Citation: Dendup, T., T. Saito (2023), Comparative study on the seismic performances of typical RC
residential buildings designed with old and new Indian codes in Bhutan, Synopsis of IISEE-GRIPS
Master's Thesis

COMPARATIVE STUDY ON THE SEISMICPERFORMANCES OF TYPICAL RC RESIDENTIAL
BUILDINGS DESIGNED WITH OLD AND NEW
INDIAN CODES IN BHUTAN
Tshering DENDUP1
MEE22709

Supervisor: Taiki SAITO2ABSTRACTIn Bhutan, the Indian seismic codes are referred for the design of reinforced concrete (RC) buildings.
These seismic codes were revised in 2016 as a result of earthquakes that took place after their publication.
The frequent and recent earthquakes in and around the Himalayan region have caused substantial
damage to RC buildings necessitating for study of seismic performance. This paper presents acomparative study on the seismic performances of typical five-story RC residential buildings designedwith old and new Indian codes. Accordingly, two three-dimensional models of a building designed withthe old and new Indian codes are developed using STERA_3D software. The seismic performances areevaluated using the Capacity Spectrum Method (CSM) and non-linear Response History Analysis(RHA) for three input ground motions. In addition, the structural damage estimates given by damage
indices are compared under scaled ground motions. Results show that the building designed with thenew Indian codes provide reduced structural responses when compared to that designed with old Indiancodes. Furthermore, the assessment of the damage indices for the building designed using the new codesshows that the building has more even damage dispersion over the floors and prevents collapse-levelstructural damage under the considered maximum scaled ground motion.
Keywords: RHA, CSM, Input Ground Motions, Damage Indices.
1. INTRODUCTION
Considering the frequent seismicity in and around the Himalayan region, in their paleoseismic research,
Le Roux‐Mallouf et al., (2016) reported the occurrence of an earthquake event with a magnitude 8±
0.5. In another study, by Roux-Mallouf et al., (2020) stated that at least five of such earthquakes struck
the country over the last 2600 years. In 2015, Nepal suffered from a catastrophic Mw7.8 magnitude
earthquake. Bhutan was spared from this disaster despite its proximity to Nepal and its comparablegeography. However, by the 2009 Mongar Earthquake (Mw6.1), which struck 180 kilometers east ofThimphu, destroyed several houses and took many lives. Similarly, the 2011 Sikkim earthquake(Mw6.9), with an epicenter near the borders of India and Nepal, also caused substantial structural
damage to the country. The study conducted by Bhagat et al., (2015), which examined the seismic

1
2

Department of Infrastructure Development, Ministry of. Infrastructure and Transport, Bhutan.
Professor, Toyohashi University of Technology, Japan.

1performance of RC building designed with Indian codes in Nepal offers valuable insight and importanceto evaluate seismic performance of buildings in Bhutan as well.
In Bhutan, two important seismic codes are generally referred for design of RC framedbuildings; Indian Standard IS 1893:2002 and IS 13920:1993. In 2016, both the codes were revised as aresult of earthquakes that took place after their initial publication. Therefore, this study primarily aims
to investigate and compare the seismic performances of a typical RC residential buildings designed
using the old Indian codes and new Indian codes. For this purpose, two models were developed using
different member sections from the design output of the former and recent Indian codes. In addition,
the damage estimate on the target models is evaluated using the Damage Indices under scaled realground motions.
2. TARGET BUILDINGS AND MODELS
Figure 1 illustrates a typical existing five-story residential RC framed building that exists in Bhutan.
The model designed using old Indian codes is hereafter referred to as “OLD Model” while that designed
using new codes is referred to as “NEW Model”. Both the models were designed as fixed-base special
moment-resisting RC famed buildings without brick infill for 500 MPa yield strength and 25 MPa
concrete compressive strength. Seismic zone V and medium soil site condition were considered for bothmodels. To evaluate the Seismic Performance of the target models, Software STERA_3D developed byanother author is used. The first mode of vibration for both the models is observed in the Y-direction.
The fundamental natural period, T1 of OLD and NEW Models are 0.897 s and 0.785 s, respectively.

(a)

(b)

Figure 1. Target Building: (a) Floor Plan; (b) Sectional Elevation.
3. SEISMIC PERFORMANCES OF THE TARGET BUILDINGS
3.1. Input Ground Motions
For Response History Analysis, three Ground Motions
are considered. Two input ground motions of duration
120 s are simulated using algorithm developed byanother author. The first simulated ground motion isscaled to match the target design spectrum (i.e.,
acceleration response spectrum, h =5% of IS 1893:2002),
with random phases and max. acceleration of 3.47 m/s2
is obtained. The second artificial ground motion issimulated by scaling to the target design spectrum withthe retained phases of the Kobe 1995 ground motionrecorded at Totoro Station, Japan and max. accelerationof 3.52 m/s2 is obtained. The third input ground motion
considered is real Nepal 2015 earthquake record withduration of 300 s and max. acceleration of 1.6 m/s2,2Figure 2. Input Ground Motions: (a)Simulated-1 (Random Phase); (b)Simulated-2 (Kobe_TOT90 Phase);(c) KATNP_360.

KATNP_360 measured at Kathmandu station. Figure 2 shows the time history of input ground motions.
3.2. RHA Results for Input Ground Motions
Prior to seismic performance evaluation, the comparison between the ultimate strengths (base shear) of
the target models is first evaluated from the pushover analysis. Pushover analysis results showed adifference of 37% in strength. The NEW model has a strength of 3405 kN whereas the OLD model has
strength of 2487 kN which is significantly larger when compared to design base shear.
The direct numerical integration of the differential equations of the motion of target models
using the input ground motions with the aid of software like STERA_3D, RHA is performed. The
maximum responses of the RHA for input ground motions obtained for both the models are as presentedin Table 1. The responses over the height of the structures are presented in Figure 8.
Table 1. Maximum Response Results for Input Ground Motions in the Y-direction.
Story Drift Angle, Ri
Maximum Response
OLD Model

NEW Model

Simulated-1

Second Floor

1/74

1/90

Simulated-2

Second Floor

1/81

1/86

KATNP_360

Second Floor

1/80

1/143

Maximum response of Story Drift Angle, 1/74 is obtained under Simulated-1 groundmotion for OLD model whereas for NEW model 1/86 is obtained under Simulated-2 ground motion. Adistinct difference in the responses of the OLD and NEW models is also observed under real groundmotion. Based on the response results obtained from the RHA under the selected input ground motions,
the NEW model yields a better performance than the OLD model.
3.3. CSM Procedure and Results for the Design Code Demand Spectrum
The CSM, a nonlinear static analysis developed by Freeman, (1998) is performed for design codedemand spectrum to obtain performance points which represent the actual response of the structure
under seismic loading.

At first, the Spectral Displacement, Sd is calculated using Eq. (1) where Sa, is spectral
acceleration values of Design Acceleration Spectrum for 5% damping and medium soil site (IS 1893).
ω is the natural frequency. From pushover analysis, the Capacity Spectrum is obtained using nonlinear
distribution of deformation along the height of the structure (Kuramoto et al., 2000). The yielddisplacement, δy is then decided for inter story drift angle of 1/200. Then the reduction of structural
response due to increasing level of damage is applied to Demand Spectrum by multiplying both valuesof Sa and Sd by a reduction factor, Fh. The reduction factor for each equivalent damping ratio, hecorresponding to ductility factor, µ =2 and µ =3 is calculated using Eq. (3). For the equivalent damping3

ratio, he Eq. (4) and for ductility factor, µ Eq. (2) is used. Finally, the performance points obtained by
CSM are presented in Figure 3 and corresponding responses are plotted in Figure 4.

(a)

(b)

Figure 3. Performance Points for Design Code Demand Spectrum in Y-direction:(a) OLD Model; (b) NEW Model.
The response results by CSM for Design Code Demand Spectrum shows that the second
floor of OLD model will sustain more damage with story drift angle 1/78 when compared to the NEW
model with less story drift angle of 1/98.
3.4. CSM Results for the KATNP_360 Demand Spectrum and Comparison with the RHA Results
Similarly, CSM is carried out for KATNP_ 360 record. The CSM response results indicated that theNEW model with maximum story drift angle of 1/128 will sustain less damage when compared to theOLD model with the higher story drift angle of 1/86.
The results are then compared with RHA results, and it shows good agreement in bothvalues and pattern as presented in Figure 5. The CSM requires less computational effort and time,making it convenient and applicable when a large number of buildings has to be studied.

Figure 5. Comparison of CSM andRHA response results(KATNP_360)

Figure 4. Response of Target
Models by CSM for Design Code
Demand Spectrum.

44. DAMAGE INDEXThe basic model developed by Park et al., (1984) torepresent the structural damage on the RC elements
such as beams and columns is given by Eq. (5) and it
is expressed simply by Figure 6.
Di=μδm
EhEh+β= m+βδu
Qy δu μu
Qy δu(5)Figure 6. Force Deformation
Relationship of an RC Element.

where, δm = μm δy , is the maximum deformation under an earthquake; δu = μu δy , is theultimate deformation under a monotonic loading; μm , is the maximum ductility factor under anearthquake; μu , is the ultimate ductility factor under a monotonic loading (STERA_3D adopts μu =15);
δy , is the yield deformation; Qy , is the yield strength; β, is the parameter related to the cumulativeloading effect given by Eq. (6) and; Eh = ∫ dE, is the dissipated hysteretic energy.
𝑙𝛽 = (-0.447 + 0.073 + 0.24n0 + 0.314 pt ) × 0.7𝜌𝑤
dl(6)

l

where, , is the shear span ratio (replaced by 1.7 if<1.7); n0 , is the normalized axial
ddstress, axial force by compressive strength (replaced by 0.2 if n0 <0.2); pt , is the longitudinal steel
ratio as a percentage (replaced by 0.75% if pt <0.75%); ρw , is the confinement ratio.
Further the RC damage index is modified to the story-level damage index given by Eq. (7),and it is calibrated into the categories of physical structural damage. The story-level damage index, Dstory< 0.4 is considered to be Reparable, whereas Dstory > 0.4 represents Damage Beyond Repair, and Dstory> 1.0 represents Total Collapse.

where, Dstory, is the story-level damage index of structure; n, is the number of elements inthe story or whole structure; Di, is the damage index of the i-th element; wi, is the weighting factor of
the i-th element given in Eq. (8).
Under different intensities of KATNP_360 ground motion, the weighted maximum storylevel damage indices of the target models are evaluated as shown in Figure 9. The results indicates that
under the 2.2KATNP ground motion, the level between base to first floor of the NEW model will sustain
damage beyond repair with a maximum story damage index of 0.497 whereas OLD model will collapse
with maximum story damage index of 1.053 which is not desirable.
5. SEISMIC PERFORMANCE OF THE RETROFITTED OLD BUILDING

Figure 7. Layout of Shear Walls.

Throughout the height of the building, designed with old Indiancodes, three shear walls are proposed, two in weaker direction
and one in stronger direction as illustrated in Figure 7. The walls
are designed for 500 MPa yield strength and 25 MPa concrete
compressive strength and the reduction of strength for openings
is applied from Seismic Retrofit of Existing Reinforced Concrete
Buildings, 2001 (JBDPA). For the Retrofitted model thefundamental natural period is 0.596 s and the first mode ofvibration is in X-direction.

5Figure 9. Damage Index Results

Figure 8. RHA Results in Y-direction.

From Figure 8, the results show that under the considered input ground motions theRetrofitted Model will demonstrate improved performance by limiting responses within 1/200 story drift.
In addition, the maximum story-level damage indices for all the models plotted in Figure 9 indicate that
the Retrofitted model will exhibit only repairable damage over all floors even under maximum
considered intensity.
6. CONCLUSIONS
The following conclusions are drawn based on the evaluation results. Although the base shear capacitiesof the target models are significantly larger than the design values, the earthquake-induced shear force
from the input ground motions was large enough to cause damage to both target models. The responses
showed that the structure designed with new Indian codes perform better. The results from CSM for
Design Code Demand Spectrum also indicated that the structure designed using the new codes provided
improved performance compared to that designed with old codes. The CSM provided reliable results
when compared to RHA results. Under maximum intensity of real-ground motion, the damage indicesindicate collapse-level damage for the building designed with old Indian codes, which is not desirable.
However, adding shear walls as retrofitting measure not only prevents collapse level damage but also
reduces the response of the structure by large.
ACKNOWLEDGEMENTSFirst and foremost, my deepest appreciation goes to Dr. Taiki Saito for his supervision and invaluableguidance during my individual study. My gratitude goes to Dr. Toshihide Kashima, Dr. Yuri Otsuka andto all the staff members at IISEE/BRI for their support and guidance throughout the training program. I
am indebted to the Japan International Cooperation Agency (JICA) and coordinators for funding mymaster’s course and providing unwavering support. Please accept my sincere gratitude to all individuals
and institutions mentioned for their invaluable contributions to my academic and personal growth.
REFERENCES
Bhagat, S., Kono, S., Wijeyewickrema, A. C., Kusunoki, K., Tajiri, S., & Kashiwa, H. (2015). New
Technologies for Urban Safety of Mega Cities in Asia, Nepal.
Freeman, S. A. (1998). Proceedings of the 11th European conference on earthquake engineering,Kuramoto, H., Teshigawara, M., Okuzono, T., Koshika, N., Takayama, M., & Hori, T. (2000).
Proceedings of the 12th World Conference on Earthquake Engineering, Auckland, New Zeland,
Le Roux‐Mallouf, R., Ferry, M., Ritz, J. F., Berthet, T., Cattin, R., & Drukpa, D. (2016). Journal ofGeophysical Research: Solid Earth, 121(10), 7271-7283.
Park, Y.-J., Ang, A.-S., & Wen, Y.-K. (1984). Civil Engineering Studies SRS-516.
Roux-Mallouf, L., Ferry, M., Cattin, R., Ritz, J.-F., Drukpa, D., & Pelgay, P. (2020). Solid Earth, 11(6),2359-2375.

6