Base Isolation for Seismic Retrofitting of Structures

Base Isolation for Seismic Retrofitting of Structures

Downloaded from ascelibrary.org by Indian Institute of Technology, Delhi on 09/16/14. Copyright ASCE. For personal use only; all rights reserved.

Vasant A. Matsagar1 and R. S. Jangid2 Abstract: Analytical seismic responses of structures retrofitted using base isolation devices are investigated and the retrofit schemes are illustrated. The retrofitting of various important structures using seismic isolation technique by incorporation of the layers of isolators at suitable locations is studied. Three specific structures such as historical buildings, bridges, and liquid storage tanks are selected to investigate the effectiveness of the base isolation in seismic retrofitting. Different types of isolation devices, such as elastomeric bearings and sliding systems are evaluated for their performance in the retrofitting works. The response of the retrofitted structural system is obtained numerically by solving the governing equations of motion under different earthquakes and compared with the corresponding conventional structure without any retrofit measures, in order to investigate the effectiveness of base isolation in retrofitting of structures. It is observed that the seismic response of the retrofitted structures reduces significantly in comparison with the conventional structures depicting effectiveness of the retrofitting done through the base isolation technique. This paper also distinctively elaborates on the methods of construction in retrofitting works involving base isolation. DOI: 10.1061/共ASCE兲1084-0680共2008兲13:4共175兲 CE Database subject headings: Base isolation; Bearings; Bridges; Earthquake engineering; Historic sites; Monuments; Rehabilitation; Tanks; Seismic effects.

Introduction Many structures have their fundamental frequencies of vibration within the band of frequencies where the energy of earthquake ground motions is the maximum. In such cases, a structure amplifies the seismic ground vibrations and produces accelerations within the structure that increase from the bottom of the structure to its top. Besides producing undesirable levels of acceleration in the structure, these amplified structural motions can cause severe distress in the structural elements and large relative motions between different parts of the structure. This may result in permanent damage to different parts of the structure, or may even lead to catastrophic collapse. The amplified accelerations throughout the structure act on the occupants and contents of the structure, causing harm and damage to the occupants and contents even when no structural damage may occur. A relatively new and costeffective procedure to mitigate such effects is to detach the structure from the earthquake ground motions by the use of base isolators provided between the base and foundation of the structure. Seismic isolation is a design technique, which proposes decoupling of a structure, part of it, or even of equipment placed in the structure, from the damaging effects of earthquakes. One of the goals of seismic isolation is to shift the fundamental frequency of the structure away from the dominant frequencies of 1

Postdoctorate Research Fellow and Adjunct Professor, Dept. of Civil Engineering, Lawrence Technological Univ., Southfield, MI 48075-1058. E-mail: [email protected] 2 Professor, Dept. of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India. E-mail: [email protected]. ac.in Note. Discussion open until April 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on July 12, 2006; approved on January 9, 2008. This paper is part of the Practice Periodical on Structural Design and Construction, Vol. 13, No. 4, November 1, 2008. ©ASCE, ISSN 1084-0680/2008/4-175–185/$25.00.

the earthquake ground motion and fundamental frequency of the fixed-base superstructure. The other purpose of an isolation system is to provide an additional means of energy dissipation, thereby reducing the transmitted acceleration into the superstructure. This innovative design approach aims mainly at the isolation of a structure from the supporting ground, generally in the horizontal direction, in order to reduce the transmission of the earthquake motion to the structure. A variety of isolation devices including elastomeric bearings 共with and without lead core兲, frictional/sliding systems, and roller bearings have been developed and used practically for seismic design of buildings during the past 20 years 共Kelly 1986; Buckle and Mayes 1990; Jangid and Datta 1995兲. There have been several existing buildings and bridges retrofitted seismically using base isolation over the last decade, particularly in the USA, New Zealand, and Japan. The building code requirements for seismic design are becoming more stringent, through which improved design methods and requirements are expected to reduce the damage in the newer buildings to acceptable levels, in the event of a moderate to strong earthquake 共Saatcioglu and Humar 2003兲. However, older buildings designed to satisfy the older codes/regulations are likely to be vulnerable to severe damage or total collapse under strong seismic excitations. Past earthquakes have demonstrated that even during small tremors older buildings have suffered extensive destruction, or collapsed, the buildings would have performed better if they had been seismically retrofitted. Hence, the retrofit option of using base isolation to improve seismic performance of existing structures is now being adopted 共Cheung et al. 1999兲. In order to protect the historical appearance of a structure, base isolation is the only way for seismic retrofitting of the structure. In addition, the retrofit works using seismic isolation can be carried out when the structure to be retrofitted is in use. To delineate the details of isolation, such as the choice of isolation systems and its placement, factors to be considered in the design of seismic isolation and the influencing dynamic properties of the structure to be retrofitted, a comprehensive study is highly necessary.

PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / NOVEMBER 2008 / 175

Pract. Period. Struct. Des. Constr. 2008.13:175-185.


Herein, the analytical response of structures retrofitted using the base isolation is investigated. The various types of structures considered are a historical building, a bridge, and a liquid storage tank. The specific objectives of the study are to: 共1兲 demonstrate the usefulness of base isolation in seismic retrofitting of the structures; 共2兲 substantiate the efficacy of different isolation devices in seismic retrofitting works; and 共3兲 study the various aspects influencing the retrofitting works using the seismic isolation technique. Because no specific study prior had addressed construction methodology to be used in retrofitting works involving base isolation, this paper delineates the construction methodology to be used in retrofitting works using the base isolation technique.

Fxb kxb uxb

(a) HDRB Fxb Fxy

(b) LRB

Essentially, there are two categories of isolation systems based on their fundamental behavior, such as elastomeric bearings and sliding systems. The choice of isolation systems and analysis procedure has to be made such as to properly predict the seismic response leading to the rational and economical design. The guide specifications for seismic design of buildings, bridges, and other structures 共NRCC 1995; BSSC 1997; ICBO 1997; AASHTO 1999兲 suggest the use of nonlinear dynamic time-history analysis of the base-isolated structures. When subjected to ground motion, the restoring forces developed in the bearings depend upon the type of isolation systems utilized; and the aforementioned documents recommend that a linear equivalent mathematical model can be adopted to carry out an equivalent static analysis for designing the bearings and the superstructure. In the present study, three representative isolation systems are considered for investigation, the force-deformation characteristics of which are as explained below. High-Damping Rubber Bearing The high-damping rubber bearing 共HDRB兲 is the most commonly adopted base isolation system. The basic components of HDRB are steel and rubber plates built in the alternate layers as shown in Fig. 1共a兲. Generally, the HDRB exhibits high-damping capacity, horizontal flexibility, and high vertical stiffness. The damping constant of the system varies considerably with the strain level of the bearing 共generally of the order of 10%兲. The system operates by decoupling the structure from the horizontal components of earthquake ground motion by interposing a layer of low horizontal stiffness between the structure and its foundation 共Kikuchi and Aiken 1997兲. These devices can be manufactured easily and are quite resistant to the environmental effects. Here, the code specified equivalent linear viscous model is adopted for the actual force-deformation behavior of the HDRB as shown in Fig. 1共a兲共ICBO 1997兲. It is to be noted that such an equivalent linear model of isolation system is quite simple and generally accurately predicts the results, except for few typical isolator parameters 共Matsagar and Jangid 2004兲. The restoring force developed in the HDRB for bi-directional excitation is given by

再冎冋册再冎冋册再冎 =

cxb 0 0 cyb

u˙xb u˙yb

uxb

q

Models of Isolation Systems

Fxb Fyb

αkxb

+

kxb 0 0 kyb

uxb uyb

共1兲

where uxb and uyb = bearing displacements, respectively in the xand y-directions; cxb = cyb and kxb = kyb = viscous damping and lateral stiffness of HDRB, respectively in the x- and y-directions, owing to the isotropic nature of individual isolation systems.

Fxb fx

kxb uxb

(c) FPS

Fig. 1. Schematics and force-deformation characteristics of various isolation systems, namely, 共a兲 HDRB; 共b兲 LRB; and 共c兲 FPS

Therefore, note that the isolation time period, Tb in the x- and y-directions is the same. The overdot denotes the derivative with respect to time. The design of isolation system requires specifying its lateral stiffness kxb and viscous damping cxb. The linear stiffness and equivalent damping of the HDRB is designed in such a way as to provide the specific values of the two parameters, namely, the isolation time period Tb and the damping ratio ␤eff expressed as

␤eff =

冑

m kxb

共2兲

cxb 2m␻xb

共3兲

Tb = 2␲

where ␻xb = 2␲ / Tb = isolation frequency; and m = total mass of the structure resting on the isolators. Lead-Rubber Bearing The second type of elastomeric bearings is lead-rubber bearings 共LRB兲 as shown in Fig. 1共b兲. This base isolation system provides the combined features of vertical load support, horizontal flexibility, restoring force, and damping in a single unit 共Skinner et al. 1975; Robinson 1982兲. These bearings are similar to the HDRB except that a central lead core is used to provide an additional means of energy dissipation. The LRB also provides energy absorbing capacity through additional hysteretic damping in yielding of the lead core that reduces the lateral displacements of the isolator, especially under ambient vibrations. The forcedeformation behavior of the LRB is generally represented by nonlinear characteristics following a hysteretic nature. For the present study, a nonlinear model 共Park et al. 1986兲 is used to characterize the hysteretic force-deformation behavior of the LRB, as shown in Fig. 1共b兲. The restoring force developed in these isolation systems for bidirectional excitation is given by

176 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / NOVEMBER 2008


再冎冋册再冎 Fxb Fyb

=␣

kx 0 0 ky

uxb uyb

+ 共1 − ␣兲

再冎再冎 Fxy

Fyy

Zx Zy

共4兲


where Fxy = Fyy = yield strengths of the bearing, respectively in the x- and y-directions; ␣ = index that represents the ratio of post to preyielding stiffness; kx = ky = preyield stiffness of the bearing, respectively in the x and y-directions; and Zx = Zy = nondimensional hysteretic displacement components satisfying the following nonlinear first order differential equation expressed as

q

再冎 Z˙x Z˙ y

=

冋

A − ␤兩u˙xb兩Zx兩Zx兩n−1 − ␶Znx − ␤兩u˙yb兩Zy兩Zx兩n−1 − ␶ZxZny − ␤兩u˙xb兩Zx兩Zy兩n−1 − ␶ZyZnx A − ␤兩u˙yb兩Zy兩Zy兩n−1 − ␶Zny

⫻

再冎 u˙xb u˙yb

册

共5兲

where q = isolator yield displacement; A, ␤, and ␶ = dimensionless parameters; and parameter n = integer constant, which controls smoothness of the transition from elastic to plastic response. These dimensionless parameters A, ␤, ␶, and n are selected such that the predicted response from the model closely matches with the experimentally obtained results. In the present study, the dimensionless parameters of the LRB are held constant with q = 2.5 cm, A = 1, ␤ = ␶ = 0.5, and n = 2. The LRB is characterized by the isolation period Tb and the normalized yield strengths Fxy / W = Fyy / W. Here, W = mg = total weight of the structure; and g = acceleration due to gravity. The bearing parameter Tb is computed from Eq. 共2兲 using the postyield stiffness of the bearing.

再冎冋册再冎再冎 Fxb Fyb

=

kxb 0 0 kyb

uxb uyb

+

fx fy

共6兲

where kxb = kyb = bearing stiffness provided by virtue of inward gravity action at the concave surface; and f x = f y = limiting frictional forces in the x- and y-directions, respectively. The system is characterized by an isolation time period Tb that depends upon the radius of curvature of the concave surface, and a friction coefficient ␮ = ␮x = ␮y. The isolation stiffness kxb = kyb is adjusted such that the specified value of the isolation period evaluated by Eq. 共2兲 is achieved. Usually, there is a large difference in the damping of the structure and the isolation system, which makes the resulting assembly nonclassically damped. In addition, the force-deformation behaviors of LRB and FPS are nonlinear. This leads to coupling of the equations of the motion of a structure, and to analyze the system correctly, complex modal analysis is required. Nevertheless, here the governing equations of motion are solved by Newmark’s method of integration using linear acceleration over a small time step ⌬t.

Retrofitting Scheme A review of a few of the major retrofitting projects successfully completed using the base isolation technique around the world is summarized here. It includes the historical buildings and bridges retrofitted using seismic isolation systems, which clearly brings about the fact that the uniqueness and aesthetics on the original heritage structure is maintained intact even after its seismic retrofitting for improved performance during the earthquakes.

Friction Pendulum System One of the most popular and effective techniques for seismic isolation is in the use of the sliding isolation devices. It is also a widely preferred isolation system during the retrofitting works. The sliding systems exhibit excellent performance under a variety of severe earthquake loading and are very effective in reducing the large levels of the superstructure acceleration. These isolators are characterized by insensitivity to the frequency content of earthquake excitation, because of the tendency of the sliding system to reduce and spread the earthquake energy over a wide range of frequencies. Another advantage of the sliding isolation systems over the conventional rubber bearings is that because of the development of the frictional force at the base, it is proportional to the mass of the structure and the center of mass and center of resistance of the sliding support coincides. Consequently, the torsional effects produced by a typical asymmetric structure are diminished. The concept of sliding bearings is combined with the concept of a pendulum type response, resulting in a conceptually interesting seismic isolation system known as the friction pendulum system 共FPS兲 as shown in Fig. 1共c兲. In FPS, the isolation is achieved by means of an articulated slider on a spherical, concave chrome surface 共Zayas et al. 1990兲. The slider is faced with a bearing material, which when in contact with the polished chrome surface results in the development of friction force, while a concave surface produces a restoring force proportional to its radius. The FPS develops a lateral force equal to the combination of the mobilized frictional force and the restoring force developed because of rising of the structure along the spherical concave surface. The force-deformation diagram of FPS is shown in Fig. 1共c兲. The resisting force provided by the FPS for bidirectional excitation is

Details of Retrofitting Strategy In order to implement the base isolation as a retrofit scheme, the following points need to be considered: 1. Evaluation of dynamic properties of the existing structure, through the ambient/forced vibration tests, essential in the design of base isolation systems; and 2. Numerical studies required to arrive at the important decisions regarding: 共a兲 the location of the isolators; and 共b兲 the choice of the type of isolation system. The choice of seismic isolation retrofitting scheme is dependent upon the type of structure to be retrofitted as explained below. Masonry Structures In most of the projects of seismic retrofitting of the old monuments, the structures are constructed of masonry in stone or brick using lime or cement mortars. These structures transmit the selfweight to the ground through masonry walls and are called load bearing structures. While incorporating the seismic isolators, underpinning is done to provide temporary supports along the masonry wall. Holes are created in the wall and a needle beam 共mounted over the seismic base isolators at a specified distance apart兲 is constructed progressively below the masonry wall as shown in Fig. 2共a兲. The temporary supports in the form of underpinning are then removed, thereby transferring the vertical load of the structure towards the foundation through the needle beam and the base isolators. Framed Structures In the case of structures constituting beams and columns, termed as framed structures, an additional floor in the form of a base raft



Needle-beam

ll wa

Column

Ma

ry so n

Base-isolator

Moat


Base-raft

Base-isolator Foundation

(a) Load bearing masonry structure

(b) Framed structure

Fig. 2. Details of isolator placement and its arrangements during retrofitting works using base isolation 共a兲 load bearing masonry structure; 共b兲 framed structure

is essential to incorporate the isolators. In a typical retrofitting work involving base isolation in the case of a framed 共beamcolumn兲 structure, near the column-foundation junction 共a preferred location兲, the column is cut using mechanical cutters and the seismic base isolator is inserted as shown in Fig. 2共b兲 in conjunction with a base raft. Alternatively, instead of constructing the base raft new, the first floor slab could be used to serve as a base raft, in which case, the isolators are placed at the top of the first story columns; thereupon, strengthening of the first story columns becomes necessary. Note that placing the isolation layer at the first story is less effective as compared to the case in which the base isolators are placed at ground level. Modalities of Isolation as Retrofit Strategy In comparison with the conventional retrofit schemes, the alternative of seismic isolation is beneficial for two basic reasons: 共i兲 conventional strengthening leaves the building susceptible to damage and potential down time after any major earthquake event; and 共ii兲 traditional retrofitting techniques have severe effects on the structure’s operations during the time that the retrofit work is in progress. Similarly, retrofit work of the bridges using a conventional strengthening approach results in unacceptably high construction costs and prolonged construction time with major disruption to the traffic flow, which can easily be eliminated by relying upon base isolation as retrofit, improving its ability to survive under the design earthquake with almost no damage. An advantage of isolation as a retrofit technique is its unaltered superstructure, especially for historic structures no variation in aesthetics of the building is required, helping in architectural preservation of these monuments. In addition, the technique is limited only to work at the base level; no superstructure activity is involved and is free of influence from the superstructure properties. In retrofitting works using isolation, the entrance and related details 共and utility services passing through the plane of isolation such as gas lines, water supply lines, sever lines, etc.兲 need to be worked out such that they should not fail during an earthquake event as well as the isolation should not get locked. Moreover, provision of separation gap distance 共moat width兲 is to be made in

order to accommodate the additional displacements occurring across the isolation layer and to avoid collision on the adjacent structures.

Retrofitting of Historical Buildings The “old, traditionally built” heritage structures, such as the monuments and traditional historical buildings, are generally more affected during earthquakes. These buildings are mainly constructed in the period before the ample use of reinforced concrete, with elements and technology based on the experience of the masons/builders alone, without any structural-seismic design. Nevertheless, interesting construction techniques could be seen in these historical buildings throughout the prehistoric period up to the first half of the twentieth century. In addition, the aging of these structures and their wearing out due to various causes such as humidity, ground settlements, pollution, earthquakes, etc. as well as the lack of maintenance make these structures much more vulnerable to earthquake action than the modern ones. Therefore, improvement in the seismic performance of the traditionalhistorical buildings is considered necessary, especially for those located in seismically active regions. The retrofitting by base isolation of such buildings becomes an obvious choice as the historical architectural characteristics of the building remain preserved. A list of a few major retrofitting building projects completed using base isolation is given in Table 1 for demonstration purposes. Fig. 3共a兲 shows a model historical building retrofitted by base isolation. The isolators are installed between the base and foundation of the building using either stone slab or reinforced beam. Since the building is quite rigid in comparison to the isolation system, as a result, the building can be idealized as a rigid block with mass m as shown in the mathematical model in Fig. 3共b兲. The simplified rigid body modeling of the superstructure of a base-isolated building was extensively used in the past 共Younis and Tadjbakhsh 1984; Chen and Ahmadi 1992; Jangid and Kelly 2000兲 and the difference between the responses of isolated structure with rigid and flexible superstructure was investigated



Table 1. List of Major Retrofitting Building Projects Completed Using Base Isolation Sr. number 1. 2. 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Project and country

Year

Salt Lake City and County Building Utah Rockwell International Corporate Headquarters—Building 80 Seal Beach, California Campbell Hall Monmouth, Oregon U.S. Court of Appeals San Francisco, California New Zealand Parliament Library and Parliament House Wellington, New Zealand Rockwell International Corporate Headquarters Seal Beach, California Oakland City Hall Oakland, California Hughes Aircraft Building El Segundo, California Caltrans Traffic Management Center San Diego, California Long Beach V.A. Hospital Long Beach, California Martin Luther King, Jr. Civic Center Building Berkeley, California Kerckhoff Hall, UCLA Campus Westwood Village, California San Francisco City Hall and Civic Center San Francisco, California Public Safety Building—911 Emergency Communications Center San Francisco, California Head office of Himeji Shinkin Bank 共Himeji Credit Bank兲 Himeji City, Hyogo, Japan Laboratory Building of Kansai University Senriyama Campus of Kansai University, Suita City, Japan Tokyo DIA Building Tokyo, Japan Shinjuku Station West Entrance Main Building Tokyo, Japan

1989 1991

Lead rubber isolators Lead rubber bearings and rubber bearing

1993 1994 1994 1994

Lead rubber isolator and rubber isolator Friction pendulum system Hybrid system: lead rubber isolators, rubber isolators, and sliding bearing isolators Lead rubber isolator

1994 1994 1994 1995 1995

Lead rubber isolator and rubber isolator Lead rubber bearing and rubber bearing High-damping rubber bearings Lead rubber isolator, rubber isolator, and sliding bearing High-damping rubber bearing and lead rubber bearing

1996

Lead rubber isolator

1998

Lead rubber isolator

1998

Lead rubber bearing and sliding system

2000

Rubber bearings and dampers

2001

Rubber bearings, sliding bearings, and oil dampers

2001 2002

Rubber bearings and viscous dampers Rubber bearings

共Kulkarni and Jangid 2002, 2003兲. The bearings considered are isotropic with similar properties in two horizontal orthogonal directions. The system is modeled as a two degree-of-freedom system subjected to two horizontal components of earthquake ground motion. The seismic response of the retrofitted building is obtained under the 1995 Kobe Earthquake motion recorded at JMA station. The EW component is applied in the x-direction of the building, whereas the other normal component NS is applied in the y-direction. The interaction of response in the x- and y-directions is duly considered; thus, the actual behavior of structures is closely modeled. For the base-isolated building, the response quantities of interest are the superstructure absolute acceleration and the relative bearing displacement. The absolute

Isolation systems utilized

acceleration developed is directly proportional to the forces exerted on the superstructure due to earthquake ground motion; these forces are maintained lower than the resisting capacity of the superstructure 共evaluated by ambient vibration tests兲 with an appropriate safety margin. On the other hand, the relative base displacement is crucial from the design point of view of the isolation system. Fig. 4 shows the time variation of the acceleration and displacement of a retrofitted historical building under EW and NS components of the 1995 Kobe Earthquake. The response is shown for HDRB with bearing properties as Tb = 2 sec, and ␤eff = 0.1 and compared with the corresponding response of the building without retrofitting 共referred to as conventional兲. The figure indicates

u yb Superstructure Baseisolator

m

u xb

kb , cb

u&&yg u&&xg (a) Retrofitted historical building

(b) Mathematical model

Fig. 3. Retrofitting of historical building with base isolation 共a兲 retrofitted historical building; 共b兲 mathematical model PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / NOVEMBER 2008 / 179



Fig. 4. Time variation of acceleration and displacement of a building isolated by HDRB under the Kobe, 1995 Earthquake 共Tb = 2 sec, and ␤eff = 0.1兲

that there is significant reduction 共by a factor of 0.5 to 0.3兲 in the superstructure acceleration of the building after being retrofitted by the base-isolation technique. This implies that the base isolation is effective in retrofitting the existing historical buildings. The peak bearing displacements are 16.59 and 32.58 cm in the x- and y-directions of the building, respectively, stipulating the extent of separation gap distance 共moat width兲 required at the foundation level to allow free movement of the base without occurrence of any impact or pounding. Similar observations can also be made from the peak responses of the building mounted over different isolation systems under the 1940 Imperial Valley, 1994 Northridge, and 1995 Kobe Earthquakes as shown in Table 2. Seismic responses are also obtained for LRB 共Tb = 2.5 sec, q = 2.5 cm, and Fxy / W = Fyy / W = 0.05兲 and FPS 共Tb = 2.5 sec, and ␮ = 0.05兲, in addition to the HDRB. Besides the 1995 Kobe Earthquake, two orthogonal components

of the Imperial Valley Earthquake recorded at El Centro and the Northridge Earthquake recorded at Sylmar Station are applied in the x- and y-directions of the building and the seismic responses are evaluated. The average reduction in acceleration imparted in the superstructure upon using base isolation as a retrofitting scheme with different isolation systems such as HDRB, LRB, and FPS is of the order of 0.5.

Retrofitting of Bridges In bridges, the base-isolation devices can rather easily be incorporated by replacing the traditional bridge bearings with the isolation systems 共Kunde and Jangid 2003兲. Base isolation bearings serve the dual purpose of providing a means for the movements due to thermal actions as well as protecting the bridge from dy-

Table 2. Peak Seismic Response of a Building Retrofitted by Various Isolation Systems Imperial Valley, 1940 Isolation system Retrofitted

HDRB LRB FPS

Conventional

Direction x-direction y-direction x-direction y-direction x-direction y-direction x-direction y-direction

Northridge, 1994

Kobe, 1995

Displacement 共cm兲

Acceleration 共g兲





14.72 14.40 13.02 11.91 10.26 10.64

0.149 0.146 0.112 0.101 0.113 0.095 0.342 0.210

34.06 49.55 46.65 55.41 51.37 66.98

0.342 0.501 0.313 0.385 0.344 0.466 0.593 0.827

16.59 32.58 17.15 28.25 16.70 25.71

0.172 0.338 0.130 0.207 0.134 0.210 0.617 0.818

—

—



—

Table 3. List of Major Retrofitting Bridge Projects Completed Using Base Isolation Sr. number


1. 2. 3. 4. 5.

Project and country

Year

Isolation systems utilized

Eel River Bridge Robinson’s Ferry, Rio Dell, California Benicia-Martinez Bridge San Francisco, California Offshore Bridge Structure Caspian Sea Highway Bridges in Illinois Million Dollar Bridge on the Copper River Cordova, Alaska

1988 2000 2000 2003 2004

Lead rubber isolators Friction pendulum bearings Spherical PTFE sliding bearing with steel hysteretic dampers Friction pendulum bearings Friction pendulum bearings

namic loads by increasing the fundamental time period and dissipating the seismic energy in hysteretic damping. A list of a few major retrofitting bridge projects that utilized the base-isolation technique is given in Table 3. In order to demonstrate the effectiveness of seismic isolation, a three-span continuous deck bridge made of reinforced concrete is considered as shown in Fig. 5共a兲. The properties of the bridge deck and piers are given in Table 4. The bridge is seismically retrofitted by using the elastomeric bearings at pier and abutment locations. The bridge is mathematically idealized as shown in Fig. 5共b兲共Tongaonkar and Jangid 2002, 2003兲. The superstructure and substructure of the bridge are modeled as a lumped mass system divided into a number of small discrete segments. Each adjacent segment is connected by a node, and two mutually perpendicular degrees of freedom are considered at each node. The masses of each segment are assumed to be distributed between the two adjacent nodes in the form of point masses. In addition, the bridge superstructure and piers are assumed to remain in the elastic range during the earthquake excitation. This is a reasonable as-

Deck

Isolation system or bearings Abutment Pier

Rock line

sumption as the base isolation attempts to reduce the earthquake response in such a way that the structure remains within the elastic range. The seismic response of a retrofitted bridge is investigated under Kobe, 1995 Earthquake motion in which the EW and NS components are applied in the x- and y-directions of the bridge, respectively. The bridge is retrofitted by providing the identical bearings at the abutment and pier locations. The total stiffness and damping of these bearings is selected in such a way as to provide specific values of the isolation period Tb and the damping ratio ␤eff based on the deck mass of the bridge; refer to Eqs. 共2兲 and 共3兲. In Fig. 6, the time variation of the base shear developed in the pier and relative displacement of the bearings of the bridge retrofitted by the elastomeric bearings is shown. The bearing is designed to provide isolation time period Tb = 2 sec 共based on rigid deck and pier condition兲 and 10% damping ratio. As can be seen from the figure, the base shear in the piers is significantly reduced 共about 80–90%兲 for the retrofitted bridge system as compared to the conventional bridge system in both the directions of the bridge. This indicates that the isolation systems are quite effective in reducing the earthquake response of the bridge. The maximum peak displacements of the bearing are 16.59 and 32.58 cm, respectively, in the x- and y-directions of the bridge. Similar observations can be made from the peak responses of the isolated bridge using different isolation systems under the 1940 Imperial Valley, 1994 Northridge, and 1995 Kobe Earthquakes as shown in Table 5. The average reduction in the base shear generated in the piers upon using base isolation as a retrofitting scheme with different isolation systems such as HDRB, LRB, and FPS is of the order of 0.8.

(a) Model of three-span continuous girder bridge

Abutment

Retrofitting of Liquid Storage Tanks

yi

y1 1

x1

2

i

Deck

Isolation system

xi Abutment

Pier yN xN

(b) Mathematical modeling of isolated bridge

Fig. 5. Typical three-span continuous bridge with seismic bearings Table 4. Properties of the Bridge Deck and Piers Properties Cross-sectional area 共m 兲 Moment of inertia 共m4兲 Young’s modulus of elasticity 共N / m2兲 Mass density 共kg/ m3兲 Length/height 共m兲 2

Deck

Piers

3.57 2.08 20.67⫻ 109 2.400⫻ 103 3@30= 90

4.09 0.64 20.67⫻ 109 2.400⫻ 103 8

Liquid storage tanks are lifeline structures and strategically very important, since they have vital use in industries and nuclear power plants. Past earthquakes have confirmed the seismic vulnerability of tanks, wherein, damage occurred in the form of buckling of a tank wall due to excessive development of compressive stresses, failure of piping systems, and uplift of anchorage system. The seismic behavior of liquid storage tanks is highly complex due to the liquid-structure interaction leading to a tedious design procedure from an earthquake-resistant design point of view. A seismic retrofit strategy in the form of using friction dampers in the tower of elevated liquid storage tanks was reported 共Hale and Pall 2000兲; however, no tank retrofit project is found by the writers in which a base isolation technique has been utilized. However, similar to as explained above, the base isolation technique can be effectively used as a retrofit scheme in the case of the liquid storage tanks as well. The structural model considered for a retrofitted cylindrical liquid storage tank is shown in Fig. 7, in which the seismic iso-




Fig. 6. Time variation of base shear and bearing displacement of bridge isolated by HDRB under the Kobe, 1995 Earthquake 共Tb = 2 sec, and ␤eff = 0.1兲

lation systems are installed between the base and foundation of the tank. This lumped mass model has been extensively used in the past to study the effectiveness of seismic isolation for liquid storage tanks 共Kim and Lee 1995; Malhotra 1997; Shrimali and Jangid 2002a,b兲. The contained liquid is considered as incompressible, inviscid, and has irrotational flow. During the base excitation, the entire tank liquid mass vibrates in three distinct patterns, such as sloshing or convective mass 共i.e., top liquid mass, which changes the free liquid surface兲, impulsive mass 共i.e., intermediate liquid mass vibrating along with the tank wall兲, and rigid mass 共i.e., the lower liquid mass, which rigidly moves with the tank wall兲. There are various modes in which sloshing and impulsive masses vibrate; however, the response can be predicted by considering the first sloshing mode and the first impulsive mode. Therefore, the continuous liquid is modeled as lumped masses with flexible tank, i.e., Haroun’s model 共Haroun 1983兲.

The sloshing, impulsive, and rigid lumped masses are denoted by mc, mi, and mr, respectively. The sloshing and impulsive masses are connected to the tank wall by corresponding equivalent springs having stiffness kc共=mc␻2c 兲 and ki共=mi␻2i 兲, respectively. The parameters ␻c and ␻i denote the sloshing and impulsive frequencies of the liquid mass, respectively. The base-isolated tank system is considered to have six degrees of freedom under bidirectional earthquake excitation. These degrees of freedom are denoted by uxc, uxi, and uxb, which denote the absolute displacement of the sloshing, impulsive, and rigid masses, respectively, in the x-direction. Similarly, the degrees of freedom in the y-direction are denoted by uyc, uyi, and uyb, which denote the absolute displacement of the sloshing, impulsive, and rigid masses, respectively. Further, self-weight of the tank is neglected since it is very small 共less than 5% of the effective weight of the tank兲. The

Table 5. Peak Seismic Response of Continuous Girder Bridge Retrofitted by Various Isolation Systems Imperial Valley, 1940 Isolation system Retrofitted

HDRB LRB FPS

Conventional


Northridge, 1994

Kobe, 1995


Normalized pier base shear





14.72 14.40 13.01 11.91 10.25 10.64

0.049 0.045 0.037 0.346 0.043 0.032 0.123 0.076

34.06 49.55 46.63 55.39 51.36 66.98

0.101 0.144 0.090 0.111 0.098 0.127 0.214 0.300

16.59 32.58 17.15 28.23 16.70 25.71

0.073 0.102 0.066 0.077 0.069 0.072 0.222 0.294

—

—



—

uyc uxc

kc

␻i =

mc

uyi uxi

ki

␻c =

H

mi

Hc

uyb uxb

Hi mr


Hr Base-isolator

u&&yg u&&xg

Fig. 7. Mathematical model of base-isolated liquid storage tank

geometric parameters of the cylindrical tank considered are liquid height, H; radius, R; and average thickness of the tank wall, th. The various masses and associated natural frequencies of the tank liquid are expressed as 共Haroun 1983兲 mc = m␥c

共7兲

mi = m␥i

共8兲

mr = m␥r

共9兲

m = ␲R2H␳w

共10兲

P H

冑

冑冉冊 1.84

E ␳s

g tanh共1.84S兲 R

共11兲

共12兲

where ␥c, ␥i, and ␥r = mass ratios associated with the sloshing, impulsive, and rigid masses of the tank liquid, respectively; ␳w = mass density of the tank liquid; E and ␳s = modulus of elasticity and density of the tank wall, respectively; S = H / R = aspect ratio of the tank 共i.e., ratio of the liquid height to the radius of the tank兲; g = acceleration due to gravity; and P = dimensionless parameter. The parameters ␥c, ␥i, ␥r, and P are functions of the aspect ratio of the tank S and th / R ratio 共Haroun 1983兲. The horizontal stiffness kb and viscous damping cb of the bearings are designed such as to provide specific values of the two parameters, namely, the isolation time period Tb and damping ratio ␤eff defined in Eqs. 共2兲 and 共3兲, respectively, in both orthogonal directions owing to the isotropic nature of the seismic isolation systems. The mass considered to determine these parameters is the total effective mass M = mc + mi + mr of the isolated liquid storage tank. Time variations of different response quantities such as normalized base shear, Fs / W 共where W = effective weight of the tank兲; impulsive displacements 共i.e., uxi = uxi − uxb, uyi = uyi − uyb兲, and relative bearing displacements 共i.e., uxb, uyb兲 of a slender tank 共S = 1.85, and H = 11.3 m兲 under EW and NS components of the 1995 Kobe Earthquake ground motion is shown in Fig. 8. The responses are shown for both conventional and retrofitted conditions. The parameters of the isolation systems considered are Tb = 2 sec, and ␤eff = 0.1. It can be observed from the figure that the

Fig. 8. Time variation of response quantities of liquid storage tank isolated by HDRB under the Kobe, 1995 Earthquake 共Tb = 2 sec, and ␤eff = 0.1兲 PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / NOVEMBER 2008 / 183


Table 6. Peak Seismic Response of Liquid Storage Tank Retrofitted by Various Isolation Systems Imperial Valley, 1940 Isolation system Retrofitted

HDRB LRB


FPS Conventional


Northridge, 1994

Kobe, 1995


Normalized base shear





11.0 9.91 7.37 7.67 5.29 7.61

0.113 0.102 0.083 0.098 0.080 0.098 0.481 0.313

26.08 42.63 23.79 42.17 35.44 48.11

0.270 0.441 0.198 0.323 0.260 0.360 0.529 0.756

15.55 30.79 12.66 23.28 13.18 22.94

0.158 0.323 0.114 0.196 0.118 0.191 0.579 0.859

—

base shear 共Fs / W兲 and impulsive displacement of the retrofitted tank are less in comparison to that of the conventional system. This indicates that the base isolation is quite effective in reducing the base shear and impulsive displacement of the liquid storage tank. Further, the peak bearing displacements in the isolator are found to be of the order of 15.55 and 30.79 cm, respectively, in the x- and y-directions. Thus, the base isolation is significantly effective in seismic retrofitting of the existing liquid storage tanks. From Table 6, similar observations can be made wherein the peak responses of the isolated liquid storage tank using different isolation systems under the 1940 Imperial Valley, 1994 Northridge, and 1995 Kobe Earthquakes are tabulated. The average reduction in the base shear generated in the tank wall upon using base isolation as a retrofitting scheme with different isolation systems such as HDRB, LRB, and FPS is of the order of 60%.

Conclusions The construction practice and effectiveness of the base isolation is investigated for retrofitting works of the existing structures. The analytical seismic responses of retrofitted structures 共such as historical buildings, bridges, and liquid storage tanks兲 using different isolation systems are obtained under different earthquakes and compared with the corresponding conventional structure 共i.e., without retrofitting兲. This study distinctively presented modalities involved in the construction technique of seismic retrofitting using the base isolation strategy. The following conclusions are arrived at from the presented study: 1. The seismic base isolation provides two important design features for the structures: it reduces the seismic forces by a factor ranging from 0.3 to 0.8 in the superstructures, and controls the distribution of these reduced lateral forces among the substructures and foundations to further enhance the overall economy and effectiveness of the retrofit designs; 2. The seismic response of the retrofitted structures is significantly reduced in comparison to the conventional structures, implying that the base isolation is quite effective in retrofitting the existing structures; 3. Both the elastomeric and sliding systems are found to be effective in retrofitting of the building, bridge, and tank structures; 4. The original uniqueness and aesthetic value of the historical monumental structures are maintained unaltered, even after retrofitting achieved through base isolation;

—

5.

6.

—

The retrofitting work can be carried out without interrupting 共or abandoning兲 the regular activities in case of any structure being retrofitted using base isolation; and The construction methodology described in this paper can be adopted in the retrofitting projects involving base isolation technique.

References American Association of State Highway and Transportation Officials 共AASHTO兲. 共1999兲. “Guide specifications for seismic isolation design.” American Association of State Highway and Transportation Officials, Washington, D.C. Buckle, I. G., and Mayes, R. L. 共1990兲. “Seismic isolation: History, application, and performance—A world view.” Earthquake Spectra, 6共2兲, 161–201. Building Seismic Safety Council 共BSSC兲. 共1997兲. “NEHRP guidelines for the seismic rehabilitation of buildings, FEMA-273, and commentary FEMA-274.” Building Seismic Safety Council, Federal Emergency Management Agency, Washington, D.C. Chen, Y., and Ahmadi, G. 共1992兲. “Wind effects on base-isolated structures.” J. Eng. Mech., 118共8兲, 1708–1727. Cheung, M., Foo, S., and Granadino, J. 共1999兲. “The use of innovative technologies in the seismic retrofit of Canadian federal buildings.” Proc., 8th Canadian Conf. on Earthquake Engineering, Vancouver, Canada, 537–542. Hale, T. H., and Pall, R. T. 共2000兲. “Seismic upgrade of the Freeport water reservoir, Sacramento, California.” Proc. 12th World Conf. on Earthquake Engineering, Auckland, New Zealand, Paper No. 269. Haroun, M. A. 共1983兲. “Vibration studies and tests of liquid storage tanks.” Earthquake Eng. Struct. Dyn., 11共2兲, 179–206. International Conference of Building Officials 共ICBO兲. 共1997兲. “Uniform building code.” International Conference of Building Officials, Whittier, Calif. Jangid, R. S., and Datta, T. K. 共1995兲. “Seismic behavior of base-isolated buildings: A state-of-the-art review.” Struct. Build., 110共2兲, 186–203. Jangid, R. S., and Kelly, J. M. 共2000兲. “Torsional displacements in baseisolated buildings.” Earthquake Spectra, 16共2兲, 443–454. Kelly, J. M. 共1986兲. “Aseismic base isolation: Review and bibliography.” Soil Dyn. Earthquake Eng., 5共4兲, 202–216. Kikuchi, M., and Aiken, I. D. 共1997兲. “An analytical hysteresis model for elastomeric seismic isolation bearings.” Earthquake Eng. Struct. Dyn., 26共2兲, 215–231. Kim, N. S., and Lee, D. G. 共1995兲. “Pseudo-dynamic test for evaluation of seismic performance of base-isolated liquid storage tanks.” Eng. Struct., 17共3兲, 198–208. Kulkarni, J. A., and Jangid, R. S. 共2002兲. “Rigid body response of baseisolated structures.” J. Struct. Control, 9共3兲, 171–188. Kulkarni, J. A., and Jangid, R. S. 共2003兲. “Effects of superstructure flex-




ibility on the response of base-isolated structures.” Shock Vib., 10共1兲, 1–13. Kunde, M. C., and Jangid, R. S. 共2003兲. “Seismic behavior of isolated bridges: A state-of-the-art review.” Electron. J. Struct. Eng., 3共3兲, 140–170. Malhotra, P. K. 共1997兲. “New method for seismic isolation of liquidstorage tanks.” Earthquake Eng. Struct. Dyn., 26共8兲, 839–847. Matsagar, V. A., and Jangid, R. S. 共2004兲. “Influence of isolator characteristics on the response of base-isolated structures.” Eng. Struct., 26共12兲, 1735–1749. National Research Council of Canada 共NRCC兲. 共1995兲. “National building code of Canada.” National Research Council of Canada, Ottawa, Canada. Park, Y. J., Wen, Y. K., and Ang, A. H.-S. 共1986兲. “Random vibration of hysteretic systems under bi-directional ground motions.” Earthquake Eng. Struct. Dyn., 14共4兲, 543–557. Robinson, W. H. 共1982兲. “Lead rubber hysteretic bearing suitable for protecting structures during earthquakes.” Earthquake Eng. Struct. Dyn., 10共4兲, 593–604. Saatcioglu, M., and Humar, J. 共2003兲. “Dynamic analysis of buildings for

earthquake-resistant design.” Can. J. Civ. Eng., 30共2兲, 338–359. Shrimali, M. K., and Jangid, R. S. 共2002a兲. “Nonlinear seismic response of base-isolated liquid storage tanks to bi-directional excitation.” Nucl. Eng. Des., 217共1–2兲, 1–20. Shrimali, M. K., and Jangid, R. S. 共2002b兲. “Seismic response of liquid storage tanks isolated by sliding bearings.” Eng. Struct., 24共7兲, 909– 921. Skinner, R. I., Kelly, J. M., and Heine, A. J. 共1975兲. “Hysteretic dampers for earthquake-resistant structures.” Earthquake Eng. Struct. Dyn., 3共3兲, 287–296. Tongaonkar, N. P., and Jangid, R. S. 共2002兲. “Earthquake response of bridges with sliding system.” Eur. Earthquake Eng., 16共2兲, 19–33. Tongaonkar, N. P., and Jangid, R. S. 共2003兲. “Seismic response of isolated bridges with soil-structure-interaction.” Soil Dyn. Earthquake Eng., 23共4兲, 287–302. Younis, C. J., and Tadjbakhsh, I. G. 共1984兲. “Response of sliding rigid structure to base excitation.” J. Eng. Mech., 110共3兲, 417–432. Zayas, V. A., Low, S. S., and Mahin, S. A. 共1990兲. “A simple pendulum technique for achieving seismic isolation.” Earthquake Spectra, 6共2兲, 317–333.



Base Isolation for Seismic Retrofitting of Structures

Base Isolation for Seismic Retrofitting of Structures

Suggest Documents

Base Isolation for Seismic Retrofitting of a Multiple Building Structure ...

Base Isolation for Seismic Retrofitting of a Multiple Building Structure ...

seismic retrofitting manual for highway structures

Natural Seismic Base Isolation of Structures Having Raft Foundations

Seismic Base Isolation in Reinforced Concrete Structures - IJRSSET

Seismic isolation retrofitting solution for an existing steel cable ... - PLOS

Seismic Retrofitting of Structures by Steel Bracings - Science Direct

Base-Isolation Techniques for the Seismic Protection of RC Framed ...

Base-Isolation Techniques for the Seismic Protection of RC Framed ...

SEISMIC ASSESSMENT, REHABILITATION & RETROFITTING - OF ...

Investigation of Seismic Base Isolated Structures for ...

''Smart'' Base Isolation Systems - Smart Structures Technology ...

seismic retrofitting steel and composite building structures - MAE Center

Benefits of Seismic Isolation for Nuclear Structures Subjected to

SEISMIC EVALUATION AND RETROFITTING ...

base isolation design for civil components and civil structures

Study on corollary of seismic base isolation system ... - Semantic Scholar

Analysis Of Seismic Performance Of Fps Base Isolated Structures ...

Seismic Response of Base-Isolated Structures with LRB and ... - Core

Seismic Retrofitting of Earthquake-Damaged Concrete ... - Quakewrap

Effectiveness of some conventional seismic retrofitting ...

performance assessment of seismic retrofitting ...

Comparison of different seismic retrofitting methods

seismic isolation of bridges