Citation: Varma, A., T. Azuhata (2020), Seismic performance evaluation for continuous use of an
existing bridge in Nepal, Synopsis of IISEE-GRIPS master report.

SEISMIC PERFORMANCE EVALUATION FOR
CONTINUOUS USE OF AN EXISTING BRIDGE IN
NEPALAmit VARMA1,2

Supervisor: Tatsuya AZUHATA3
ABSTRACTRoad network is still the primary transportation system in Nepal, and bridges are the most sensitive partof that system. Most of the bridges were constructed about 40-50 years ago based on the design life of
50 years and traffic forecast for 30 years. The design was based on an allowable stress method where asmall value of seismic design force was considered. Therefore, it is necessary to evaluate the seismic
performance for the continuous use of an existing bridge. The structure was evaluated separately for theallowable stress method and ultimate capacity method as per the revised code. The seismic performanceof the bridge was then estimated with the Capacity Spectrum Method and the Response History Analysis.
These analyses imply that the structure has enough capacity to meet the seismic demand as per therevised code or when the double lane standard deck replaces the existing one. However, the failure mode
is a shear failure, so retrofitting should be done for the continuous use of an existing structure.
Keywords: Allowable stress, Ultimate capacity design, Steel jacketing, Non-linear earthquake
responses.
1. INTRODUCTION
Nepal is situated upon the Alpine-Himalayan or Alpine belt (the second most active seismic zone
following the Pacific ring of fire), where 17 percent of the world's most massive earthquake occurs.
Nepal is a landlocked developing country with an 83% hilly area and 17% plain area. Road network is
still the predominant public transportation system in Nepal, with the limited railways and air services tosome region. Road and rail networks can be realized only by constructing bridges over the obstacles.
Several bridges were constructed not only in Nepal but also around the world when the seismic design
method was in the development phase or was insufficient in comparison with the new standards. Before
2010, Bridge designers referred to the Indian Road Congress (IRC) guidelines and their own experience
to design the bridges in Nepal based on the design life of 50 years and traffic forecast for 30 years. At
several locations, those bridges are found incapable of addressing the current traffic flow. In those IRC
codes, a small value of seismic design force was considered, and also there was no provision for shear
design of a compression member. Therefore, their seismic capacity was difficult to estimate.
In Nepal, till now, bridges are designed using traditional methods, i.e., allowable stressmethod. In this method, the structural components are designed for a linear elastic case, and adequate
safety is achieved by assuming significantly less value of allowable stresses, which results in relatively
large sections. Due to these reasons, most of the structures designed under allowable stress have been
performing satisfactorily for many years (Pillai and Menon, 2005). However, the response level of astructure due to an earthquake is poorly estimated by allowable stress method, and thus lesser value ofdisplacement was considered in design (Teresa R. A., 2012).
1

Road Division, Palpa, Department of Roads, Kathmandu, Nepal.
IISEE-GRIPS master course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute.
2

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The concepts of capacity design and ductility design are needed for the analysis of
structural response under the severe seismic case (Priestley et al., 1996). Due to the high importance ofbridge structures, even functional damage of that structure is not acceptable in case of major earthquakes.
Therefore, the structures designed according to allowable stress should be evaluated by non-linear
analysis such as capacity spectrum method or response history analysis to check its seismic performancein severe earthquake conditions and propose retrofitting technique if needed. The retrofitting technique
should be acceptable for developing countries like Nepal in terms of economy and technology. Therefore,
steel jacketing is used here in this study as retrofitting methods of the existing structure.
2. TARGET BRIDGE STRUCTURE
The target bridge structure is a reinforced concrete cast in situ bridge of three spans, and each span is 20
m. This bridge was designed by using the working load/allowable stress method under the Indian Road
Congress (IRC) guidelines (IRC 6:2000) with a pile foundation over the Dhanewa river in the
Nawalparasi district of Nepal.

Figure 1. Bridge cross-section (longitudinal sectional elevation).
The existing bridge deck is of 6.0 m width, and the new double lane deck proposed to meetthe social traffic demand is 7.5 m carriageway width with 1.5 m footpath on both sides, as shown below
in Figure 2. The total height from the pier base to the acting point of the inertia force of the superstructureis 7.332 m. The identical elastomeric bearing is used on both sides of the span, which performs as free(longitudinally free, transversely restrained) and restrained (longitudinally and transversely restrained).
The elastomeric rubber bearing of size 400 x 300 x 52 mm (having a damping ratio of less than 0.06,
designed according to IRC 83 Part II) was used for this target structure. The bearing has lateral stiffnessof 2700 kN/m, vertical stiffness of 851269 kN/m, and rotational stiffness of 8300 kNm/rad.
Table 1. Pier details.
Diameter of circular Pier
Height of Pier
Height of Pier Cap
Total number of main rebar
Diameter of main rebar
Spacing of lateral ties
Diameter of lateral rebar
Compressive strength of
concrete
Yield strength of rebar

Figure 2. New superstructure section detail.

1600 mm
4900 mm
1200 mm
47 nos.
32 mm
150 mm
12 mm
25 MPa500 MPa

The seismic design code was recently updated as IRC: SP: 114-2018 based on the principleof capacity design. The fundamental difference in seismic design described in IRC 6:2000 and IRC: SP:
114-2018 are listed below in Table 2.

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Table 2. Seismic design criteria between IRC 6:2000 and IRC: SP: 114:2018.
Description
IRC 6:2000
Design Method
Allowable Stress Design Method.
Horizontal Seismic αβλCoefficient
α= coefficient based on location
β= coefficient based on soil
λ= Importance factor (1 or 1.5)
Verticalseismic 1/2
x
Horizontal
coefficient
coefficient

Seismic force dueto the live load
DesignSeismic
Forces(F)/
Moments (M)seismic

Considered only in the transverse
direction.
Same
as
the
seismic
forces/moments obtained for
longitudinalandtransverse
direction. (Flongitudinal = Fx and
Mlongitudinal= Mx).

IRC: SP: 114-2018
Ultimate Capacity Design Method(Z/2) I (Sa/g)/R
Z= Zone factor
I= Importance factor (1, 1.2 or1.5)
Sa/g= Average acceleration coefficient
R= Response reduction factor
Time period of the superstructure in thevertical direction is determined. Zone
factor as 2/3 of zone factor considered in
the horizontal case.
Considered in the transverse direction
and vertical direction.
Determined by combining the seismic
forces/moments obtained for longitudinal
and transverse direction.
(Flongitudinal=Fx+0.3Fy+0.3Fz
and
Mlongitudinal= Mx+0.3My).

In the allowable stress design method, a high factor of safety (3 for concrete and 2.5 forreinforcement) is considered. It is based on the concept that the allowable stress should be higher than
the working stress. In the ultimate capacity design method, the load factor is considered a factor of safety.
The ratio of the ultimate load to the capacity should be less than 1 to verify the ultimate design method.
According to the revised IRC design codes, the seismic force and the total forces were evaluated for the
existing superstructure and the new proposed bridge deck.
Table 3. Load Summary.
Description
Existing
deck withtheolddesign code
Existing
deck withthe revised
design code
Existing
deck withthe revised
design code

Verticalload (kN)
Seismic load

Horizontal Force (kN)
Design Moment (kNm)
Transverse Longitudinal Transverse Longitudinal-

312.72312.721803.141,803.14

Total load

3,550.98

355.48670.312,076.56

4,805.31

Seismic load
Total load594.294,245.85

714.73750.43575.81
941.85

4,124.60
4,188.95

2,904.15
5,924.05

Ultimate load

4,799.55

1,107.80

1,052.48

6,251.25

5,613.77

Seismic load
Total load

858.896,084.68

1,038.93
1,074.63

762.53
1,103.37

6,344.82
8,188.77

3,934.30
6,759.54

Ultimate load

7,123.87

1,594.09

1,324.63

9,937.51

7,111.48

From this table, we can see that what should be the capacity of the existing substructure sothat it can meet the current seismic demand as per the revised code and if the new deck replaces theexisting one to fulfill the social need.
3. SEISMIC EVALUATION
To evaluate the non-linear earthquake responses of the target bridge, numerical analyses were conducted.
At first, both the fixed-base model and soil-structure interaction model was created, and the time periodof the target structure was determined. The target structure is modeled as:

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Pier: Beam element with its axial stiffness, bending stiffness, initial yield moment capacity and ultimate
moment capacity (Takeda model)
Bearing: Spring element (Kh, Kv, Kθ) (Linear model)
Superstructure: Beam element (Takeda Model)
Expansion Joint: GAP Element
Pile: Beam element with its axial and bending stiffness (Takeda model)
Soil: Equivalent linear soil model with its lateral stiffness

Figure 3. Numerical model.
Table 4. Fundamental natural period of the target structure.
Description(based on complete structure modeling)
Fixed Base Model
Soil Structure Interaction Model

Existing Superstructure
New SuperstructureLongitudinal Transverse Longitudinal Transverse
1.099 sec
0.398 sec
1.403 sec
0.471 sec
1.114 sec
0.440 sec
1.423 sec
0.523 sec

The seismic performance of any structure will be greatly affected if soil-pile structureinteraction is considered for analysis as it affects the natural period and damping of that structure.
However, here we find that the fundamental natural period does not change significantly in the case of
the SSI model, as the pile group with a cap gives a very rigid support condition. The rotational stiffnessof the group piles is very high. That is why the study mainly focuses on the fixed base model.
3.1. Capacity spectrum method

Figure 4. Capacity curve versus demand
spectrum (existing superstructure).

Figure 5. Capacity curve versus demand
spectrum (new superstructure).

For the existing superstructure, the bridge pier can resist up-to 1.0G, while the bridge wasdesigned under a seismic coefficient of 0.12G only. The maximum displacement is 0.33 m in thelongitudinal direction, while 0.05 m in the transverse direction. Thus, the pier behaves stiffer in thetransverse direction comparatively. For the new proposed structure, the pier section passed into a plastic
state before reaching the maximum demand though the capacity is 0.67G.
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3.2. Response history analysis
The following earthquake ground motions based on ground type (Type II soil) are considered for
response history analysis as per JRA Specification. The STERA_WAVE (Dr. Taiki Saito,
http://www.rc.ace.tut.ac.jp/saito/index.html) software is used to generate artificial ground motion fromthese earthquakes.
(a) 2011 Great East Japan (Tohoku) Earthquake(b) 1995 Hyogo-ken Nanbu (Kobe) Earthquake(c) 2015 Nepal (Gorkha) Earthquake(d) Random phase earthquake
3.2.1. Analysis of longitudinal direction

Figure 6. Displacement at bearing top.

Figure 7. Time history of bending response of pier.

The bending response is in an elastic state for the longitudinal direction. The bearing top has a maximum
displacement of 150 mm, while the shear displacement limit of that bearing is only 36.4 mm. It means
the bearing will fail, and then the pounding will occur. The pounding force is calculated by inserting the
gap element at abutment and pier location. The displacement of the pier top and the bending responseof the pier are decreased when the gap element is used. It means there is no damage to the pier. As theabutment restricted the movement of the deck, the pounding force shall be borne by the superstructure
only and thus damages. Therefore, restrainers or dampers should be used to limit that displacement.
3.2.2. Analysis of transverse direction
Both free and fix bearings are restrained in the transverse direction. Shear blocks are used near to bearing
to restrain it. It means the complete structure behaves rigidly in the transverse direction. Due to thislarge stiffness, the time period of the structure becomes small, and hence the seismic load increases inthat direction. The pier passed into a plastic state but still safe as the maximum curvature is in the limit.
Table 5. Summary results for the Tohoku Earthquake.

Figure 8. Maximum bending moment
response under simulated the Great East
Japan (Tohoku) Earthquake.

Description
Existing deck New deck
Permissible
Shear
1807.89 kN
1807.89 kN
strength of pier (Ps)
Permissible
Ultimate
0.015 m-1
0.015 m-1
moment curvature
Pier top displacement
0.069 m0.121 mUltimatehorizontal
2297.18 kN
2472.19 kN
Strength (Pu)Maximum
moment0.0083 m-1
0.0138 m-1
curvature
Failure mode (Pu > Ps)
Shear Failure
Shear Failure

From the above analysis, it can be summarized that the pier has a shear failure, even the
maximum curvature response is less than the ultimate curvature of the pier. The pier top displacement
is also less than the limit displacement (137.9 mm).
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Therefore, the structure is safe for both cases, i.e., for the load according to the new seismiccode and when the new one replaces the existing superstructure. However, the failure mode is a shear
failure in a transverse direction, which is not acceptable for safety purposes, and thus steel jacketing isused to increase its shear capacity. The grouting material (non-shrinkage mortar or epoxy) is used to fill
the gap between the steel plate and the existing concrete pier. The shear strength (determined as per
Seismic Rehabilitation of Concrete Structure, JCI, and JRA, Part V Seismic Design) increased to3122.51 kN by using a steel plate of 2mm thickness having tensile yield strength as 345 MPa. Thus, thefailure mode changed to flexure failure.
4. CONCLUSION
The seismic capacity of the target structure was evaluated according to the revised IRC code (forcebased analysis) and the JRA specification (displacement-based analysis). From these analyses, thefollowing results could be summarized as:
1. The existing bridges of Nepal, designed with allowable stress method, seem to satisfy theseismic demand as per the revised code. It is because the factor of safety for concrete and steel
was considered as 3 and 2.5, respectively.
2. The existing intermediate lane deck can be replaced with the standard two-lane deck having
sufficient footpath to meet the present traffic demand as the pier has relatively higher bending
capacity. However, the failure mode is a shear failure, so a steel plate of 2mm thickness andtensile yield strength of 345 MPa is proposed for the continuous use of the existing substructure.
3. The maximum pier top displacement in the transverse direction was found to be 87.74% (121mm) of its ultimate limit displacement (137.9 mm) in the case of the Great East Japan (Tohoku)
earthquake for the new superstructure.
4. There will be a pounding effect due to earthquakes in a longitudinal direction, so some restrainer
or damper should be used to decrease the movement. If an expansion joint of 25cm will be
provided with the new standard superstructure, the pounding force can be neglected. The
bearings also should be replaced with the bearings having large shear deformation capacity.
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. I would like to express my heartiest gratitude to my supervisor Dr. Tatsuya Azuhata(Director of IISEE), for providing me encouragement, invaluable supervision, immense guidance, andconsistent support since I came to BRI. I am indebted thankful to my advisor Dr. Haruhiko Suwada for
his valuable suggestions and encouragement during this study. Lastly but not least, I would like toexpress my acknowledgment to all the persons who helped me directly and indirectly in this study course.
REFERENCES
Indian Roads Congress, New Delhi, 2000, IRC:6, Standard Specifications and Code of Practice for Road
Bridges, Section: II, Loads and Stresses.
Indian Roads Congress, New Delhi, 2018, IRC: SP: 114, Guidelines for Seismic Design of Road Bridges.
Japan Concrete Institute, 2007, Seismic Rehabilitation of Concrete Structures", International Publication
Series, Editor: Shunsuke Sugano, 12-14, 65-70.
Japan Road Association, 2012, Specifications for Highway Bridges, Part V, Seismic Design.
Pillai S. U. and Menon D., 1998, Reinforced Concrete Design, Tata McGraw-Hill Publishing Company
Limited, New Delhi, 77-93.
Priestley M. J. N., Seible F. and Calvi G. M., 1996, Seismic Design and Retrofit of Bridges, A WileyInterscience Publication, John Wiley & Sons, Inc., 1-60, 160-170, 585-590.
STERA_WAVE Ver. 1.0, Artificial Ground Motion Tools, coded by Dr. Taiki Saito(http://www.rc.ace.tut.ac.jp/saito/index.html).
Teresa R. A., 2012, Analysis of T-Beam Bridge for Seismic Characterization, Anna University, India.
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