ............................................................................... 2 2b. Base Isolating Systems....................................................................................................... 2 2c. Benefits of Base Isolation................................................................................................... 4 2d. Base Isolation for Seismic Protection of Equipment ......................................................... 4 3. Methods .................................................................................................................................. 6 3a. Modeling ............................................................................................................................ 8 3b. Instrumentation................................................................................................................. 9 4. Results/Discussion ................................................................................................................. 11 5. Conclusion ............................................................................................................................. 13 6. Contact Information .............................................................................................................. 13 7. Acknowledgments ................................................................................................................. 13 References ................................................................................................................................. 14

1 1. Introduction: The main goal of the project Passive Seismic Protective Systems for Nonstructural Systems and Components in Multistory Buildings led by Principal Investigator Dr. Claudia Marin-Artieda, is to validate the effectiveness of isolation devices and energy dissipation mechanisms for the seismic protection of equipment and components in essential multistory facilities such as hospitals, telecommunication centers, and power plants. The seismic protective options include flexible mechanisms designed to control or avoid earthquake-caused damage or malfunctioning of equipment and components—these mechanisms work by reducing accelerations and by controlling deformations and displacements. Validated seismic options for local protection of sets and pieces of equipment and components in multistory facilities are currently limited. Equipment such as vital sign monitors, defibrillators, and infant incubators are just a few examples of equipment that need to maintain function during and after natural disasters. The loss of functionality of hospitals, power plants, and telecommunication centers, to name a few, could be harmful to emergency operations and to the ability of communities to respond and recover when an earthquake hits. It is critical to limit these types of losses and to avoid malfunctioning or damage of specialized equipment in these key facilities. Minimizing the effects of damaging earthquakes could be accomplished by using preventive strategies that should be secured globally by engineering the seismic resilience of facilities; and secured locally by implementing seismic mitigation mechanisms within the equipment and components—to date, however, local protective options are very limited (Marin-Artieda). The main project scope includes theoretical analysis and real-environment full-scale laboratory tests to: 1) examine the applicability of isolation and energy dissipation devices in the form of platforms, floors, or individual systems to protect different equipment sets; 2) study the optimal characteristics of the seismic protective options of equipment and contents; 3) study the interaction of seismic protective options and the effects of floor accelerations on the response of sensitive equipment and their protective measures; 4) validate the equipment-passive systems arrangements via fullscale experiments; and 5) apply a framework for analysis, design, and implementation of the protective measures (Marin-Artieda). Full-scale tests were performed at University of Buffalo in July and August of 2014. 1a. NEES-REU Project Scope: The scope of the Network for Earthquake Engineering Simulation Research Experience for Undergraduates (NEES-REU) project includes numerical predictions and support of the experimental testing program of a platform supported on four seismic isolators to protect pieces of simulated equipment under simulated floor earthquake shaking on various fixed-based buildings. The isolation systems consisted of lead rubber bearings

2 supporting simulated equipment. Through numerical analysis using SAP2000 and MATLAB the seismic responses of the isolated platform-equipment sets are predicted. The scope of this project also includes the support of the seismic isolator system tests at NEES@Buffalo, which is one of the NEES-REU sites at the University at Buffalo. The experiment support includes configuration of test specimens and instrumentation. 1b. Objectives of NEES-REU Project The objectives of this NEES-REU project are: 1) To analyze the specific performance of the lead rubber elastomeric bearing system. 2) To determine the seismic performance of the lead rubber elastomeric bearing under various input floor accelerations. 3) To study the mechanical characteristics of the elastomeric pad. 4) To support the experimental settings including configuration of test specimens and instrumentation. 2. Literature Review 2a. Introduction: What is Base Isolation? Base isolation is achieved by providing a flexible interface between the building and its foundation to isolate the building structure from the ground (Murota et al.,2005). This flexible interface allows the building or structure’s seismic responses to be separated from the ground motion during an earthquake (Murota et al., 2005). Base isolation controls the seismic effects on building and contents, including equipment and secondary systems by reducing accelerations and deformations. By isolating the building from the foundation during earthquakes, the building is not directly attached or connected to the ground; therefore it does not directly experience the seismic energy from the ground. The isolator, which is in contact with the ground, will absorb the seismic energy, and reduce both the acceleration and displacements of the building. 2b. Base Isolating Systems There are different types of base isolation systems discussed by Murota et al. (2005). The sliding bearing, which can operate as a friction pendulum system, when activated moves within a curved surface allowing the motion of the building to act as a pendulum. It is this pendulum action that enables the bearing to elongate the period of the isolated structure. The elastomeric bearing systems include: natural, high damping-rubber bearing, and lead core rubber bearings. Natural rubber bearings have low damping capacity and a closely linear relationship with respect to the horizontal force and displacement. The high damping rubber bearing is made up of a special rubber that allows for dissipation of seismic energy during deformation. Lead rubber bearing is a system that is comprised of rubber and stiffening plate layers with a mechanism at the center of the isolators made of lead to dissipate energy.

3 Fan et al. (1988) reported test results on several types of base isolation systems including: 1) pure-friction/sliding-joints, 2) laminated rubber bearings, 3) a resilientfriction based isolation system, 4) a French system based on energy dissipation combining the friction between steel reinforced neoprene padding and bronze plates, 5) a New Zealand system based on a lead core rubber bearing system, and 6) a sliding resilient-friction system based on two-component frictional elements. Each of these systems was subjected to a horizontal harmonic simulated ground motion. Numerical evaluations of the responses were derived. In this study the absolute acceleration, maximum displacement of structure and the base displacement were evaluated. The findings were compared to the responses of a fixed-base structure. Descriptions of the systems are below (Fan et al., 1988). a.) Pure Friction/Sliding Joint (PF): the mechanism of the isolator is sliding friction; horizontal friction provides resistance to motion and dissipates energy. b.) Laminated rubber bearing (LBR): made of alternating rubber and steel layers, which allows the bearing to be flexible in the horizontal direction while stiff in the vertical direction. The horizontal flexibility reduces the frequency of the structure to avoid the dominant frequency of the ground motion. c.) The Resilient-Friction Base Isolation system (R-FBI): made up of several layers of Teflon-coated friction plates with a rubber core. The rubber gives the damping and restoring forces and the friction mechanism dissipates energy. d.) New Zealand base isolation system (NZ): is a laminated rubber bearing system that has a central lead core. The lead core reduces the displacement by dissipating energy, while the rubber allows lateral flexibility. e.) The Electricite de France (EDF): is a system that was designed for the seismic protection of nuclear power plants in regions that are subjected to high seismic activity. This system is made up of steel reinforced neoprene pads that are topped with lead-bronze plates. The pads and lead-bronze plate are in frictional contact with a steel plate, which is connected to base raft of the structure. When there is no sliding between the plates, the EDF acts as a LBR, and when the structure is shaken, sliding occurs between the friction plates to dissipate energy. f.) Sliding Resilient-Friction (SR-F) is a combination of the R-FBI and the EDF. This system has two friction components. When sliding is not occurring in the upper friction plates, the isolator acts as an R-FBI but when it is subjected to an intense earthquake, the upper friction plates begin to slide. This allows the additional energy to be dissipated and therefore making the system more effective. Experimental results from Fan et al.’s (1988) study demonstrated that the transmitted acceleration and the stresses experienced by the structure are significantly reduced by the base isolated systems. The experimental results demonstrated that when the dominant period of the ground was close to the natural period of the isolation system for LRB, NZ and the R-FBI, all systems amplified the ground accelerations, and the responses of the friction type systems were slightly sensitive to the changes of the friction coefficient.

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It is important to have an understanding of the effects of different base isolation systems on global building protection to provide background of the possible isolation options that may be applicable for the local protection of equipment. 2c. Benefits of Base Isolation It is well documented that base isolators are implemented on many concrete and steel structures such as San Francisco City Hall, Long Beach V.A. hospital, Caltrans Traffic Management Center, Pasadena City Hall and others (Namie, 1999). Base isolation systems not only improve the seismic performance of concrete and steel structures, but also they may be beneficial for wood structures (Symans et al., 2002). 2d. Base Isolation for Seismic Protection of Equipment Base isolation is not only beneficial for the performance of the load carrying structure and nonstructural elements such as walls, ceilings and facades during earthquakes; it also provides local protection to large equipment and non-building type structures (Murota et al., 2005; Marin-Artieda, 2014). Several experimental and analytical studies have been done to test the effects of base isolation on equipment (Murota et al., 2005; Marin-Artieda, 2014). Below are some results of base isolation for the protection of equipment such as transformers, cooling towers, and secondary systems. 1.) Murota et al. (2005) performed an experiment on an electric power transformer to test a seismic isolation system. The isolation consisted of combining a sliding bearing, a rubber bearing, and a segmented high damping rubber system. The equipment isolation systems set was subjected to tri-axial earthquake shaking. The studies demonstrated the efficiency of base isolation. Some of the conclusions of this experiment include: a base isolation system using conventional rubber bearing is not an adequate solution for lightweight structures because the natural period cannot be lengthened. The rubber bearing performed effectively in the uni-axial (x-direction) and the bi-axial (x and y direction). The ground motion’s vertical component caused the response accelerations of the bushing support on the sliding system to be higher than the fixed based system and the frequency characteristics depended on the compressive stiffness of isolator. No coupling effects were observed in the system using the segmented high damping rubber bearing. Rubber rings were developed in order to add flexibility to the fix end joints. They were also placed between the bushing and transformer top to help with reducing the acceleration during testing. 2.) In Marin-Artieda (2014) case studies conducted on the seismic protection of equipment in essential buildings through numerical analysis and verification with experimental data. The results confirmed that for buildings with fixed bases, the

5 highest floor level of the building would experience a higher level of seismic amplification. This amplification is due to the characteristics of the building and it is independent of the type of ground motion. Also for fixed-base structures, the acceleration responses potentially could cause overturning of equipment. A way to reduce the responses of the equipment to the acceleration of the top floor is by shifting the fundament period of the equipment away from the resonance frequency of the building (Marin-Artieda, 2014). Equipment with short periods had a response that is a function of the ground frequency whereas the response of equipment with relatively long periods depended upon the dynamic properties of the building. A major benefit of seismic isolation is that it reduces the response of equipment on all levels of a structure. 3.) Kelly (1982) studied the influences of base isolators on seismic responses of light secondary equipment. Different base isolation systems were tested to determine which system is the most effective in protecting the secondary system such as pumps, valves, control devices, and piping systems against seismic damage. The base isolation systems were tested on a large-scale structural model using a shake table. The systems included a fully isolated system of elastomeric bearings, a friction-damped system, and a system incorporating elastomeric bearing and an energy-absorbing device. These systems were able to reduce the responses of the secondary system substantially. The fully isolated system of elastomeric bearing provided the most protection to the equipment. However, large displacements were experienced between the structure and the ground. The friction damped system controlled displacements but it generated high frequency responses in the structure and this response was observed in the equipment as well. The energy absorbing base isolation system was the most effective in controlling the displacement and minimizing the increase in acceleration (Kelly, 1982). 4.) Cooling towers are important equipment that needs to maintain operation in essential buildings. Research focusing on the seismic protection system of cooling towers is reported by Marin et al. (2014). In urban areas, most cooling towers are located on the roofs of buildings. The placement of the cooling towers puts them in a position to be easily affected by seismic effects due to the amplification of top floors of fixed based structures. The research focused on the effects of isolation/restraint systems on controlling the seismic responses of rooftop equipment, understanding the main factors that control the acceleration of top floors of fixed and isolated based buildings, how seismic responses of rooftop equipment are affected by the amplified acceleration of roofs, and the limitation and capabilities of commercially available computer programs in modeling isolation and restraint systems. Research and application has shown that base isolation system perform well with protecting building non-structural elements. Because of the advancement in the

6 implementation of seismic isolation systems this research continued to look into the response of equipment that is connected to base isolation systems and will provide more information on safer ways to protect equipment within essential buildings in seismic areas. 3. Methods The primary analytical tools that were used in the process of this research include SAP2000 and MATLAB. SAP2000 was used to create a numerical model of a base isolated platform supporting a piece of equipment The SAP2000 model provided predictions of the performance of the base isolation system during experimental lab testing. A MATLAB code was used to create response spectra for floor accelerations to study the characteristics and variation of floor accelerations with building floors With SAP2000 a model was created using a concentrated mass. The mass was a general representation of a piece of sensitive equipment that could be located in a building such as a hospital or telecommunication. The modeled equipment was connected to a platform that was directly attached to four lead rubber bearings on each corner of the platform. In the model, the equipment was represented using a concentrated load of 5.7 kips, the platform represented using four point that were placed 96 inches apart from one another in a square shape. The lead rubber bearings were connected to those points using a bilinear link element. In those bilinear link elements the properties such as effective stiffness, yield strength, stiffness, and post yield ratio of the lead rubber bearings were inputted; the values of properties can be seen in Table 1. The parameters were extracted from the displacement loop that can be seen in Figure 1. The model was tested using the floor motions of the White Ridge earthquake; the data was from the PEER ground motion data base. These floor motions were the same motions that were used during experimental testing. Table 1: Input parameters and values Effective stiffness (kips/in) Yield strength (kips) Stiffness (inches) Post yield Ratio

0.56 0.50 0.38 0.75

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Figure 1 Force displacement loop of rubber bearing In order to determine the effectiveness of the lead rubber bearing ability to reduce the seismic effect to the equipment, the accelerations of accelerometers at the top of the steel plate, were compared to accelerometers attached to table extension. Figure 2 shows the comparison of the x components of the table acceleration (aextx) and the accelerations of the top of the steel plates (acc31x). Figure 2 shows an example of the response spectrum for the table accelerations in the x, y, and z direction. These motions of the table extension were needed for an input into SAP2000.

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Acceleration X 1.5

acceleration

1 0.5 acc31x 0 0

1000

2000

3000

4000

5000

aextx

-0.5 -1

time (s)

Figure 2 Time history graph of experimental data (floor acceleration vs. table extension) 3a. Modeling In order to create and run the analytical model the geometry of the platform, system, and equipment were inputted. The floor motions were inputted as well in order to run the analysis. The base isolation was placed on the corners of an 8 feet by 8 feet platform. The elastomeric bearing base isolators were represented using bilinear link elements and length of the link was 0.0 feet in order to represent a direct connection to platform. A body constraint was applied on the nodes at the platform to simulate the platform acting a rigid body. Parameters, which can be seen in Table 1, from the displacement loop (Figure 1) had to be extracted in order to properly model the bearing in SAP2000. In order to extract the parameters a line was drawn between the two extended points on the loop, the slope of the line was the effective stiffness. In order to properly simulate the equipment the mass was need as well as the mass moment of inertia. They were calculated using Equations 1 and 2. In Figure 3 is a representation of the numerical model in SAP2000 that was used in order to run analysis of the system. 𝑀=

𝑊

𝑅3 =

𝑔

𝑀(𝑏 2 +𝑣 2 ) 12

(1) M – mass W – weight G – gravitational force

M – mass b – length v – width

(2)

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Figure 3 SAP2000 model of final numerical model 3b. Instrumentation Testing was performed at the University at Buffalo Structural Engineering Earthquake Simulation Laboratory. The laboratory is equipped with two 3.6 meter by 3.6 meter shake tables with extension. These tables are capable of 6 degrees of freedom movement and can hold 100 metric ton. Figures 4 and 5 gives a picture of the set up lab testing. For testing the acceleration were measured with accelerometers and the displacement with string potentiometers. The values from the experiment were compared to those of the expected result from models.

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Figure 4 shake table test set up before the specimen

Figure 5 shake table test set up after the specimen on the table extensions

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4. Results/Discussion The experimental testing and SAP2000 model showed the isolators to be effective in reducing the acceleration for the x and y component of the motion but for the vertical component the isolators were not as effective in reducing the accelerations. For the experimental testing showed the x direction acceleration decrease to be 59.5%, y 50% and the z direction to be a 61% amplification. For the numerical model the x direction decreased accelerations 49.2%, the y 53.3% and the z direction amplified accelerations 82%. For equipment within essential facilities, the reduction of accelerations in the horizontal direction means there is a possibility that equipment can continue to function during seismic activities and if damage occurred it could be minimized. However, for equipment that is sensitive to vertical accelerations this system would not be effective at protecting it. This could mean that equipment that are on higher floors could possibly cause damage to floors, causing them to collapse onto lower levels. More research would be needed for isolators that would be effective in reducing vertical accelerations. Figure 6 shows the spectrum response of the components of the table acceleration these spectrums are a reflection of the initial floor accelerations and Figure 7 are the response spectrum of the steel plates. Figure 8 shows results of the analytical model. Figure 8 is the comparison of the table accelerations compared to the accelerations of the concentrated load analytical model. The difference between the experimental testing and the numerical modeling for the x direction was 10.3%, y direction 3.3% and for the z direction 20.5%. Pseudo Acceleration Spectrum

Displacement Spectrum 4

0.01 X Y Z (UP)

0.009

X Y Z (UP)

3.5

0.008 3 0.007 2.5

Spa (g)

Sd (in)

0.006

0.005

2

0.004 1.5 0.003 1 0.002 0.5 0.001

0

0

0.5

1

1.5

2

2.5

0

Period (sec)

Figure 6 response spectrum of the floor acceleration

0

0.5

1

1.5

Period (sec)

2

2.5

12

Displacement Spectrum

Pseudo Acceleration Spectrum

0.018

2.5 X Y Z (UP)

0.016

X Y Z (UP)

2

0.014

0.012

Spa (g)

Sd (in)

1.5 0.01

0.008

1 0.006

0.004

0.5

0.002

0

0.5

0

1

2

1.5

0

2.5

0

1

0.5

Period (sec)

1.5

2.5

2

Period (sec)

Figure 7 Response spectrum of steel plates

X direction

Acceleration (g)

-3

2

x 10

1 0 -1

0

5

10

15

Acceleration (g)

-5

2

x 10

25

30

35

40

25

30

35

40

25

30

35

40

1 0 -1 -2

0

5

10

15

4

20

Time (s) Z direction

-4

Acceleration (g)

20

Time (s) Y direction

x 10

2 0 -2

0

5

10

15

20

Time (s)

Figure 8 acceleration time history of table extension and numerical model

13 Table 2: Table of the maximum accelerations for the SAP model input motions compared to the accelerations of the concentrated load Max Acc.

x

y

z

Concentrated load (black line) (g)

0.520

0.008

0.236

Input motion (red line) (g)

1.057

0.015

0.194

5. Conclusion Base isolation has shown to be effective in protecting building from the harmful effects of seismic energy. The same principles that protect building from seismic energy are used in this research to further study ways to reduce the effect of seismic energy on non-structural components such as equipment within essential buildings. The research showed 1) the lead rubber elastomeric bearing system performed effectively in reducing acceleration in the horizontal direction, 2) the performance of the lead-rubber bearings were effective in reducing the seismic effect onto equipment under various floor accelerations and 3) experimental settings including configuration of test specimens and instrumentation was supported using an analytical model in SAP2000. Further studies on different isolation systems are necessary for possible solution for the reduction of acceleration in the vertical direction. 6. Contact Information For more information on this report you can contact Paquilla Jones at [email protected] for more information on the project “Passive Seismic Protective Systems for Nonstructural Systems and Components in Multistory Buildings” please contact Dr. Claudia Marin at [email protected]. 7. Acknowledgments The author would like to give special thanks to principal investigator Dr. Claudia Marin, graduate student mentors Diego Buitrago and Amdedebrhan for assistance with research. Also would like to thank Dr. Han and technicians and the University at Buffalo Structural Engineering and Earthquake Simulation Laboratory (SEESL). This research was conducted as a part of the NEES and NEES-REU grant number EEC-1263155 and the NEES operation award number CMMI-0927178.

14 References Fan, F. and Ahmadi, T. (1988). "Base isolation of a multi-story building under a harmonic ground motion." Report No. NCEER-88-0010, National Center for Earthquake Engineering Research, State University of New York at Buffalo French, C. NEES EOT (2011). “Base isolation,” http://nees.org/rresources/3832 Kelly, J (1982)."The influence of base isolation on the seismic responses of light secondary equipment." Report No. UCB/EERC-81/17, University of California, Berkeley, Earthquake Engineering Research Center Marin, Claudia., "Performance assessment of seismic protective systems for sensitive equipment in multistory buildings." Section Track: Non-building structures. Design, Analysis, and testing of non-structural components. NB214, ASCE 2012 Structures Congress, ASCE, March, 29-31, 2012, Chicago, IL. Marin-Artieda, C., Kea, K., and Valencia, T., (2014). “Studies of a rooftop cooling tower and its seismic protective system.” Structures Congress 2014, 1892-1903. Marin-Artieda, C.C. (2014). "Case studies on the seismic protection of equipment in essential buildings." Proc. 10th U.S. National Conference on Earthquake Engineering,NCEE Anchorage, Alaska. Murota, N., Feng, M., Liu, G. (2005). "Experimental and analytical studies of base isolation systems for seismic protection of power transformers." Report No. MCEER -05-0008, Multidisciplinary Center for Earthquake Engineering Research. University at Buffalo, State University of New York Naeim, F and Kelly, J.M. (1999). Design of seismic isolated structures. New York: John Wiley & Sons. (pp. 6-17). Symans, M., Cofer, W.F., and Fridley, K.J. (2002). Base isolation and supplemental damping systems for seismic protection of wood structures: Literature Review. Earthquake Spectra, 18(3), pp. 549-572.