Green leaf engineers white paper seismic protection of structures with modern base isolation technol

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SEISMIC PROTECTION OF STRUCTURES WITH

MODERN BASE ISOLATION TECHNOLOGIES

A Green Leaf Engineers White Paper

by Luis Andrade and John Tuxworth

Day Month Year

mailto:[email protected]

SEISMIC PROTECTION OF STRUCTURES

WITH MODERN BASE ISOLATION TECHNOLOGIES Page 2

ABSTRACT

Increased resistance to earthquake forces is not always a desirable solution for buildings which house

contents that are irreplaceable or simply more valuable than the actual primary structure (e.g. museums,

data storage centres, etc). Base isolation can be employed to minimize inter-story drifts and floor

accelerations via specially designed interfaces at the structural base, or at higher levels of the

superstructure.

This paper presents the design comparison of two isolation systems (lead-rubber bearings, and friction

pendulum bearings) for a five-story reinforced concrete framed building. The response of the base-case,

fixed-structure, and isolated systems is compared for dynamic analysis to actual historical records for five

significant seismic events.

KEYWORDS : BEARING, CONCRETE, DAMPING, DISSIPATION, DRIFT,

ISOLATION, INTER-STOREY, LEAD-RUBBER, PENDULUM, SEISMIC.



CONTENTS

ABSTRACT 2

CONTENTS 3

INTORDUCTION 4

1. BASE ISOLATION SYSTEMS 5

1.1 Friction Pendulum System (FPS) 5

1.2 Lead-Plug Bearings (LPB) 6

2. MODEL-BUILDING CONFIGURATION 8

3. DESIGN PARAMETERS 9

5. MODAL ANALYSIS 12

6. TIME HISTORY ANALYSIS 13

CONCLUSIONS & RECOMMENDATIONS 17

REFERENCES 17



INTORDUCTION

Conventionally, seismic design of building structures is based on the concept of

increasing resistance against earthquake forces by employing the use of shear walls,

braced frames, or moment-resistant frames. For stiff buildings these traditional methods

often result in high floor accelerations, and large inter-story drifts for flexible buildings.

With both scenarios building contents and non-structural components may suffer

significant damage during a major event, even if the structure itself remains basically

intact. Obviously this is an undesirable outcome for buildings which house contents that

are irreplaceable, or simply more costly and valuable than the actual primary structure

(e.g. museums, data storage centres, etc).

The concept of base isolation is increasingly being adopted in order to minimize inter-

story drift and floor accelerations. In this instance the control of structural forces and

motion is exercised through specially designed interfaces at the structural base — or

potentially at a higher level of the superstructure — thus filtering out the actions

transmitted from the ground. The effect of base isolation is to essentially uncouple the

building from the ground.

This paper presents the design comparison of two isolation systems — Friction Pendulum

System (FPS) and Lead-Plug Bearings (LPB) — for a five-story reinforced concrete framed

building. The response of the fixed-base structure is compared to base-isolated cases for

five different historical time-history records for significant earthquake events.



1 BASE ISOLATION SYSTEMS

There are two common categories of large-displacement base (or seismic) isolation

hardware: Sliding Bearings and Elastomeric Bearings. This paper considers Friction

Pendulum Systems (FPS) and Lead-Plug-Bearings (LPB), which belong to the first and

second categories respectively.

1.1 Friction Pendulum System (FPS)

A FPS is comprised of a stainless steel concave surface, an articulated sliding element,

and cover plate. The slider is finished with a self-lubricating composite liner (e.g. Teflon).

During an earthquake, the articulated slider within the bearing, travels along the concave

surface, causing the supported structure to move with gentle pendulum motions as

illustrated in Figure 1(a) and 1(b). Movement of the slider generates a dynamic frictional

force that provides the required damping to absorb the earthquake energy. Friction at the

interface is dependent on the contact between the Teflon-coated slider and the stainless

steel surface, which increases with pressure. Values of the friction coefficient ranging

between 3% to 10% are considered reasonable for a FPS to be effective, Wang (1).

The isolator period is a function of the radius of curvature (R) of the concave surface. The

natural period is independent of the mass of the supported structure, and is determined

from the pendulum equation:

gRT /2 (1)

where g is the acceleration due to gravity.

The horizontal stiffness (KH) of the system, which provides the restoring capability, is

provided by:

RWkH / (2)

where W is the weight of the structure.

The movement of the slider generates a dynamic friction force that provides the required

damping for absorbing earthquake energy. The base shear V, transmitted to the structure

as the bearing slides to a distance (D), away from the neutral position, includes the

restoring forces and the friction forces as can be seen on the following equation, where is the friction coefficient:

DR

WWV (3)

The characterised constant (Q) of the isolation system is the maximum frictional force,

which is defined as:

WQ (4)

The effective stiffness (keff) of the isolation system is a function of the estimated largest

bearing displacement (D), for a given value of and R, and is determined by:

R

W

D

WDVkeff

/ (5)



A typical hysteresis loop of a FPS can be idealized as shown in Figure 1(c).

(a)

(b)

(c)

Figure 1. Motion in a FPS (a) initial condition, (b) displaced condition at maximum

displacement, (c) Idealized Hysteresis Loop of a FPS

The dissipated energy (area inside the hysteretic loop) for one cycle of sliding, with

amplitude (D), can be estimated as:

WDED 4 (6)

Thus the damping of the system can be estimated as:

RDDk

E

eff

D

/

2

4 2

(7)

1.2 Lead-Plug Bearings (LPB)

The elastomeric LPB which are generally used for base isolation of structures consist of

two steel fixing plates located at the top and bottom of the bearing, several alternating

layers of rubber and steel shims, and a central lead core as shown in Figure 2(a). The

elastomeric material provides the isolation component with lateral flexibility; the lead

core provides energy dissipation (or damping) component, while the internal steel shims

provide the vertical load capacity whilst minimizing bulging. The steel shims, together

with the top and bottom steel fixing plates, also confine plastic deformation of the central

lead core. The rubber layers deform laterally during seismic excitation of the structure,

allowing the structure to translate horizontally, and the bearing to absorb energy when

the lead core yields.

The nonlinear behavior of a LPB isolator can be effectively idealized in terms of a bilinear

force-deflection curve, with constant values throughout multiple cycles of loading as

shown overleaf on Figure 2(b).

k

e

f

f

1

k

H

Q

D

i

s

p

l

a

c

e

m

e

n

t

F

o

r

c

e

V

m

a

x

1

D

m

a

x



(a) (b)

Figure 2. LPB isolator (a) components, (b) Idealized Hysteresis Loop of a LPB

The natural period of the isolated LPB system is provided by:

gk

WT

eff

2 (8)

The Characterised strength (Q) is effectively equal to the yield force (Fy,) of the lead plug.

The yield stress of the plug material is usually taken as being around 10MPa. The

effective stiffness (keff ) of the LPB, at a horizontal displacement (D) being larger than the

yield displacement (Dy) may be defined in terms of the post-elastic stiffness (kd,) and

characteristic strength (Q), with the following equation:

DQkk deff / (9)

As a rule of thumb for LPB isolators, the initial stiffness (ki) is usually taken as 10 x kd ,

Naeim et al (2).

The energy dissipated for one cycle of sliding, with amplitude (D) can be estimated as:

)(4 yD DDQE (10)

Following on from this assumption, it has been shown by Naeim et al (2) that the

effective percentage of critical damping provided by the isolator can be obtained from:

DQDk

kQDQ

Dk

E

i

I

eff

D

)(

9/(2

4 2

(11)

k

e

f

f

1

k

d

F

o

r

c

e

V

m

a

x

k

i

1

D

i

s

p

l

a

c

e

m

e

n

t

Q

D

m

a

x

D

y



2. MODEL-BUILDING CONFIGURATION

A reinforced concrete moment-resisting frame was adopted as the structural system for

the analysis building. Figure 3 (a) and 3(b) show the structural configuration of the

building in plan.

Figure 3. Structural configuration plans (a) 1st to 3rd floors. (b) 4th and 5th floors.

Self weight of the structure was based on a concrete density ()= 24 kN/m3. Super-dead

loads of 1 kN/m2 was also applied to represent floor finishes, and 140 mm thick, 2.5-m

high hollow masonry partitions with a density of () = 15 kN/m3 were considered to

contribute as a line-load along beams of 4.9 kN/m. The imposed (live) load applied in

each floor was taken as 2 kN/m2. Story heights were taken as 3 m.

The Universal Building Code was considered in relation to seismic classification and

variables, so as to enable consistency of symbols and nomenclature throughout the

paper. Most international standards including AS 1170.4:2007 are either based on, or

align significantly with, UBC 1997(3). It was assumed that the building ‘model’ was

located in a Seismic Zone 4 of source Type A, and rests on a soil profile Type C.



3. DESIGN PARAMETERS

According to Mayes et al (4), an effective seismic isolation system should have the

following characteristics:

sufficient horizontal flexibility to increase the structural period and accommodate

spectral demands of the installation (except for very soft soil sites),

sufficient energy dissipation capacity to limit displacement to a practical level,

adequate rigidity to enable the building structure to behave similarly to a fixed base

building under general service loadings.

As recommended by both Naeim et al (2) and Mayes et al (4), a target period (T) of 2.2

seconds was adopted for the isolated structure — approximately 3 times the fixed-base

fundamental period (TF ) of 0.7 seconds.

Following UBC 1997, the target design displacement can be calculated as:

D

VD

DB

TCgD

)4/( 2 (12)

where CVD is a seismic coefficient, and BD is a damping coefficient which is a function of

the effective damping .

From UBC 1997 Table 16-R, CVD = 0.56. An affective damping of 15% was assumed for

both LPB and FPS — to be confirmed at the end of the design. From Equation 12, the

design displacement = 220 mm.

The effective stiffness for both bearing types was calculated following the formulas

presented previously. Properties including damping, hardness, modulus of rigidity,

modulus of elasticity and poisons ratio (for LPB), and friction coefficient (for FPS) were

adopted from manufacturer’s data.

As the performance of LPB isolators is weight dependant, three different sizes were

incorporated in the model. The positions nominated in Figure 5 were adopted to promote

an economical design. Final design parameters and details for each isolator type are

provided following.

Detailed design calculations have been omitted for clarity, however iterative calculation is

required to ascertain effective stiffness and effective damping as both are typically

displacement dependent.

Figures 4(a) & 4(b) display cross-sectional details for isolator characteristics summarised

in Tables 1 and 2 respectively.



(a) (b)

Figure 4. Geometrical characteristics of Base Isolators (a) FPS. (b) LPB Type A

Table 1. Design Parameters of FPS isolators.

Symbol Value Nomenclature

T (sec) 2.2 (Design Period)

(%) 15 (Effective damping)

BD 1.38 (Damping factor)

DD (mm) 220 (Design displacement Eq. 12)

R (mm) 1200 (radius of curvature, calculated from Eq. 1)

0.057 (friction coefficient)

RI 2.0 (Force reduction factor, UBC 1997 Table A-16-E, Concrete special

moment resisting frame)

Wi (kN) 7318 (Total weight of the building)

Keff (kN/m) 7961 (Total effective stiffness Eq. 5)

kH (kN/m) 6085 (Non-linear stiffness Eq. 2 )

ki (kN/m) 3103

30 (Elastic stiffness, taken as 51kH)

Q (kN) 416 (Frictional force Eq. 4)

Dy (m) 1.4 (Yield displacement calculated as Q / ( ki- kH )

14.9 (Check of assumed effective damping Eq. 7)

R

=

1

2

0

0

m

m



Table 2. Design Parameters of LPB isolators.

Parameter Value Nomenclature

T (sec) 2.2 (Design period)

(%) 15 (Effective damping)

BD 1.38 (Damping factor)

DD (mm) 220 (Design displacement Eq. 12)

G (MPa) 0.45 (Shear modulus)

T (sec) 2.2 (Design period)

Parameter Isolator Nomenclature

Type A Type B Type C

Wi (kN) 1030 740 510 (Axial load on isolator)

Keff (kN/m) 840 604 416 (Effective stiffness calculated from Eq. 8)

ED (kN-m) 38.9 28.0 19.3

(Global energy dissipated per cycle, calculated

from Eq. 11)

Q (kN) 43.9 31.5 21.7 (Short term yield force, calculated form Eq. 10)

Kd (kN/m) 642 461 318 (Inelastic stiffness, calculated form Eq.9)

Ki (kN/m) 6422 4614 3180 (Elastic stiffness, taken as 10kd)

Kd / Ki 0.10 0.10 0.10 (Stiffness ratio)

Dy (mm) 7.6 7.6 7.6 (Yield displacement, calculated as Q/9 kd)

Fy (kN) 48.8 35.0 24.1 (Yield Force calculated as kiDy)

Figure 5. Location of LPB isolators Type A, B and C.



5. MODAL ANALYSIS

SAP2000 structural analysis software is capable of Time History Analysis, including

Multiple Base Excitiation. SAP2000 facilitates the dynamic modeling of base isolators as

link elements, which can be assigned various stiffness properties. This stiffness values

for both FPS and LPB isolators were calculated as detailed in previous sections of this

paper. Calculations associated with the following summary and totaling some one-

hundred pages have been excluded from the paper.

Table 3 provides the fundamental period for the three cases studied: structure with fixed

base; with FPS isolators; and with LPB isolators, as derived from an SAP2000 modal

analysis. It can be seen that the periods obtained for both types of isolator are close to

the target period (T = 2.2 sec) recommended by Naeim et al (2) and Mayes et al (4).

Figure 6 shows the shape of the first mode of vibration for the 3 models. In addition to

influencing fundamental period Figure 6 shows the isolators’ influence on modal shape.

Table 3. Fundamental Periods

Model Fundamental Period, T

(sec)

Fixed Base 0.73

LPB 2.23

FPS 2.05

(a) (b) (c)

Figure 6. First mode of vibration for (a) fixed base building, (b) FPS isolated building and

(c) LPB isolated building.



6. TIME HISTORY ANALYSIS

A nonlinear analysis was carried out in SAP2000 in order to test the response of the

structural systems, and to validate isolator functionality. The models were subjected to

the following historical seismic time-history records:

• 1940 Imperial Valley Earthquake, El Centro Record (Richter Scale 7.1),

• 1979 Imperial Valley Earthquake, El Centro Record, Array #5 (Richter Scale 6.4),

• 1989 Loma Prieta Earthquake, Los Gatos Record (Richter Scale 7.1),

• 1994 Northridge Earthquake, Newhall Record (Richter Scale 6.6),

• 1995 Aigion Earthquake, Greece (Richter Scale approx. 5)

A seismologist is of invaluable assistance when selecting applicable time-histories,

however guidance for selecting scaling records can be gleaned from codes, Kelly (5). The

events chosen for consideration in this paper represent several of the major earthquakes

in recorded history, with the 1995 Aigion Earthquake in Greece being of similar

magnitude to the Newcastle earthquake of 1989 (Richter Scale 5.6)

Figure 7 shows maximum response values for each of the earthquake records for roof

acceleration, base shear, inter-storey drift, and isolator displacements.

Maximum roof acceleration is dominated by the 1989 Loma Prieta earthquake record

which yields a value of about 36 m/sec2 for the fixed base structure, while for the

isolated structures is in the order of 8.5 m/sec2 (76% reduction) (see Figure 7(a)).

Maximum isolator base shears are dominated also by the 1989 Loma Prieta earthquake.

A shear of approximately 8900 kN for the fixed base building is reduced to 2800 kN

(68% reduction) and 3300 kN (63% reduction) for LPB and FPS isolators respectively

(see Figure 7(b)).

Maximum Inter-storey drifts for fixed base and isolator cases are again generated by the

1989 Loma Prieta Earthquake, with values of about 129mm for the fixed base structure

and 25mm (81% reduction) and 35mm (73% reduction) for LPB and FPS respectively

(see Figure (c)). The drift ratio derived for Level-1 of the fixed base structure is 4.3%,

about twice the maximum limit of 2% imposed by the UBC 1997. The FPS isolated

structure displays a value of 1.15% which is well under the limit.

Figure 7(d) shows maximum isolator displacements in the order of 473mm and 469mm.

It can be seen in Figure 7(e) that these values are round 215% of the isolator design

displacement of 220 mm, indicating that both isolator systems would fail during the

1989 Loma Prieta Earthquake.

Force-Displacement hysteresis loops for the FPS and LPB isolator (Type A), as subjected

to the 1989 Loma Prieta earthquake record, are provided in Figures 8(a) and 8(b). These

curves follow the mathematical models presented in section 2 of this paper. Elastic and

post-elastic stiffness can be obtained as the slopes of the first two initial segments.

The energy dissipated by each isolator is provided by the area inside each loop cycle.

Effective damping can be calculated using Equations 7 or 11 and compared with the



0

5

10

15

20

25

30

35

40

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Acc

eler

ati

on

(m

/sec

/sec

)

Earthquake Record

Roof Acceleration

LBS

FPS

Fixed Base

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Base

Sh

ear

(kN

)

Earthquake Record

Base Shear

LRB

FPS

Fixed Base

0

50

100

150

200

250

300

350

400

450

500

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Iso

lato

r D

isp

lace

men

t (m

m)

Earthquake Record

Isolator Displacement

LRB

FPS

0

20

40

60

80

100

120

140

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Dri

ft (m

m)

Earthquake Record

1st Floor Inter - Story Drift

LRB

FPS

Fixed Base

0

20

40

60

80

100

120

140

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Dri

ft (m

m)

Earthquake Record

1st Floor Inter - Story Drift

LRB

FPS

Fixed Base

assumed design value. Note that there is seemingly an anomaly present in Figure 8 (a),

as maximum ‘-ve’ deflection for the FPS isolator corresponds to a reduction in base

shear. This anomaly was evident only for the Loma Prieta earthquake, and further study

is required to ascertain why this issue occurred.

Finally, time-history results for the Loma Prieta earthquake record are shown in Figure 9.

It can be noticed from Figures 9(a) and 9(b) how the response in time of the isolated

system is considerably less than the fixed base structure, specially between the 10 and

the 15 first seconds of the beginning of the seismic excitation. Figure 9(c) compares the

two types of isolators’ lateral displacements, which appears to be less for the FPS.

Figure (7a) Figure (7b)

Figure (7c) Figure (7d)



0%

50%

100%

150%

200%

250%

1940 El Centro

1979 El Centro

1989 Loma Prieta

1994 Northridge

1995 Aigion

Earthquake Record

Time History Displacement / Design Value

LRBFPS

Figure (7e)

Figure 7. Comparison of Results for the 5 earthquake records (a) roof accelerations, (b)

base shear (c) 1st floor inter-story drift, (d) isolator displacement, (e) time history

displacement / design value utilization ratio.

Figure (8a) Figure (8b)

Figure 8. 1989 Loma Prieta Earthquake Record. Force-displacement hysteresis loops for

(a) FPS isolator (b) LPB isolator Type A.



Figure 9. Time-history results for 1989 Loma Prieta earthquake record. (a) Roof

acceleration, (b) base shear, (c) isolator displacement.

-40.0

-20.0

0.0

20.0

40.0

0 5 10 15 20 25 30Acc

eler

ati

on

(m

/sec

/sec

)

Time (sec)

Lead Plug Bearing Friction Pendulum System Fixed Base

-9000

-4500

0

4500

9000

0 5 10 15 20 25 30

Base

Sh

ear

(kN

)

Time (sec)

Lead Plug Bearing Friction Pendulum System Fixed Base

-500

-250

0

250

500

0 5 10 15 20 25 30

Iso

lato

r D

isp

lace

men

t

(mm

)

Time (sec)

Lead Plug Bearing Friction Pendulum System



CONCLUSIONS & RECOMMENDATIONS

It can be seen that resultant accelerations, base shears and inter-storey drifts were all

effectively reduced by the adoption of Lead-Plug and Friction-Pendulum isolator systems,

resulting in significant improvement in modeled building performance, and a very likely

minimisation of post-event losses. For the ground conditions and sway-frame structural

system adopted, LPB & FPS base isolation would be excellent options to reduce

structural and non-structural damage, and to protect building contents. Both the LPB and

FP systems provided a comparative reduction in roof level accelerations (up to 76%);

however the LPB provided the best reduction in base shear, and inter-storey drift (at first

floor). For the adopted bearing characteristics, the FPS provided greatest control of

isolator displacement — a significant serviceability constraint with respect to boundary

conditions.

Response of the isolated structural framing systems was dominated by the time-history

record of the 1989 Loma Prieta Earthquake. The second highest intensity experienced by

the test structure was due to 1994 Northbridge earthquake. The isolator design

displacement (being a function of the nominated isolator characteristics) of both systems

was exceeded by these earthquakes, indicating alternate properties/sizes would be

required to accommodate higher intensity events.

Further work is recommended to establish applicability of these base-isolation systems

for the common braced-frame structural framing paradigm, and also to confirm suitability

(or lack thereof) for high-rise construction, and or use on deep alluvial soil strata as

evident in Australian centres such as Newcastle.

REFERENCES

1. Wang, Yen-Po, “Fundamentals of Seismic Base Isolation”, International Training programs for Seismic Design of Building Structures.

2. Naeim, F. & Kelly, J. M., “Design of Seismic Isolated Structures: From Theory to Practice”, John Wiley & Sons, Inc. 1999.

3. International Conference of Building Officials, ICBO (1997), “Earthquake Regulations for Seismic-Isolated Structures”, Uniform Building Code, Appendix Chapter 16, Whittier, CA.

4. Mayes, R. & Naeim, F., “Design of Structures with Seismic Isolation”, Earthquake Engineering Handbook, University of Hawaii, CRC Press, 2003.

5. Kelly, T. E., “Base Isolation of Structures Design Guidelines”, Holmes Consulting Group Ltd, July 2001.

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