Citation: Pema, T. Aoki (2020), Seismic evaluation and retrofitting of traditional Bhutanese stone
masonry residential house, Synopsis of IISEE-GRIPS master report.

SEISMIC EVALUATION AND RETROFITTING OF
TRADITIONAL BHUTANESE STONE MASONRYRESIDENTIAL HOUSE
PEMA1,2Supervisor: Takayoshi AOKI3
ABSTRACT

Traditional houses form an integral part of the unique landscape of Bhutan. Unfortunately, thesestructures are highly vulnerable to seismic activities, but there are only limited research works in thisfield. Thus this paper deals explicitly on the development of seismic technology for strengthening suchrandom rubble masonry structures in mud and cement mortars. This study’s basis is the full-scale quasistatic test conducted on two-storied residential houses designed based on a representative sampling oftraditional houses for unreinforced and retrofitted types. The proposed seismic intervention is the meshwrap retrofitting technique. Furthermore, we developed a numerical model based on the Finite Element
Method and conducted non-linear pushover analyses, which was capable of simulating the experiment.
Finally, this study presents the first-of-its-kind conventional fragility curves for traditional Bhutanese
residential houses, generated based on the Capacity Spectrum Method.
Keywords: Stone masonry, Retrofitting, Experimental works, Finite Element Method, Fragility curves.
1. INTRODUCTION
The masonry is the earliest and oldest form of construction material in the history of humanity. Today,stone masonry is still one of Bhutan’s indigenous construction practices, which constitutes of historicalmonuments to a simple traditional house. Over the past decade, Bhutan has witnessed seismic events (ofa magnitude not exceeding M7). Such seismic events have manifested into the damage of traditional
houses (load-bearing structures), thus raising critical concerns about its structural integrity and seismic
capacity. On the other hand, there is a rich repository of traditional knowledge systems in Bhutaneseindigenous construction materials, techniques and methods. Therefore, it is important to evaluate its
seismic performance through an engineered lens considering its relevance to the social and economic
aspects of the rural community residing in such seismic prone areas. This study focuses on clarifying
the seismic performance of such traditional stone masonry structures and proposes seismic retrofitting
measures, i.e., mesh-wrap retrofitting technique. It confirms the efficacy of the proposed method, bothexperimentally and numerically.
2. EXPERIMENTAL PROGRAM
2.1. Test specimen
The test specimen was designed based on the representative sampling of all architectural forms of
Bhutanese house. The technique for weaving the stone and construction of walls follows the current
trend of construction practice. The specimen has a floor area of 8.1m x 5.4m and a floor height of 3.15m
and 3.75m for the first and second floors, respectively (Figure 1). The thickness of the wall was 600mm
throughout the wall height. The characteristic feature of floor plans found in Bhutanese house resulted
in differential wall distribution in the consequent floors due to the large opening of the front façade on
1

Department of Culture, Ministry of Home and Cultural Affairs, Bhutan.
IISEE-GRIPS master course student.
3
Professor, Nagoya City University, Japan.
2

1the second floor. We constructed two structures following this design only to be different by binding
mortar, i.e., in mud mortar (USMM) and cement mortar (USMC).

Figure 1. Plan of the target specimen.
2.2. Retrofitting measure
For the community to accept the proposed technique and achieve widespread utilization of the proposed
retrofitting methods, it is essential to ensure that the materials are readily available. The processes need
to be practical, pragmatic, and user-friendly. It is also crucial to consider the economic viability, and it
does not obstruct the community’s social perception. Considering all the reasons mentioned above, we
decided to design, check on the feasibility and structural performance of the proposed mesh-wrap
techniques at a smaller scale, and then gradually adopted the same to full-scale specimens. Two typesof mesh with different sizes of 12 gauge/ 1.83mm with 34x34mm2 openings (primary mesh) and 16
gauge/ 1.45mm with 28x28mm2 openings (secondary mesh) were selected. The primary mesh was fixed
to the stone masonry substrate, both externally and internally. For the limitation of the sizes, the primarymesh overlapping was unavoidable for both vertical and horizontal directions. It required further
intervention for continuity and eradication of the creation of a weak joint, thereby secondary meshes
were necessary. The requirement of minimum overlap is 300mm. This series of work encapsulates the
background idea for the retrofitting technique: Fixing of primary mesh, overlapping at corners and joints,
application of two coats of cement plaster- 30mm thick finished surface and provide timber bracings at
floor diaphragm (Figure 2). The retrofitted counterpart of the USMM and USMC specimens are
hereafter referred to as RSMM and RSMC.

Figure 2. Design of mesh-wrap technique applied to the full-scale specimen.
2.3. Test-set up and capacity curves
The quasi-static test setup utilizes two 1000kN and two 500kN capacity static jacks, positioned at firstand second floors. There are two types of displacement transducers: ten laser transducers and four linear
variable displacement transducers (LVDTs) at three levels of the test specimen placed at the loading andthe free side. We used a multi-channel dynamic strain-meter DS-50A for data logging with the LANinterface setting, and the data sampling rate was 100 Hz. The monotonic static lateral loads applied tothe specimen was the displacement-controlled loading with manual synchronization of the hydraulic2

pumps (Figures 3-a and 3-b). The load transfer mechanism involves using a built-up H-section from thestatic jack to the specimen, to achieve uniform load application throughout the point of application.

(b)

(a)

(c)

Figure 3. (a) Schematic full-scale quasi-static test setup, (b) Real scenario experimentation setup and (c)FE-numerical model with loading constraints.
The specimen was subjected to one-half cycles of displacement-controlled loading(monotonic static lateral force) for each specified drift target value. The USMC specimen was tested fora drift of 1/1000 (0.01%), 1/750 (0.13%), 1/400 (0.25%) up to 1/300 (0.33%) and the RSMC specimen
was further subjected to a drift of 1/750 (0.13%), 1/500 (0.2%), 1/250 (0.4%), 1/150 (0.67%), 1/100(1%), 1/75 (1.33%) and 1/50 (2%). The RSMM specimen was subjected to a drift of 1/2000 (0.05%),
1/1000 (0.1%), 1/750 (0.13%), 1/500 (0.2%), 1/250 (0.4%), 1/150 (0.67%), 1/100 (1%), 1/90 (1.11%)
and 1/80 (1.25%). Here, the storey-drift is the ratio of the lateral displacement to the floor height. The
peak base shear recorded for USMC was 765 kN, and that for RSMC and RSMM was 1200 kN and 783
kN, respectively (Figure 4-a). An increment in the base shear capacity of 56.6% was achieved with the
adopted mesh-wrap retrofitting in the stone masonry house in cement mortar. Here, the base shear is thesummation of the storey shears at first and roof floors.
3. FINITE ELEMENT ANALYSISThe numerical simulation is made in a finite element program DIANA 10.4. DIANA offers many
constitutive material models depending on the nature of the analysis, and here we use the total strain
rotating crack model. The basis for selecting this constitutive model is a cautious compromise between
the accuracy of the results and computation time. After selecting the non-linear material model, we needto define the stress-strain relationships for different types of behavior clearly. The adopted hysteretic
behavior takes into account the exponential tension-softening for the tensile behavior and parabolic
hardening with subsequent softening for the compressive behavior, which is considered only for themasonry (Angelillo et al., 2014). In this model, the shear behavior is adequately captured and updated
as the damage progression happens. The FE simulation follows the macro-modeling approach for the
full-scale house.
The numerical FE-model has 9070 numbers of finite elements. The structural walls of thisFE model consist of 3492 numbers of an eight-node isoparametric solid brick element (HX24L), whichconstitutes of non-linear property. The rest of the material has only linear material properties considered.
The adopted mesh size for all the FE elements is 300mm. In this pushover analysis, the target structure’s
non-linear behavior was evaluated by applying lateral loads in user-controlled successive load steps. A
multi-point tying option available in DIANA, i.e., connecting all the slave nodes (yellow dot circles) tothe master mode (red dot circle) as shown in Figure 3-c, was used to simulate the actual loading conditionof the experiment. The iterative process for solving the system of equations follows the Newton-Raphson
method and convergence criteria set in the form of energy, displacement, and forces. A line searchalgorithm was also adopted for the improvement of the convergence. Table 1 summarizes the material
properties adopted for FE numerical simulation for the full-scale tests. The mesh-wrap components’properties follow the material characterization tests of the uniaxial compression test of the cement mortar
and tension coupon test of mesh composite. The FE-numerical model was in good agreement with theexperimental campaign. The capacity curve of the FE-model for RSMC response simulated well in the3

linear elastic region where the capacity curves are parallel to each other, which then diverged in the nonlinear region.
The peak load of FE numerical analysis is 845.70kN. The recorded peak load from theexperiment is 765kN, but this is not the ultimate load since we stopped the investigation at a drift of
1/300 (0.33%) for retrofitting purposes (RSMC). At 0.2% storey drift, the corresponding base shear is716.25kN and 736.30kN for the experiment and FE model, respectively (Figure 4-b). The capacity
curve with a 50% reduction of the tensile property of mesh composite, indicated by the black dottedcurve “FE-RSMC-50%” and “FE-RSMM-50%, is the best-fit capacity curve (Figures 4-c and 4-d). Thebase shear for USMM from the FE analysis is 438.38kN.
Table 1. Adopted material and physical properties for full-scale specimen numerical FE simulations.
Material Properties
Mass density, ρ (kg/m3)
Elastic Modulus, E (MPa)Poisson’s ratio, υCompressive strength, fc (MPa)Compressive fracture energy, Gfc (N/mm)
Tensile strength, ft (MPa)Tensile fracture energy, GfI (N/mm)

Specimen with mud mortar
2300
150
0.20
0.60
0.96
0.02
0.0016

Specimen with cement mortar
2300
350
0.20
3.00
4.80
0.15
0.012

Figure 4. (a) Experimental capacity curves; Comparison of FE generated capacity curve toexperimentally obtained capacity curve for (b) USMC (c) RSMC and (d) RSMM.
4. VULNERABILITY ASSESSMENT
The concept of a vulnerability assessment is related to Probabilistic Safety Assessment (PSA) andfragility assessment. A fragility curve is a statistical tool that utilizes the results obtained from anappropriate structural response assessment. It is capable of presenting complex engineering results ingraphical formats for general-purpose. Here, the seismic performance assessment utilizes the Capacity
Spectrum Method (CSM). The CSM is a simplified non-linear static analysis procedure, and nowadays,
extensively used to explain the structural performance of a structure. Freeman et al. (1975) first proposed
the CSM method, and it was subsequently included in ATC-40. It has also become a subject of study in4the last two decades. In Japan, the seismic design requirement in the Building Standard Law of Japan(BSLJ) was made inclusive of a performance-based design framework in June 2000 (Otani et al., 2000).
In this study, the CSM analysis follows the procedure specified by the BSLJ. For the CSM method, it isnecessary to convert the capacity curve, generally expressed in terms of base shear and roof displacement,
to a capacity spectrum. Capacity spectrum is the representation of the capacity curve in the AccelerationDisplacement Response Spectra (ADRS) format (i.e., Spectral acceleration SA versus Spectral
Displacement SD), after Mahaney et al. (1993). The pushover analysis (using Finite Element solutions)with external force distribution proportional to the first mode, provides the SA - SD curve (Capacity
spectrum) typically. Here, the author has an advantage with the availability of experimental pushover
curves based on a full-scale quasi-static test of building prototype.
Every country’s seismic code presents a traditional response spectra SA versus T (Timeperiod) for building design purposes. This response spectrum also requires the conversion in the ADRS
format. In addition to the conventional response spectra obtained from building codes and standards, wehave selected appropriate ground motion records for the structural response assessment. We chose
cautiously seven (7) earthquake ground motions (Figure 5-right), exclusive of response spectra of IS
1893, and NBC-105. All input demand spectrums were scaled appropriately to different levels as per thecapacity of the structures. The intersection of the capacity curve and the demand spectrum for an
appropriate equivalent damping ratio is called Performance point (Figure 5-left). We get a single
performance point for one PGA level of the particular EQ spectrum, thus obtaining multiple performancepoints for USMC, RSMC and RSMM structures, for a suite of EQ earthquake response spectra, whichforms the basis for the generation of fragility curves.

Figure 5. Performance point for PGA of 0.4g of IS 1893-2016 for RSMC house using the CSM method(Left) and Acceleration response spectra for the selected earthquake records at 5% damping (Right).
The fragility curve expresses the probability that the target structure will exceed a specificdamage state as a function of earthquake intensity parameters (such as PGA, PGV, SA, etc.). In thisstudy, the author uses the methodology proposed by Wen et al. (2004) for the generation of fragility
curves. The probability that the given structure exceeds the limit-damage state for given ground motion
intensity is given by Eq. (1).
𝜆𝑖 −𝜆𝐷⁄𝐺𝑀𝐼)
𝛽𝐷⁄𝐺𝑀𝐼(𝐿𝑆𝑖 ⁄𝐺𝑀𝐼 ) = 1 − 𝜙 ( 𝐶𝐿(1)where 𝑃(𝐿𝑆𝑖 ⁄𝐺𝑀𝐼 ) is the probability of exceeding a particular limit state given ground motion intensity(GMI), 𝜙 (•) is the standard normal cumulative distribution function, 𝜆𝑖𝐶𝐿 is ln (median storey drift fora particular limit state, i ), 𝜆𝐷⁄𝐺 𝑀𝐼 is ln (calculated median demand storey drift given the GMI from thebest fit power-law line) and 𝛽𝐷⁄𝐺 𝑀𝐼 is the demand uncertainty. The two parameters 𝜆𝐷⁄𝐺 𝑀𝐼 and 𝛽𝐷⁄𝐺 𝑀𝐼
are given by Eqs. (2) and (3):𝜆𝐷/𝐺𝑀𝐼 = 𝐼𝑛𝑎1 + 𝑎2 ln(𝐺𝑀𝐼)
5

(2)

∑𝑛

2

[𝐿𝑁(𝐺𝑀𝐼𝑘 )−𝜆𝐷⁄𝐺𝑀𝐼 (𝐺𝑀𝐼𝑘 )]𝛽𝐷⁄𝐺 𝑀𝐼 = √ 𝑘=1

𝑛−2

(3)

The constants 𝑎1 and 𝑎2 are obtained through linear regression analysis, by plotting natural
logarithmic values corresponding to storey drift and PGA (which is the GMI). Following the statisticalanalysis, three fragility curves are generated (Figure 6) for USMC, RSMC and RSMM, a plot showing
the probability of exceeding the damage limit-state threshold values as a function of ground motion
intensity (PGA). Three categories of damage limit states, i.e., Immediate Occupancy (IO), Life Safety(LS), and Collapse Prevention (CP), is defined based on the observation of the full-scale quasi-static test.

Figure 6. Fragility curves for traditional Bhutanese residential houses.
5. CONCLUSION
The need for more experimental studies to clarify traditional structures and materials’ behavior is ofparamount importance. This study is one such effort to preserve and promote traditional structures. The
full-scale quasi-static test shows that the proposed mesh-wrap retrofitting technique effectively increases
the seismic capacity of such structures, i.e., 1.58 times the base shear of conventional (unreinforced)
structures. The study shows that there is a need to increase the seismic capacity of stone masonry in mud
mortar, and more scientific studies are required. Finally, the first of its kind conventional fragility curves
are developed based on experimental capacity curves for traditional Bhutanese houses.
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 want to express my sincere gratitude to my supervisor Prof. Takayoshi Aoki and cosupervisor Asst. Prof. Kshitij Charana Shrestha of Nagoya City University, for their valuable advice andthoughtful guidance. I will always remain grateful to Dr. Tatsuya Azuhata, Director and Dr. Mai Ito of
the International Institute of Seismology and Earthquake Engineering (IISEE), who are my advisors. I
would also like to express my sincere gratitude to the National Graduate Institute for Policy Studies(GRIPS) and Japan International Cooperation Agency (JICA) for providing me with this opportunity toundertake the integrated master’s program. I want to thank the Royal Government of Bhutan and, inparticular, the Department of Culture under the Ministry of Home and Cultural Affairs for allowing me
to pursue further studies.
REFERENCES
Angelillo, M., Lourenço, P. B., and Milani, G. (2014). In CISM International Centre for Mechanical Sciences,
Courses and Lectures.
Freeman, S. A., Nicoletti, J. P., and Tyrrell, J. B. (1975), Proceedings of US National Conference on
Earthquake Engineering, 113–122.
Mahaney, J. A., Paret, T. F., Kehoe, B. E., and Freeman, S. A. (1993). National Earthquake Conference.
Otani, S., Hiraishi, H., Midorikawa, M., and Tashigawara, M. (2000). ACI Special Publication, 197, 87–104.
Wen, Y. K., Ellingwood, B. R., and Bracci, J. (2004). Mid-America Earthquake Center Project, 1–101.
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