Citation: Ramos, W., S. Sugano (2020), Evaluation and retrofitting of a historic adobe masonry building,
Synopsis of IISEE-GRIPS master report.

EVALUATION AND RETROFITTING OF A HISTORICADOBE MASONRY BUILDING
William Alexander Ramos Hernandez1,2Supervisor: Shunsuke SUGANO3ABSTRACT
Most historic buildings in El Salvador are made out of adobe, a type of earth masonry that is nonengineered and extremely weak against earthquakes. This study was conducted to find the best solution
for the retrofitting of these buildings. A 150-years-old adobe building was evaluated considering three
retrofitting solutions: Polypropylene band wrapping, Backside wall affixing, and RC frame affixing.
The analysis was conducted using three methods (JBDPA Guidelines, capacity spectrum method, andresponse history analysis), and two levels of seismic demand. The Polypropylene band wrapping
solution was not enough for a building this weak and a site with such seismicity. The RC frame affixing
solution worked, but it requires partial demolition works and causes extensive cracking on the walls.
Finally, the Backside wall affixing technique presented a failure mode that protects the walls fromdamage while avoiding an aggressive alteration of the building in the low seismic demand level.
Keywords: Adobe, Retrofitting, Historic building, PP-band, Backside Wall.
1. INTRODUCTION
Adobe buildings have an innately low seismic capacity, which makes them unattractive as legitimatedwellings. However, even though public opinions on adobe buildings are generally negative, thosebuildings can’t be demolished because El Salvador’s Cultural Heritage law protects them. Therefore,
the only option left is to retrofit them. Even though retrofitting is necessary to bring a building to an
acceptable seismic performance level, the works allowed by the Ministry of Culture are severely limited
because most of the techniques preferred by contractors are aggressive and affect the cultural value ofthe buildings. There is a real need to find the most appropriate retrofitting techniques that can increase
the seismic performance of existing adobe buildings without compromising their historical value.
Therefore, the objectives of this study are: 1) To apply the results of Salvadorian adobe masonry research,building codes, and Japanese evaluation, analysis, and retrofitting practices into the study of a real
historic building; 2) To determine the most appropriate retrofitting technique for a historic adobe
masonry building, to increase its seismic performance, without compromising its historic value.
2. DATA FROM THE TARGET BUILDING
The target building is a 150 years old adobe structure located in the city of Izalco, in western El Salvador.
It has only one story, with a size of 31 m x 30 m, and it is C shaped (Fig. 1). All of the walls have astructural thickness of 75 cm (without plaster), a height of 3.80 m, and lack any reinforcement. The roof
has a two-way shape and contains clay tiles resting on a timber structure. The structural analysis of thetarget building was conducted using Taiki Saito’s Stera3D. A simple model was evaluated in both theN-S direction and the E-W direction, considering only in-plane responses in the walls. Therefore, theretrofitting techniques chosen for this study were those that could increase the in-plane capacity of thewalls., the retrofitting techniques were designed to limit the eccentricity of the building to a minimum
to mitigate any possible torsional effects caused by the irregularity of the building. The results of the1

Department of Built Cultural Heritage, Ministry of Culture, El Salvador.
IISEE-GRIPS master course student.
3
Professor Emeritus, Hiroshima University, and Visiting Research Fellow, IISEE, BRI.
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Figure 1. Architectural plan of the building.

TAISHIN Project, conducted by the universities
UCA and UES, were used to model the structural
behavior of the adobe walls. Material and-full scale
test results from the first and second phase of theTAISHIN Project were taken, obtaining four
idealized curves, for which the lower envelope waschosen as the model skeleton curve for unreinforced
adobe masonry. This model had the maximum drift
value of the adobe walls equal to 1/2000. The
skeleton curve of every wall in the target building
was calculated using this model, and the seismicindex of the original building was calculated,
obtaining that the Is values (Table 1) were lower thanthose required by the JBDPA standards. Thus, thebuilding needs retrofitting.

Table 1. Seismic Index of the target building.
Result
1st level
2nd level
Direction Story
IsoIsoIs
Is0.60FAILN-S10.12 0.80 0.14
E-W
10.12 0.80 0.17 0.60 FAIL
3. METHODOLOGY
The first step in determining whether a retrofitting technique is successful or not is to set the analysis
methods to be used in the analysis, followed by the seismic demand, and finally, by setting the criteria
for a successful retrofitting. The seismic capacity of the retrofitted building was set to be calculatedusing three methods: 1) Capacity Spectrum method, where a pushover analysis was conducted on theretrofitted model in Stera3D and compared to the spectrum determined by the demand level; 2) JBDPA
Guidelines, where the strength factor (C) and the ductility factor (F) were to be determined by thestructural retrofitting model and compared to the Iso values determined by the demand level; and 3)
Response History Analysis, where the demand spectrum would be utilized to generate two waveforms
using both a crustal earthquake (1995 Kobe eq.) and a subduction earthquake (2001 El Salvador eq.) asenvelope functions. The next step is to determine the seismic demand level (Fig. 2).

Figure 2. High vs low demand spectra.

Figure 3. Criteria for a successful retrofitting.

The demand level depends on whether or not reducing the impact of the retrofitting is more crucial
than ensuring its habitability. For this study, two levels were considered: 1) The high demand level isdesigned to ensure that the building withstands all the strong motions that could appear in its lifetime,making it fit for human habitation, but resulting in a more expensive and aggressive retrofitting. This2demand level is obtained according to the Salvadorian Seismic Code, consisting of a design spectrumwith a PGA based on the country’s seismic zoning. The building is located in the Seismic Zone I. For aground motion with a probability of exceedance of 10% in 50 years, PGA is equal to 0.4g. 1) The low
demand level is designed to ensure that the retrofitting works are more respectful of the historic valueof the building, but this results in a design that doesn’t ensure habitability because there is a risk that astrong motion can cause the collapse of the building during its lifetime. For buildings whose historicvalue is more important than its habitability, this demand level can be used. The reduced demand wasestimated using a Probabilistic Seismic Hazard Analysis (PSHA) of earthquakes in the region, and theground motion with a probability of exceedance of 50% in 50 years was obtained (PGA=0.24g). As forthe retrofitting strategy, the following process was adopted: 1) Improvement of irregularity by adding asouth wall; 2) Stiffening of the roof level of the walls by adding an RC head beam anchored to the adobewalls; 3) Retrofitting of the walls using three retrofitting techniques and comparing them: -Polypropylene band wrapping (Fig. 4): Plastic bands are wrapped in a mesh around the walls and tied
to the head beam and foundation. The retrofitted building is expected to show a significant increase inductility. --Backside wall affixing (Fig. 5): RC wall built next to the adobe wall, tied to the head beamand foundation. The retrofitted building is expected to show a significant increase in strength. --RC
frame affixing (Fig. 6): RC frame embedded in the adobe wall, encased in a clay brick casing. The
retrofitted building is expected to show a considerable increase in ductility and strength, making it amixed technique. All retrofitting techniques were evaluated according to the procedure shown in Fig. 3.

Figure 4. PP-band wrapping.

Figure 5. Backside wall.

Figure 6. RC Frame affixing.

4. RESULTS AND DISCUSSION
Polypropylene band wrapping technique: Due to the lack of experimental data, a literature search on thestructural behavior of polypropylene band wrapped masonry buildings subjected to in-plane motionswas conducted. Three different research papers were selected: 1) Zhou et al., 2) Zohreh et al., 3) Mayorca& Meguro. To obtain the retrofitting model, first, the skeleton curves in all of the specimens werenormalized to compare them to those of the non-retrofitted adobe building. Afterward, the retrofitted
adobe skeleton curve was inferred by observing the changes in each of the reference studies andreplicating the same changes in the original adobe skeleton curve. Out of the three results, the lowerenvelope was chosen as the model skeleton curve. This model had the maximum drift value of theretrofitted adobe walls equal to 1/215. This model was first evaluated using the capacity spectrum
method. The capacity curve never reached the demand line, except for the E-W direction in the low
demand level. After this, the JBDPA method was employed to determine the seismic capacity of theretrofitted building. The seismic indexes (Is) only reached values higher than the seismic demandindexes (Iso) for the E-W direction, low demand level, and only in the second level screening. Finally,
the response history analysis was conducted, in which the only favorable results obtained were in the EW direction, low demand level. The results are summarized in Tables 2 and 3.
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Table 2. Results of the Polypropylene bandwrapping technique, N-S direction.

Table 3. Results of the Polypropylene bandwrapping technique, E-W direction.

Backside wall affixing technique: The first step in developing the backside wall affixing model was tofind the skeleton curve of each adobe wall and each RC overlay. The skeleton curves of the adobe walls
were identical to those obtained for the Polypropylene band wrapped walls because this method requires
the wrapping of the walls before applying the overlay walls. For the estimation of the skeleton curvesof the RC overlays, the method recommended by the AIJ guidelines was used. Then, the combined
skeleton curve was equal to the skeleton curve of the adobe wall plus the overlays. This model had themaximum drift value of the retrofitted adobe walls equal to 1/250, therefore ensuring that the adobewalls don’t fail before the overlays do.

Figure 8. Backside walls, low demand level.

Figure 7. Backside walls, high demand level.

Afterward, the distribution of overlay walls (Figs. 7 and 8), dimensions, and reinforcement for bothdemand levels was chosen. The location of the walls was chosen to counteract the irregularity of thebuilding, keeping the eccentricity levels under 5%. This model was first evaluated using the capacity
spectrum method. The capacity curve reached the demand spectrum in both directions and both demandlevels. After this, the JBDPA method was employed to determine the seismic capacity of the retrofitted
building. The seismic indexes (Is) reached values higher than the seismic demand indexes (Iso) for bothdirections, both demand levels, and both 1st and 2nd level screenings. Finally, the response history
analysis was conducted, in which all of the results obtained, in all levels and directions were less thanthe maximum allowable by the material (1/250). The results are summarized in Tables 4 and 5.

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Table 4. Results of the Backside wall affixing
technique, N-S direction.

Table 5. Results of the Backside wall affixing
technique, E-W direction.

RC frame affixing technique: The first step in developing the RC frame affixing model was to find thecapacity curve of the adobe walls and the RC frames. The capacity curve of the adobe building wasidentical to that obtained for the Polypropylene band wrapped walls because this method requires thewrapping of the walls after constructing the embedded frames. The capacity curve of the RC frame wascalculated directly using Stera3D. Then, the combined capacity curve was equal to the capacity curveof the adobe building plus that of the RC frame. This model had the maximum drift value of theretrofitted adobe walls higher than 1/67, which is the maximum drift allowable by the Salvadorian
Seismic Code for regular buildings with fragile non-structural elements (1.5%). Thus the adobe walls
suffer damage before the overall building collapses.

Figure 9. RC frame, high demand level.

Figure 10. RC frame, low demand level.

Afterward, the distribution of the beams and columns (Figs. 9 and 10), dimensions, andreinforcement for both demand levels were chosen. The structure was designed to counteract theirregularity of the building, keeping the eccentricity levels under 5%. The beams were designed to carryvertical loads after the adobe walls lose its vertical carrying capacity (at a drift of 1/215). This model
was first evaluated using the capacity spectrum method. The capacity curve reached the demand
spectrum in both directions and both demand levels. After this, the JBDPA method was employed todetermine the seismic capacity of the retrofitted building. The seismic indexes (Is) reached values higher
than the seismic demand indexes (Iso) for both directions, both demand levels, and both 1st and 2nd levelscreenings. Finally, the response history analysis was conducted, in which all of the results obtained, in
all levels and directions were less than the maximum allowable by the material (1/67). The results are
summarized in Tables 6 and 7.

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Table 6. Results of the RC frame affixing
technique, N-S direction.

Table 7. Results of the RC frame affixing
technique, E-W direction.

5. CONCLUSION
For this target building, the Polypropylene band wrapping technique was not enough to reach therequired capacity. Both demand levels were too high, and the strength of the adobe masonry was tooweak. Thus, the Polypropylene band wrapping technique was not feasible. The RC frame affixing
technique increased strength and significantly increased ductility, making the building reach the required
capacity. However, the adobe walls failed before reaching this point. This technique was feasible but
required considerable structure, and the failure mode doesn’t protect the adobe walls from extensive
cracking. The Backside wall affixing technique increased ductility and strength before the failure of theadobe walls, making the building reach the required capacity. This technique was feasible, required lessstructure, and its failure mode protects the adobe walls from extensive cracking. Therefore, the backside
wall affixing technique was the best retrofitting technique for this target building. As for the selectionof the demand level, it depends on the projected habitability of the building. A low design level canprovide a more economical and respectful retrofitting, but the building wouldn’t be habitable. A higher
design level can ensure the building’s habitability but at a costlier and more aggressive solution.
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 heartfelt gratitude to Dr. Shunsuke Sugano for his supervision
throughout my entire Individual Study. I want to thank Dr. T. Azuhata and Dr. H. Suwada, for theirvaluable guidance. Thanks are also given to Dr. E. Peña (University of El Salvador, TAISHIN Project)and Dr. K. Meguro (University of Tokyo, PP-Band Study), for their valuable research in masonry thatserved as a reference in this study. I am also grateful to Dr. T. Saito (Toyohashi University ofTechnology) for allowing me to use his software tools for my study.
REFERENCES
ASIA, MOP. (1994). Norma sismica Para Diseño por sismo y sus comentarios. San Salvador.
Japan Building Disaster Prevention Association. (2004). Standard for Seismic Evaluation of Existing
Reinforced Concrete Buildings (English Version ed.). (B. R. Institute, Trans.) Tokyo, Japan: JBDPA.
Mayorca, P., & Meguro, K. (2004, August). Proposal for an Efficient Technique for Retrofitting
Unreinforced Masonry Dwellings. Vancouver, B.C., Canada: 13 WCEE. Retrieved May 2020
Peña, E., Evelio, R., & Guevara, N. (2008). Informe de Resultados de la Investigacion del Sistema
Constructivo Adobe Sismo-Resistente. TAISHIN Project. San Salvador: VMVDU.
Saito, T. (2019). Procedure of Capacity Spectrum Method. Tsukuba: IISEE.
Zhou, Q., Yang, L., & Zhao, W. (2020). Experimental Analysis of Seismic Performance of Masonry
Shear Wall Reinforced with PP-Band Mesh and Plastering Mortar under In-Plane Cyclic Loading.
Zohreh, J., Ghaednia, H., Nayak, S., Das, S., Bhattacharya, S., & Chandra Dutta, S. (2019).
Experimental and Field Performance of PP Band–Retrofitted Masonry.
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