Citation: Oropel, J., T. Azuhata (2020), Earthquake performance evaluation of typical bridge structureswith seismic isolation and soil structure interaction in the Philippines, Synopsis of IISEE-GRIPS master
report.

EARTHQUAKE PERFORMANCE EVALUATION OF
TYPICAL BRIDGE STRUCTURES WITH SEISMIC
ISOLATION AND SOIL STRUCTURE INTERACTION
IN THE PHILIPPINES
Joseph Christopher A. OROPEL1,2

Supervisor: Tatsuya AZUHATA3
ABSTRACTThe current seismic code for bridges in the Philippines adopts concepts from the American Associationof State Highway and Transportation Officials’ (AASHTO) Load and Resistance Factor Design (LRFD)
Bridge Design Specifications, 2012. It utilizes the Force-Based Design (FBD) approach, which has been
applied in bridge seismic design since the 1990 Luzon Earthquake. However, AASHTO recently
recommends the use of Displacement-Based Design (DBD) as an alternative approach in defining
seismic performance levels. The study attempts to adopt the concept from DBD to investigate the effects
of seismic isolation together with soil-structure interaction (SSI) towards typical bridge structures in thePhilippines. The approach for evaluating the structure’s limit state and seismic performance wasconducted following the provisions of the Japan Road Association Specifications for Highway Bridges(JRASHB), 2012. The application of isolation bearings on structures with distinctive natural periods
was evaluated based on its effectivity and applicability. The earthquake response analysis was also
performed to analyze its influence on the structure to develop a more accurate representation of its
seismic response. Furthermore, the effects of the individual and combined contribution from theparameters were evaluated on its significance in affecting seismic performance.
Keywords: Seismic Isolation, Soil-Structure Interaction, Displacement Based Design.
1. INTRODUCTION
The current seismic code for bridges in the Philippines adopts concepts from the American Associationof State Highway and Transportation Officials’ (AASHTO) Load and Resistance Factor Design (LRFD)
Bridge Design Specifications, 2012. It utilizes the Force-Based Design (FBD) approach, which has been
applied in bridge seismic design since the 1990 Luzon Earthquake. However, AASHTO recently
recommends the use of Displacement-Based Design (DBD) as an alternative approach in defining
seismic performance levels.
The study of Panaligan (2019) investigated the seismic performance of a typical bridge in
the Philippines designed using FBD and evaluated by the alternative approach – DBD. One of the main
conclusions was strengthening the pier’s sectional properties by increasing the column diameter from1000 mm to 1500 mm and increasing the amount of longitudinal reinforcements. The revised section
enhanced the original performance point to satisfy the limit state requirements of the Capacity Spectrum
Method (CSM). However, it inadvertently increases the substructure’s stiffness, causing a shorter
fundamental natural period, which consequently amplifies the earthquake response towards the structure.
This study aims to reduce the seismic demand brought by the heightened stiffness using seismic isolation.
The bridge is located on alluvial ground and is supported by deep pile foundations a depthof 30 m. The soil condition’s influence was beyond the scope of Panaligan’s study. Therefore, the effectsof Soil-Structure Interaction (SSI) was not considered. It depicts that the initial assumption on the____________________________________________
1

Department of Public Works and Highways Bureau of Design, Philippines.
IISEE-GRIPS master course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute.
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foundation condition could not portray the structure’s actual seismic behavior. The influence of SSI
could also inadvertently affect the applicability of seismic isolation as it is much dependent on thenatural period of the structure. This study aims to accommodate soil amplification and soil spring effects
brought by local soil conditions to develop a more accurate representation of the structural seismic
behavior. Furthermore, the attributes introduced by SSI and seismic isolation are expected to produce
large displacements; therefore, using the DBD approach is advantageous in addressing the heightened
parameter. This method is regarded as a more accurate representation of damage; hence it provides amore reliable measure to depend upon than the FBD approach.
2. BRIDGE STRUCTURE AND MODEL
The target structure is a typical bridge that is being widely used in the Philippines. It is a two-lane, threespan prestressed concrete girder (PSCG) bridge supported by two-column bent piers. It was designedby the Department of Public Works and Highways (DPWH) in accordance with the Design Guidelines,
Criteria, and Standards (DGCS) 2004 Edition, and the Bridge Seismic Design Specifications (BSDS)2013 Edition. The seismic design utilizes the FBD method or more commonly known as the R-factor
method. The bridge’s design was recently evaluated through the DBD method in the study of Panaligan(2019). This study utilizes the same bridge structure, although applying both the original and revised
column sections to investigate SSI and seismic isolation effects.

Figure 1. General plan and elevation of the bridge.
Original Column Section

Revised Column Section

Figure 2. Original and revised column sections.
23. METHODOLOGY
The study focuses on two main parameters, which are the foundation condition and seismic isolation.
These parameters are investigated as individual and combined components on their respective influence
towards the bridge structure’s seismic performance. Afterward, seismic response analyses were
performed, such as the static capacity spectrum method (CSM) analysis and the dynamic nonlinear
response history analysis (RHA).
Several geophysical tests can obtain the low-strain shear modulus of soil either in thelaboratory or in situ. The most convenient method in obtaining this parameter is defined in as a functionof the soil’s bulk density and shear wave velocity. However, this relationship represents the first tangent
shear modulus, where it is at its maximum and can only be found in normal conditions or low strain
circumstances. The soil properties alter when considering high-strain situations found in strong groundmotion. Various cyclic tests have shown that soil stiffness is directly dependent on the strain amplitude
and the number of loading cycles. Therefore, in conducting seismic analysis, the reduced soil parameters
are considered to account for this phenomenon. This study adopts the Hardin-Drnevich Model as thenumerical soil model.
The seismic performance of seismically isolated bridges depends primarily on the energy
absorption capacity coming from its isolation bearings. Therefore, it is necessary to define an adequate
nonlinear hysteric model that best represents the isolation system’s attributes. Typically, isolators anddampers work together to achieve seismic isolation based on hysteric and viscous damping theories toimprove the overall damping performance of the structure. This study utilizes Lead Rubber Bearing(LRB) as the seismic isolation bearing.
4. RESULTS AND DISCUSSION
Three analyses were conducted towards the structure. First, an eigenvalue analysis to obtain thefundamental natural period of the structure. Second, a static analysis to determine the structure’s
performance point. Third, a dynamic analysis to verify the results from the static analysis. Furthermore,these analyses were conducted along two orthogonal directions – longitudinal and transverse. The three
analyses considered the individual and combined effects of the parameters on the structure and eachother.
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12

6

Sa-Sd h =5%
Sa-Sd h =10.9%
1500 LRB
1500 NB

10Sa (g)

81500 NB vs. LRB

12

Sa-Sd h =5%
Sa-Sd h =10.4%
Sa-Sd h =15.4%
1000 LRB
1000 NB

10Sa (g)

14

1000 NB vs. LRB

8
6

4

4

2

2

0

0
0

0.2

0.4

0.6

0.8

1

1.2

0

Sd (m)

0.2

0.4

0.6

0.8

1

1.2

Sd (m)

Figure 3. Capacity curve vs. Demand spectrum of the two column sections on
fixed support with and without seismic isolation along the longitudinal
direction.
It is observed in the Figure below that the 1000 NB model’s capacity curve does not
generate an intersection point. The structure had been pushed up to the allowable limit displacement of
378 mm to incur minimal or repairable damages according to its seismic performance level. If thestructure were to be displaced further, the limit displacement would not be satisfied, and subsequently,
3neither will the seismic performance level. On the other hand, when introducing LRB towards the
structure, the limit displacement is theoretically increased due to the additional limit displacement
capacity of the isolation bearing. However, it is observed that the capacity curve indicated by the 1000
LRB and 1000 NB models are seemingly identical. Furthermore, there is no noticeable change instiffness between the two models indicating that there is no significant contribution from the isolation
bearing.
It can be noticed that there is a significant change between the capacity curves of the 1500
NB and 1500 LRB models shown in the Figure above. The 1500 NB model’s yield point of the capacity
curve indicates that the member starts to yield at the plastic hinge location. In the case of 1500 LRB, theyield point occurs much lower and continues to reach the target spectrum without any reduction factor.
The lower yield point is assumed to be caused by the introduction of the isolation bearing. However, toverify this assumption, the F-D relationship of the structural members and the isolation bearings needs
to be investigated.
The left figure below shows the F-D behavior of the column at the plastic hinge. It can beseen that the performance point, indicated by the red dot, is under yielding. It should be noted that thispoint has not reached the intersection point of the capacity spectrum. However, this point defines thelimit displacement of the section with respect to the seismic performance. Further displacing this point
would not satisfy the limit state requirements of Seismic Performance Level 2. The center figure shows
the F-D behavior of the 1000 LRB model at the same location. Notice that the performance point is still
under yielding. It also indicates that the point has surpassed the allowable limit state, thereby notsatisfying seismic performance requirements. Also, the Right figure illustrates the F-D relationship ofthe LRB. Notice that the performance point has occurred during the isolator's initial stiffness andconcludes that the LRB is not functioning for the original section when applying CSM analysis.
1000-F-LRB

Curvature, Ø (1/m)

LRB

Force (kN)

Moment (kNm)

Moment (kNm)

1000-F-NB

Curvature, Ø (1/m)

Displacement (m)Figure 4. F-D relationship from CSM of the 1000-F-NB and 1000-F-LRB
models along the longitudinal direction.
1500-F-LRB

Curvature, Ø (1/m)

LRB

Force (kN)

Moment (kNm)

Moment (kNm)

1500-F-NB

Curvature, Ø (1/m)

Displacement (m)Figure 5. F-D relationship from CSM of the 1500-F-NB and 1500-F-LRB
models along the longitudinal direction.
Figure 5 shows the original section’s F-D relationship on fixed-support subjected to 2011
Great East Japan earthquake ground motion. The left figures show the column is incurring somedamages from the earthquake, and it can be seen, that the column is behaving nonlinearly through theTakeda-Model relationship. It is also noted that all three earthquake ground motions did not surpass thelimit curvature of the original column section. The center figures show the F-D relationship of thecolumn when the model is incorporating isolation bearings. It shows a minimal change from the previous
4model due to the linear behavior from the isolation bearings, as shown in the right figures. The LRB
does not exhibit any bilinear deformation and therefore was not able to dissipate any seismic energy.
The RHA results from the revised section on fixed supports generated parallel results whencomparing to the static analysis results. The left figures show the F-D relationship of the 1500 section
with no isolation bearing. It can be seen that the column is experiencing some damages indicated by the
Takeda-Model relationship. In applying LRB towards the model, the behavior significantly changes.
The column section does not show any yielding and is kept in the linear region. The deformation was
accommodated by the LRB, indicated in the right figures. It can be observed that the LRB is performing
its function as it shows the bilinear deformation relationship. It can be said that the functionality of the
LRB was achieved as it maintained the linear behavior of the column section.
1500-F-LRB

LRBForce (kN)

Moment (kNm)

Moment (kNm)

1500-F-NB

Displacement (m)Curvature, Ø (1/m)

Curvature, Ø (1/m)

Figure 6. F-D relationship from RHA of the 1500-F-NB and 1500-F-LRB
models using the 2011 Great East Japan Eq along the longitudinal direction.
The results of the RHA on pile foundations closely resemble the results from the fixedsupport models. However, in carefully comparing the maximum attained curvatures, the pile foundation
models yielded higher values than the fixed-support models. It could be attributed to the soil
amplification as well as the soil-structure interaction. The seismic forces may have potentially increased
due to the difference in support conditions. Furthermore, the inertial forces from a more flexible
structure may have caused an increase in the members’ deformation. It can also be verified by comparing
the maximum deformation of the isolation bearings from the pile foundation models to the fixed supportmodels shown in Figure 7. It illustrates the amplified displacements caused by the pile foundations
through the rocking and swaying motion of the structure. Furthermore, it consistently shows that the
maximum attained deformation is indeed higher than that of the fixed-support models. Therefore, theinertial forces occurring from the superstructure is higher than the fixed-support models.
12
10
8
6
4
2

Swaying000.2

1500 LRB
Rocking

10

Height (m)Height (m)121000 LRB
Rocking

Fixed
P1 w/o SoilP2 w/o SoilP1 w/ SoilP2 w/ Soil

0.4
0.6
Displacement (m)

8
6
4
2

Swaying

0
0.8

0

Fixed
P1 w/o SoilP2 w/o SoilP1 w/ SoilP2 w/ Soil

0.1
0.2
Displacement (m)0.3

Figure 7. Longitudinal horizontal displacements of the 1000-LRB and 1500LRB models from the 2011 Great East Japan Earthquake.
5. CONCLUSIONSThe study was attempted to investigate the effects of seismic isolation together with soil-structureinteraction towards typical bridge structures in the Philippines. The displacement-based method forevaluating the structure’s limit state and seismic performance was conducted and is considered as a more
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convenient approach to investigate the effects of seismic isolation and soil-structure interaction. The
limit displacements were determined in reference to the JRASHB, 2012.
Two types of analysis were performed in the study: a static analysis from the capacity
spectrum method and dynamic analysis from the response history analysis. The static and dynamic
analyses generated parallel results and are in good agreement with each other. The conclusions from theseismic analyses are summarized as follows:1.
The seismic isolation bearings exclusively functioned when applied to the revised column
section. This section having a shorter natural period is a more rigid structure for which the
LRB could effectively function.
2.
The soil-structure interaction amplified the effects of the ground motion through theswaying and rocking motions of the structure, which inadvertently increased horizontal
displacement responses. This displacement, however, was lowered when effectively
introducing isolation bearings.
3.
The LRB continued to function towards rigid structures with pile foundations despite theincreased natural period and added flexibility at the pier.
4.
The maximum and residual displacements at the top of the superstructure remained within
practical design limits. The response displacement is accounted for by providing sufficient
clearances for the functionality of the isolation bearings.
5.
Installment of expansion joints that can accommodate the horizontal displacement should
confirm not hindering the isolation bearing’s functionality and maintaining safe traffic
conditions during the bridge’s service state.
6.
The revised column section satisfies the current seismic code of the Philippines. Since theR-factor was recently revised in the BSDS from 5 to 3.5 for the same bridge with anoperational category as ‘Essential’, it confirms the revised section has higher strength
capacity than the required.
7.
The transverse direction is sufficient to maintain a linear response from strong groundmotions. The ramen-type pier together with superstructure’s lateral stiffness contributed inresisting the lateral force demand.
ACKNOWLEDGEMENTS
This research was conducted during the individual study period of the training course “Seismology,
Earthquake Engineering and Tsunami Disaster Mitigation” by the Building Research Institute, JICA,
and GRIPS. First and foremost, I would like to express my sincere gratitude to my adviser, Dr. Tatsuya
Azuhata, for the continuous guidance and suggestions towards the completion of the research. His
knowledge and expertise have indeed been invaluable in providing me the technical understanding in
the field of earthquake engineering. My appreciation extends to the staff members at IISEE/BRI to theirsustained support and assistance during the course of the program. I am incredibly grateful to JICA and
the Japanese government for letting me participate in this training program. I thank them for the financial
support and the outstanding response with COVID-19. It has been a life-changing experience foreveryone and their service, allowed me to continue with the course despite uncertainties from thepandemic. Also, I would like to express my deepest appreciation to our beloved coordinator, Tomoe
Koyano. She has shown unrelenting compassion and encouragement throughout the program. Her warm
personality has brought smiles and laughter, making our stay in Japan as comfortable as possible.
REFERENCES
Higashino, M., and Okamoto, S., 2006, Response Control and Seismic Isolation of Buildings, Taylor
and Francis, New York, USA.
Japan Road Association (JRA), 2012, Specification for Highway Bridges, Part IV: Substructures andPart V: Seismic Design.
Kowalsky M.J., Priestly M.J.N., Macrae, G., 1995, Displacement-Based Design of RC Bridge Column
in Seismic Regions, Published by John Wiley & Sons, Ltd.
Kramer, S.L., 1996, Geotechnical Earthquake Engineering, Prentice-Hall.
Panaligan R.J.N., 2019, Earthquake Performance Evaluation of Typical Bridge Structures Designed by
A Force-Based Design Method in The Philippines, Individual studies by participant at the IISEE.
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