Citation: Tenzin, K., M. Miyamoto, T. Aoki (2022), Dynamic behavior of traditional composite masonrybuildings in Bhutan, Synopsis of IISEE-GRIPS Master’s Thesis.

DYNAMIC BEHAVIOR OF TRADITIONALCOMPOSITE MASONRY BUILDINGS IN BHUTAN
Kunzang Tenzin1
MEE21713

Supervisor: Mitsuhiro MIYAMOTO2, Takayoshi AOKI3

ABSTRACTThe traditional construction methods in Bhutan consist of rammed earth and stone masonry structures
where most of the population live. However, during the 2009 Eastern Bhutan earthquake and the 2011
Sikkim, India earthquake huge numbers of these traditional buildings suffered severe damages.
Therefore, it is important to evaluate the seismic performance of these buildings to prevent futuredisaster. Several studies have been carried out by numerous authors and particularly full-scale specimens
representing traditional Bhutanese buildings were studied through quasi static pushover tests.This study
aims to discuss the dynamic characteristics of composite masonry structures by shaking table tests on
1/6 reduced scale models under scaled earthquake motions recorded in Bhutan. A total of four specimens
comprising of two rammed earth (unreinforced &retrofitted) and two stone masonry (unreinforced&retrofitted) were tested to understand change in dynamic properties and damage propagation. Further
numerical analysis was carried by transforming the models to two-degree lumped mass system based on
microtremor measurements. The time history analysis was executed both in linear and nonlinear ranges
using the input motion from the experimental tests and dynamic behaviors were further studiedcomparing with the experimental results.
Keywords: Bhutan, composite masonry, dynamic behavior. shaking table, lumped mass.
1. INTRODUCTION
The Kingdom of Bhutan lies in the eastern Himalayan range, bordering India in the south and China in
the north. The traditional construction practices are still prevalent in the country and depending on the
climatic condition and material availability, construction techniques and methods vary from region toregion. The most common forms of construction are rammed earth in the western region and stone
masonry in central and eastern parts of the country. General architecture styles and designs remainsimilar in both areas, and the number of floors varies from two to three stories. The rammed earth
construction technique mainly involves compacting the earth in several layers inside the wooden
formwork and forming the blocks of walls whereas stone masonry construction involves constructing
walls using stones stacked together and bonded by mortar (mud). The materials such as stones, soils andtimbers required in both the construction are usually collected from nearby area. However, thesecomposite masonry buildings are vulnerable to the earthquake force due to its poor seismic resistantproperties and lack of proper design. The 2009 eastern Bhutan earthquake with magnitude of Mw 6.3
caused severe damages to the stone masonry houses and magnitude Mw 6.9, 2011 Sikkim, India
earthquake has devasted rammed earth houses in western region of country.
In the past years, several research have been undertaken to examine the effectiveness ofretrofitting measures on rammed earth structures, Yang Wang et al. (2016) by shaking table test.
1

Department of Culture, Ministry of Home and Cultural Affairs, Bhutan.
Associate Professor, Faculty of Engineering and Design, Kagawa Univ.
3
Professor, Graduate School of Design and Architecture, Nagoya City Univ.
2

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Mazzon et al. (2009 ) evaluated the performance of the 2:3 scaled stone masonry models and influenceof natural hydraulic lime injection by shaking table test. Further several researchers a has also
undertaken shaking table test as well as numerical analysis using various Finite Element method to study
the dynamic behaviors of stone masonry and rammed earth structures. Shrestha C et al. (2019) and
Wangmo et al. (2019). based on a push-over test on full-scale two-story stone masonry and rammed
earth house, proposed effective retrofitting measures to increase the seismic strength by mesh-wrapping
technique. In this research, the seismic performance of the traditional composite masonry houses,
rammed earth, and stone masonry are further evaluated based on the shaking table test on reduced scalemodel specimens with similar retrofitting techniques proposed by Shrestha K C et al. (2019) and
Wangmo P et al. (2019). This research aims to study the dynamic characteristics of composite masonry
structures while assessing the effectiveness of proposed retrofitting methods by conducting shaking table
test on 1/6 reduced scale models. Further, numerical analysis will also be carried out by developing twodegree lumped mass model system to simulate the shaking table test and study the changes in dynamic
parameters.
2. EXPERIMENTAL PROGRAM
2.1. Model specimen
Models are prepared to reproduce the prototype’s geometrical, physical, and dynamic characteristics.
Considering the limitation of the shaking table capacity and difficulties in moving the specimens on the
shaking table, the test specimens are geometrically reduced to a scale of 1:6. In total, two rammed earth,
Unreinforced (URE) and Retrofitted(RRE) and two stone masonry, Unreinforced(USM) andRetrofitted(RSM) model specimens are constructed which represents the common typologies found in
Bhutan. All the models have the same size with a floor plan of 1.35m x 0.9m and a total height of
1.137m. The wall thickness is 100mm. The specimens are constructed by local craft men using thematerials locally available and the one each specimen of stone masonry and rammed earth is retrofitted.
The retrofitting technique follows the wrapping of the walls by steel wire mesh from both interface andapplying 10mm thick cement plaster of 1:3 cement sand ratio. The material properties remain same as
prototype buildings. To reproduce the dynamic behavior of the prototype by the scaled model, the simple
model similarity (Tomazevic. (1992)) is considered such that acceleration is amplified and time iscompressed following the scale factor.
2.2. Test Setup and Instrumentation
Tests were performed on two specimens at the same time: one was unreinforced, and the other was
retrofitted, and its comparisons were made on the spot. The response of the structure is measured using
the accelerometer STP-300S and the sampling frequency was set at 200 Hz. A total of sixteen
accelerometers were used to measure the response of the two specimens, eight on the unreinforced andeight on the retrofitted. For each specimen, 1 sensor was mounted at the base, 4 at the second-floor leveland 3 at the roof level. Digital cameras were placed in strategic locations to record the visual shaking
behavior of the structure and analysis of damage propagation after the tests. Two types of dynamic
excitations were performed, sweep sine waves and real earthquake inputs. A sweep test was carried out
with a low intensity of 0.03g by gradually increasing the frequency from 1Hz to 25Hz to obtain thecharacteristics of a model in the elastic range. Following the sweep test, a series of earthquake motions
with increasing intensity were used. The earthquake ground motion recorded in Thimphu, Bhutan on
September 12, 2018, was used as an input motion. The original wave was scaled following the similitude
rule to suit the scaled model. The test was performed with increasing the intensity of a ground motion
at 0.2g in each succeeding step, where 8 sequential runs for the rammed earth and 7 sequential runs forthe stone masonry were employed.

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Table 1. Damping factor
Damping
URE
RRE
factorX
0.018 0.0119
Y
Figure 1. Rammed earth

0.02

0.0127USM
0.01550.0152

Figure 2. Stone masonry
3. METHODOLOGY

3.1. Experimental data processing and analysis
The shaking table test data for acceleration are obtained directly from accelerometers and the dataobtained by accelerometers are base-line corrected and filtered using the band pass filtering techniquewith cut-off frequencies of 1Hz to 50Hz. The response of the models is evaluated based on change
natural frequencies, acceleration factors and damage propagation at each input Peak GroundAcceleration (PGA). The damping of the specimen is also estimated by Random Decrement Techniqueusing microtremor measurement carried out prior to experiment. The result is shown in Table 1
The natural frequency of a model at each input PGA is estimated from a spectral ratio of
the top vs base by Fast Fourier transform method. The natural frequencies of specimens at each input
level are the average natural frequency of three accelerometers at the roof level. While in case of theunreinforced stone masonry specimen one sensor at the roof level was damaged at PGA 0.6g, therefore
only two sensors are considered.
Acceleration Factor is the ratio of the absolute maximum acceleration response of any
above story to the maximum input acceleration at the base. This factor describes how many times the
input acceleration is amplified at each floor level. The factors at the roof level are average of three
sensors while at second floor average of four sensors.
3.2. Numerical analysis
The numerical analysis was carried out considering the lumped mass
system. The two-story model is transformed into a lumped mass andshear spring model using the microtremor measurement results tosimulate responses of shaking table test. Lumped mass represents thestory mass, and the shear spring represents the story stiffness. The
Linear Time History Analysis (LTHA) and Nonlinear Time History
Analysis(NLTHA) was performed for each model using Software
SNAP ver. 8. Analysis was executed applying the acceleration time
histories from experimental tests. The method of analysis involves the
direct integration of dynamic equations of motions (Eq. 1).
𝑀𝑈̈(𝑡) + 𝐶𝑈̇(𝑡) + 𝐾𝑈(𝑡) = −𝑀1𝑢̈ 𝑔 (𝑡)Figure 3. Takeda model(1)

The non-linearity behavior of the stiffness is described based on the Takeda model and the yield strength(Qy) are determined from capacity curve of the static test on full-scale buildings. The constant damping
ratio of 2% is adopted for the analysis and the damping matrix [C] in the structure is calculated byRayleigh damping. The relation is shown in Equation 2.
C = αM + β

3(2)

4. RESULTS AND DISCUSSION
4.1. Natural Frequency
The variation of natural frequencies of rammed earth specimen (URE &RRE) at each input PGA isshown in Figure 4. The natural frequencies decreased with the increase in input ground motion due tothe degradation of stiffness caused by damages. The natural frequency of the unreinforced rammed earth
specimen before the test was 13.79Hz and for retrofitted was 16.2Hz. The frequency dropped to 3.17
Hz and 4.33Hz respectively for URE and RRE at the last test with the specimen showing varying rangeof damages on the walls.
The change of natural frequency for stone masonry specimen is shown in Figure 5. The
natural frequency of the unreinforced stone masonry before the test was 13.18 Hz. The natural frequency
dropped to 2.5Hz at nominal PGA of 1.4g with several diagonal shear cracks on the walls. The natural
frequency of the retrofitted stone masonry was 38.26 Hz The natural frequency dropped suddenly to 4.3
Hz at nominal PGA of 1.4g with large shear cracks on the side walls.

Figure5. Natural frequency ofstone masonry

Figure 4. Natural Frequency oframmed earth
4.2. Acceleration factors

The variation of the acceleration factors at different floor levels indicates the distribution of lateral
stiffness. Furthermore, acceleration factors also reflect the damage condition on the structures andusually when damages are increasing the stiffness is reducing leading to elasto plastic phase resulting inlower acceleration amplification factor at above floors.
The acceleration factors at the roof level varied more than the second-floor level, which
was caused by irregularity of walls in upper floors. While at second floor level the variation is lessindicating the proper distribution of stiffness by wall in all sides. The acceleration factor for rammed
earth specimen is shown in
Figure 6. The value shows
decrement after 0.6g at roof
level of URE is due to morecrack
occurrence
withincreased PGA. In the
retrofitted specimen theacceleration factors gradually
decreased. At second floor
level the changes in thefactors are not large in both
specimen which is due to
Figure 6. Rammed earth specimen (URE & RRE)
proper distribution of stiffness bywalls in lower floor.

4Figure 5 shows theacceleration factors of
the
stone
masonry
specimen.
The
acceleration factor at theroof level showed adecreasing trend in USM
whereas in RSM thefactor increases, andpeak response wasrecorded at nominal
Figure 7. Stone masonry specimen (USM & RSM)
PGA of 1.0g. At second
floor the factor increases gradually till 0.6g in USM and then drops with occurrence of cracks on thewall whereas in RSM the factors increase till the end of test indicating less damages and proper
distribution of stiffness by walls at lower floor.
4.3. Damage Propagation and crack patterns
The structural system under the study shall beconsidered as box system. The in-plane walldevelops shear resisting mechanism while outof plane walls develops bending or flexuremechanism. The in-plane wall provides thelateral resistance by their self-weight andresist the force transmitted by out of plane towall during the earthquake excitation. Due tohigher flexure moment at the end of in plane
Figure 8. Rammed earth (URE & RRE)
wall, corner cracks are induced to structure.
While the shear failure in plane wall withdiagonal cracks are due development ofprinciple tensile stress because for lateral andvertical load. Due to opening in the front
façade at second floor level the in-plane wallis reduce to half which accordingly the shear
strength is also reduced. Therefore, the cracks
started at the junction of 2fl and propagated tolower floor and lots of cracks at the corners.
Figure 9. Stone masonry (USM & RSM)
4.4. Numerical Analysis results
The experimental results are compared with numerical analysis results in terms of acceleration responses
at each floor level. The result is shown in Figure 10 and 11 for rammed earth and stone masonry
respectively. In case of the USM only linear analysis was executed as there is lack of capacity curveof full-scale test.
At lower input PGA the acceleration responses in all the specimens are in close range. In
addition, during the experimental test there was no significant damage detected on the specimen
indicating the structure remains in elastic state. For instance, in URE roof level there is small damages
on the wall at 0.6 and 0.8g and the results of all three remains in closed range. The difference between
LTHA and NLTHA increases at higher input PGA wherein the specimens have more damages showing
the nonlinear behavior. Further, the results of nonlinear analysis agrees with the damage propagation as
well as follows similar trend with experimental results.

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Figure 10. Rammed earth (URE & RRE).

Figure 11. Stone masonry (USM & RSM)

5. CONCLUSIONSIn this study the seismic performance of the composite masonry buildings in Bhutan is evaluated byshaking table test and numerical analysis. The shaking table test was executed on four 1/6 reduced scalemodels which comprises of two rammed earth models (unreinforced and retrofitted) and two stone
masonry models (unreinforced and retrofitted). The numerical analysis was carried by transforming themodel specimen into the lumped mass models of the two-degree freedom system. The linear andnonlinear time history analysis was executed using the input motion from the experimental test.
The experimental results show that natural frequency of the specimens decreases with anincrease in nominal PGA, reflecting the degradation of stiffness due to damage. Comparing the natural
frequencies, the retrofitted models showed a higher value than unreinforced models. The proposed
retrofitting measures improved the seismic performance of the structures by preventing the occurrenceof cracks and limiting the damage propagation.
The comparative analysis was made between the numerical analysis and experimental test
results in terms of the maximum acceleration response at each floor level at each input ground motion.
From the results we conclude the numerical simulation results are in close range to the experimental test.
Further, the good correlation between the damage occurrence and results of time history analysis isobserved in all the models.
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 sincere gratitude to my supervisors Dr. Mitsuhiro Miyamoto and
Dr. Takayoshi Aoki for continuous guidance and support. I am grateful to Dr. Tatsuya Azuhata for his
support and advice throughout my training program. I would also like to acknowledge Dr. Toshihide
Kashima for his valuable comments and feedbacks.
REFERENCES
Mazzon, N, Valluzzi, M R, Aoki, T, Garbin, E, De Canio, G, Ranieri, N and Modena, C (2009). “Shaking
table tests on two multi-leaf stone masonry buildings,” in Proc. of the 11th Canadian Masonry
Symposium, Toronto (Canada), May 31st - June 3rd, 2009, Mc Master University.
Shrestha K C et al. (2019). Study on earthquake resistance technology of composite masonry buildingsin Bhutan Part 14: Full scale tests on composite masonry buildings: Mesh-wrapped stone masonrywith mud mortar construction, Summaries of Technical Papers of Annual Meeting, AIJ, StructuresIV, 863-864.
Wangmo P et al. (2019). Study on earthquake resistance technology of composite masonry buildings in
Bhutan Part 13: Full scale tests on composite masonry buildings: Unreinforced and mesh-wrapped
rammed earth construction, Summaries of Technical Papers of Annual Meeting, AIJ, Structures-IV, 861862.
Wang Y, Wang M, Liu K, Pan W, Yang X (2016): Shaking table tests on seismic retrofitting of rammed–
earth structures. Bulletin of Earthquake Engineering, 15, 1037–1055.

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