During the 16th and 17th centuries, Latin American cities adopted earthen construction techniques from European colonizers. As a result, rammed earth (RE) buildings now occupy an important place in Latin America's cultural heritage. However, earthquakes around the world have shown that unreinforced earthen constructions are highly vulnerable. For several years, researchers in northern South America have been proposing a technique that consists of installing confining steel plates (or wooden elements) on both sides of the RE walls forming a grid. This system has shown excellent performance in controlling seismic damage and increasing strength and ductility capacity. Although researchers have tested full-scale one- and two-story earthen walls under pseudo-static loading in the laboratory, and one- and two-story earthen walls at 1:1 and 1:2 scale on uniaxial and biaxial shaking tables, the behavior of a complete reinforced module (one- or two-story) on a shaking table has never been assessed. The present study presents the results of shaking table tests performed on two-story RE modules at 1:4 scale. The experimental data indicate that the retrofit system with confining steel strips was effective in reducing the seismic damage of earthen constructions. In addition, the comparison of the results of the 1:4 scale tests with the 1:2 and 1:1 scale tests previously conducted by the researchers, shows that the acceleration levels of the equivalent prototypes are in the same order of magnitude for the three scales.

Keywords:

Subject: Engineering - Civil Engineering

1. Introduction

Earthen construction techniques have a rich and extensive history, dating back to the Neolithic period when humans first settled and began constructing permanent structures [1]. Throughout history, civilizations such as the Sumerians, Egyptians, and Babylonians, among others, have used various forms of earthen structures for dwellings, temples, and government buildings [2]. Today, earthen buildings serve as homes for at least 33% of the world´s population and make up one-tenth of World Heritage Monuments [3]. For instance, in regions influenced by European colonial architecture, such as Colombia (northern South America), rammed earth (RE) buildings hold immense cultural value and most of them are considered heritage constructions [4]. The preservation of these architectural legacies becomes crucial, not only to maintain their cultural significance for future generations, but also because today, earthen buildings represent an ideal sustainable construction system due to their recyclability, and their low energy and water footprints. However, the deterioration of earthen buildings has accelerated due to natural and human impacts, including earthquakes, urbanization, and the adoption of industrialized building technologies [5].

Seismic performance has been a particular concern for unreinforced RE constructions due to their deficient structural behavior during past earthquakes. Events like the Peru 2007 and the Chile 2010 earthquakes highlighted the vulnerability of historic earthen buildings to seismic forces [6,7]. The deficiencies in their seismic behavior can be attributed to factors such as poor material properties, irregular distribution of walls and openings, weak connections between walls, inadequate wall-floor-roof connections, heavy roofs and floors, and deficient out-of-plane strength [8,9,10,11,12,13,14]. Earthen buildings have evidenced partial collapse for 0.5% of inter-story drift levels [8,13,15]. For this reason, in the last four decades, there has been a growing motivation to improve the seismic performance of earthen buildings (RE and adobe), driven by the need for conservation of historical sites built with sustainable construction practices. Researchers worldwide have made significant efforts to develop retrofitting techniques for existing RE buildings. Various countries, including Peru [16], Australia [17], Germany [18], and more recently, Colombia [19], have established regulations and guidelines for seismic rehabilitation of earthen structures. The current rehabilitation techniques include bonded fibers, mesh reinforcement, tensors, polyester fabric strips, straps, steel cables, concrete beams, mortar with textiles, and confining elements (wood and steel strips) [6,7,8,9,12,13,14,15,20,21,22,23,24,25,26,27,28,29].

Among the retrofitting techniques proposed in the literature, confinement with wooden elements or steel plates have emerged as a prominent alternative for restoration purposes in order to preserve heritage building in the long term [8,9,15,21,28,29]. These reinforcements have demonstrated outstanding results in improving the overall capacities and seismic behavior of earthen walls in comparison to other rehabilitation techniques; for instance, the maximum residual drift is at least three times lower than the classic mesh reinforcement technique [8] ensuring the integrity of the building after large earthquakes. Both approaches (confinement with wooden elements or with steel plates) involve installing vertical and horizontal reinforcement elements approximately each 1000 mm on the inner and outer faces of the wall and interconnected with steel rods each 500 mm. However, the reinforcement with wooden strips has significant drawbacks due to the large section of wooden strips (180 mm x 40 mm) and the durability issues of the material. To address these challenges, the use of steel plate confinement has gained attention in northern South America as it offers advantages such as equal or superior efficiency compared to wooden elements, improved durability against external agents, less invasive installation, and smaller reinforcement element size (100 mm x 6.35 mm). Figure 1a shows a typical façade of an Andean historic earthen house, and Figure 1b allows a visualization of the wall with the reinforcement based on steel strips. It is very important to note that once the steel plates are installed on both sides of the walls, they are covered with a layer of earth, lime, and sand so that the aesthetic of the building is not affected by the seismic reinforcement.

Despite the progress in this retrofit technique with research projects including full-scale pseudo-static tests (one- and two-story earthen walls) and one- and two-story earthen wall tests on uniaxial and biaxial shake tables (1:1 and 1:2 scale), there is a notable lack of studies concerning multi-story earthen houses (with interconnected walls) reinforced with steel plate confinement. The importance of this study arises from the prevalence of multi-story earthen heritage buildings throughout the world. For instance, in the case of the historic center of Bogota (the main city of Colombia, South America), which is one of the areas with the highest density of earthen buildings per square meter in the country, it has been reported that nearly 50% of the earthen historic constructions have two stories [30]. Therefore, this research aims to contribute to the understanding of the seismic behavior of earthen historic structures reinforced with steel plates in order to preserve the cultural heritage and promote sustainable construction buildings.

To evaluate the effectiveness of the steel plate retrofit technique, two shaking table tests of reduced scale (1:4) two-story houses were performed at the Structures Laboratory of the Pontificia Universidad Javeriana. In addition, the acceleration records and the damage patterns (reinforced and unreinforced earthen houses) were compared with previous seismic tests on 1:2 and 1:1 scale specimens [8,31]. The main results show that the steel plate reinforcement significantly improves the seismic performance, demonstrating its effectiveness for two-story rammed earth houses. There is also a good agreement between the 1:4 scale test results and the larger scale tests previously conducted by the authors. This study will complement existing historic building codes with information from experimental evidence for two-story buildings, as well as lay the groundwork for new earthen standards.

2. Material properties

In order to accurately represent a historic earthen building, rammed earth (RE) material obtained from a 120-year-old historic Colombian building, that was demolished due to lack of maintenance, was used to construct the specimens. The granulometry of the material was passed through a #10 sieve, taking into account the reduced scale of the specimens to be tested. The RE material was subjected to laboratory tests to determine its physical and mechanical properties and then compared with the literature. The physical properties are shown in Table 1. The granulometry, Atterberg's limits and maximum dry density along with the optimum moisture content were evaluated according to the ASTM D422, ASTM D4318 and ASTM D698 standards, respectively [32,33,34]. Table 2 shows the compressive strength obtained from compression tests on rammed earth cylinders (300 mm height and 150 mm diameter). Both tables include literature results for similar materials for comparison.

The data presented in Table 1 and Table 2 include both the commonalities and the variations of the physical and mechanical properties, highlighting the distinctive attributes of the RE material used in historical constructions in the Andean region. The physical properties determined in the present research are similar to the values reported in the technical and scientific literature, but the compressive strength shows diverse average values and remarkable variability. Despite the dispersion found in the references, the average compressive strength values in the present research fall within the range of the data in Table 2. The unit weight of the RE specimens is 18.5 kN/m³, which is in the order of magnitude presented in [31] (18.9 kN/m³ ~ 19.5 kN/m³). However, it is important to note that in some cases the current results show a difference due to the granulometry adjustment made according to the 1:4 scale of the samples. This adjustment allows a better compaction of the particles, resulting in higher values. It should be emphasized that the collapse mechanisms observed in the present research are similar to those reported for 1:1 and 1:2 scale specimens [8,31]. Finally, the ASTM A 424 steel plates used in the research, had a yield stress of 194 MP, a tensile strength of 284 MPa and a Young's modulus of 201 GPa according to ASTM tests performed [35].

3. Reduced Scale Specimens

In the Structures Laboratory at the Pontificia Universidad Javeriana, two 1:4 scale RE models were constructed and tested on a biaxial MTS shaking device. One of the specimens was constructed without reinforcement, while the other was reinforced with steel strips. Both specimens were two-story, square-shaped rammed earth models. The square typology was chosen to verify the performance of the reinforcement in an earthen system that simulates the continuity of the walls. Figure 2 shows an example of a typical earthen historic Andean house and a module with this typology. The module allows the study of the interactions between the walls and the diaphragm of the structure.

The choice of the 1:4 scale is determined by two main factors: First, the options for anchoring the house foundation to the shaking table given the square floor plan configuration. Second, it addresses the capacity constraints of the MTS biaxial shaking table in terms of overturning moment and maximum load (ensuring that the weight of the reinforced model does not exceed 100 kN). To the best of the authors' knowledge, there have been few examples of reduced-scale two-story earthen houses (constructions of the northern region of South America) being tested on biaxial shaking tables. In particular, this experiment is the first to subject a two-story earthen module reinforced (with steel plates), with windows and door, to a seismic-dynamic test.

The dimensions of the 1:4 models were determined according to the main characteristics of typical Andean earthen heritage constructions. The authors conducted a dimensional study focusing on walls, openings, and floors of the historic city center of Bogotá, Colombia called “La Candelaria” [30]. Figure 2 shows an example of one of the analyzed heritage houses. Through the analysis of planimetric information and field visits, it was possible to obtain statistical data on the geometric characteristics of the earthen heritage buildings. The results of the analysis showed that the average wall thickness was 640 mm and the wall height per floor was typically between 3000 mm and 3330 mm. It was also found that earthen walls usually have symmetrical openings on both floors, to accommodate windows or doors. Based on these findings, the characteristics of the reduced-scale samples were determined as explained below.

3.1. Unreinforced model

The model constructed for the experiment was a two-story RE module, consisting of four interconnected walls with 5 windows and 1 door (Figure 3). The specimen consists of one solid wall (with no openings), two walls each with a window on both floors, and one wall with both a door and a window. This configuration is intended to accurately represent typical architectural features (such as those shown in Figure 2), while allowing for potential differences in damage patterns and collapse mechanisms between walls with and without openings. In addition, 40 mm x 50 mm wooden scaled beams (160 mm x 200 mm in real scale) were included to represent the second floor and roof.

All components and features, including construction tools, foundation stones and the rammed earth particle size, were adjusted to a scale of 1:4. The walls were constructed with compacted layers of 25 mm (100 mm at 1:1 scale), the wall thickness was 160 mm (640 mm at 1:1 scale), with a total height of 1560 mm (6240 mm at 1:1 scale). The foundation of the specimens consisted of a U-shaped steel box to simulate the boundary conditions of an earthen building. Above the foundation, an 80 mm (320 mm at 1:1 scale) height stem wall was constructed, followed by the six rows of rammed earth blocks that make up the rammed earth module (Figure 3). The row configuration was adopted to represent existing walls, creating interlocking connections between blocks of different rows, and ensuring structural integrity. Upon completion of construction, the soil material of the earthen blocks was allowed to dry for 30 days prior to testing.

3.2. Reinforced module with steel plates

After testing the first unreinforced 1:4 scale RE model, an identical specimen was constructed using the same labor and materials used previously. After four weeks of natural drying under the controlled laboratory conditions, the second model was reinforced with confinement steel plates. This reinforcement follows the guidelines of the AIS 610 - EP17 standard [19], which systematically outlines the basic requirements for the reinforcement of earthen heritage constructions in Colombia. The main objective of this reinforcement is to significantly increase the structural stiffness and strength of the walls, both in-plane and out-of-plane, by improving their ability to resist bending and shear loads.

The reinforcement consists of steel strips on both sides of the walls, forming horizontal and vertical rings that confine the walls. To address the scale reduction, the steel plates were 25 mm width and 1.6 mm thickness, corresponding to a full-scale prototype in which the steel plates are 100 mm width and 6.35 mm thickness. Following the AIS 610 guidelines [19], the horizontal spacing of the reinforcement rings was limited to 250 mm (1000 mm in the full-scale prototype). An exception of this rule are the first three rings at the base, that were spaced at 125 mm (500 mm in the full-scale prototype). The vertical plates were placed 330 mm apart, a larger spacing than AIS 610 [19] (1200 mm in the prototype), based on the results of 1:2 shaking table tests performed by the authors in reference [31]. Consequently, the model had a total of ten horizontal and twenty vertical reinforcement rings (four vertical rings per wall). Compatibility between the steel plates and the earthen material was maintained through the use of threaded rods. In addition, these rods ensured direct contact between the plates and the surface of the walls. These rods were placed at the intersection of the horizontal and vertical rings according to AIS 610 (2017). Finally, to further strengthen the earthen house, the diaphragm of the first floor was reinforced by adding two diagonal plates that were welded to the vertical plates.

Figure 4. Reinforced 1:4 model with steel plates.

4. Reduced scale (1:4) biaxial shaking table test

In order to extend the test results to the full-scale prototype, similarity ratios were established based on a 1:4 geometric scale neglecting gravitational forces during dynamic testing as described by the reference [37]. Previous researchers investigating the seismic performance of RE or adobe buildings have also made similar assumptions for gravitational forces, see for examples references [13,31]. Specific similarity equations for physical properties between the scale model (1:4) and the corresponding full-scale prototype are shown in Table 3. It is important to consider that the subscript

" m "

refers to the model, and the subscript

" p "

refers to the prototype.

For example, for displacement-related variables the equation

L_{m} = \frac{L_{p}}{4}

implies that the model dimensions are one-fourth the dimensions of the prototype. Based on the similarity ratio rules, the real acceleration records were adjusted according to Table 3 in order to control the shaking table tests by accelerations (

x

and

y

directions). It is important to emphasize that to avoid confusion, all results presented in the present research are converted with similarity equations to the prototype values (i.e. the full-scale 1:1).

4.1. Test setup

The 3m x 3m shaking table was equipped with two dynamic actuators capable to generate displacements of ±250 mm in the

x

and

y

directions with a maximum acceleration of 10 g and a peak velocity limit of 1 m/s. The primary objective of these tests was to simultaneously evaluate the seismic performance of the models in both the out-of-plane and in-plane directions. The weight of the reinforced 1:4 scale model was approximately 30 kN. In addition, the model included 4.0 kN of concrete weights (at the roof) to simulate the vertical load of 5.3 kN/m per meter of the full-scale walls (gable roof) [15]. In addition, 8 kN of earth bags were used to represent the weight of the first floor. To anchor the earthen house to the shaking device platform, the steel box containing the foundation was anchored with 12 pre-stressed bolts. To enhance the understanding of the experimental setup, QR codes have been incorporated into Figure 5 to provide access to 360° panoramic views.

The 1:4 scale models were instrumented with a total of 18 high sensitivity piezoelectric accelerometers. Sixteen of these were strategically placed on the walls of the earthen models, while the remaining two were placed on the shaking table. The accelerometers were placed to cover both the first and second floor walls. Each of these points was equipped with two accelerometers, oriented in both the

x

and

y

directions. These locations are shown in Figure 6 with labels from A to H.

4.2. Test Protocol

The loading protocol for this test was based on a seismic disaggregation analysis following the procedure described by [8,31,38]. The analysis covers periods in the range 0.3 seconds to 1.0 seconds. For the seismic simulation, specific ground motions corresponding to return periods of 2500, 475, 225 and 31 years were used. These return periods are related to 2%, 10%, 20%, and 80% exceedance probabilities in 50-years, respectively. Table 4 presents the

P G A

values for the prototype (full scale) and for the scaled models.

5. Crack and damage patterns

The damage progression of the unreinforced and reinforced houses after the seismic motions is shown in Figure 7. The unreinforced specimen partially resisted earthquake phases with

P G A_{y}

of 0.14g, 0.38g, 0.43g and 0.53g, and showed irreparable damage and partial collapse during the event with

P G A_{y} = 0.53 g

followed by a complete collapse during the earthquake with

P G A_{y} = 0.76 g

. In contrast, the reinforced house withstood the entire sequence of ground motions up to an earthquake with

P G A_{y} = 1.02 g

(a total of 6 ground motions), with damage in some points of the stone stem wall, but with little damage in the rammed earth walls. It is noteworthy that in the reinforced house, the diaphragm works adequately to transfer the seismic forces to the reinforced walls, providing structural integrity to the system.

Figure 7 also shows that the second floor of the models had fewer cracks than the first floor in both models. However, the damage in the unreinforced model was extensive, with vertical and diagonal cracks in the walls originating at the joints of the rammed earth blocks, due to block detachment and corner earth separation in the structural assembly. In contrast, the reinforced model showed localized and repairable damage manifested as limited vertical or diagonal cracking. It's important to emphasize that, the damage at the base in the reinforced model, particularly at the rock-wall junction, was significantly more pronounced compared to the unreinforced structure. At the same time, the buckling of the strips induced damage at the stem wall for a

P G A_{y} = 0.53 g

. This buckling was also observed in reference [31], which tested a two-story RE wall, while it was not evident in the test of the one-story earthen walls in reference [8]. Therefore, this damage pattern can be attributed to the additional overturning moment at the base of the walls due to the mass of the second story. In conclusion, the test results indicate that steel plates prevent wall failure, delay cracking, and improve the displacement capacity of the model.

In contrast to what was reported in references [8] and [31], the modules tested in the actual research (both reinforced and unreinforced) withstood an additional increase in the loading protocol defined in Table 4. The authors hypothesize that this better behavior may be due to the redundancy of the four-wall modules tested in the present research compared to the C-shaped wall (from reference [31]) and the L-shaped wall from reference [8]. Having four walls with two diaphragms may reduce the possibility of dislocation of the RE blocks, which was one of the failure mechanisms reported in both references.

6. Acceleration records

Figure 8 shows an example of the acceleration records at one of the points of the second floor (point D) for both the unreinforced and the reinforced prototype. In order to facilitate the interpretation of the measurements made, all the results presented refer to the prototype (scale 1:1). For this purpose, the experimental results have been adjusted according to the similarity laws presented in Table 3. For each data set, the peak acceleration value is highlighted. Accelerations in two mutually perpendicular directions (

x

and

y -

directions) were recorded at each point, but only the movements for the y- direction (critical direction) are shown in Figure 8. For all the data sets analyzed, it was found that the peak accelerations at all the points of the reinforced prototype are higher than those of the unreinforced prototype. In addition, the reinforced specimen withstood all 6 intensities of ground motions, while the unreinforced specimen showed partial collapse for

P G A_{x} = 0.55 g

and

P G A_{y} = 0.76 g

. The reinforced prototype with steel plates experienced a maximum acceleration of

1.24 g

at point D for the maximum

P G A_{y} = 1.02 g

Figure 9 and Figure 10 show the profile of the maximum accelerations for each of the ground motions of the shake table protocol. The value reported for each floor is the average of the four accelerometers installed at each level of the specimen (reinforced and unreinforced). The selected motions had a direction of greater intensity, which is the

y

- direction for the present research. For the unreinforced prototype (Figure 10), the results are shown only for the first four intensity levels because the unreinforced specimen was severely damaged after the ground movement with

P G A_{y} = 0.53 g

. Based on the results, it can be concluded that the acceleration of the upper floors was always higher than that of the lower floor in both

x

and

y

directions. The maximum acceleration of the unreinforced prototype was

0.76 g

(for

P G A_{y} = 0.53 g

) in the y-direction. For the same earthquake motion, but in the

x

direction (

P G A_{x} = 0.39 g

), the average maximum acceleration of the second floor was

0.55 g

. According to the results of the acceleration profiles of the reinforced prototype, a maximum acceleration of

1.22 g

and

0.92 g

was reached in the

y

and

x

directions, respectively. These maximum accelerations were reached for the last movement at the base of the loading protocol.

Despite these high levels of

P G A

and accelerations at the first and second floors, the reinforced specimen showed little damage (as shown in Figure 8) and the steel plates confined the rammed-earth blocks and absorbed the flexural tensile stresses, reducing the damage of the specimen. At these maximum acceleration levels, the second floor of the reinforced specimen showed less damage and cracking than the first floor. As reported by [31], the steel plates confining the RE blocks reduce the possibility of dislocation. Based on the experimental results, the reinforcement system reduces the fragility of the rammed earth walls and gives the structural system an inelastic displacement capacity.

7. Comparison of accelerations between scale reinforced models

Table 5 and Table 6 present a comparison of the maximum accelerations determined for the present study (1:4 scale two-story reinforced earthen module) with the maximum accelerations measured in two previous tests conducted by the authors ([8,31]). The comparison of maximum accelerations is presented for both out-of-plane (Table 5) and in-plane (Table 6) directions for five ground motion intensities. For in-plane actions, the comparison specimen is a 1:2 scale model with a two-story C-shaped wall [31]. In addition, for out-of-plane actions, Table 6 includes data on the accelerations of a one-story steel plate reinforced RE wall whose shaking table results (uniaxial ground motions) are reported in reference [8]. Table 6 also shows the out-of-plane results for the two-story C-shaped wall of the reference [31]. The current research analyzes walls with openings, while the references [31] and [8] tested walls without doors or windows. Both two-story scale models were built with RE walls and included a floor system with crown and load-bearing wooden beams. Figure 11 shows a scheme and the main characteristics of the three models of rammed earth walls/models retrofitted with steel strips which will be compared in terms of maximum accelerations. This figure shows relevant information such as the scale, the dimensions of the prototype they represent, and the number of stories.

The three earthen reinforced models (this study, reference [31] and reference [8]) were subjected to ground motions with the same loading protocol defined in Table 4, although in each case adapted to the scale of the test according to the similarity rules presented in Table 3. The model form reference [8] was tested with the ground motions of the

y

direction of Table 4. The last column of Table 5 and Table 6 shows a ratio between the acceleration estimated in the 1:2 scale model and the acceleration of the 1:4 scale model. The experimental data suggest that there is a reasonable agreement between the results of the accelerations of the equivalent prototypes of each test; the average of the ratios for the out-of-plane and in-plane directions of the two floors is close to 100% (96% and 98%). However, the ratio is higher for higher intensity earthquakes. The above evidence suggests that RE models following the guidelines of reference [37] (neglected gravitational forces) allow to reasonably estimate the maximum accelerations of large prototypes. Finally, and considering the data in Table 6 (out-of-plane acceleration), although the 1:1 scale model tested in reference [8] has only one floor, the results for the maximum accelerations recorded are in the same order of magnitude as the acceleration values of the second floor of reference [31] and the present study.

6. Conclusions

The steel plate reinforcement system reduces the cracking and the possibility of dislocation of the earthen blocks that conform RE walls. Additionally, the retrofitting system provides the earthen buildings energy dissipation capacity, good behavior in the inelastic range, and confinement of the earthen constructions.

The failure mechanism of the unreinforced earth model subjected to the seismic shaking table test is the appearance of flexural and shear cracks that induce the dislocation of the earth blocks and, consequently, the instability and collapse of the walls. According to the experimental results, the cracks appear to be critical at

P G A_{y} = 0.53 g

and

P G A_{x} = 0.39 g

Effectively, the steel plate reinforcement system provides continuity, connection, and confinement to the rammed earth walls, and as a consequence, the reinforced model withstood the entire load protocol up to peak ground acceleration values of

P G A_{y} = 1.02 g

and

P G A_{x} = 0.78 g

. For these

P G A

values, the maximum average roof acceleration was

1.22 g

with no signs of structural collapse. After the loading protocol, the reinforced module presented repairable damage in the zone of the stem wall. This evidence confirms that the steel plate reinforcement system provides resilience to RE buildings even during high intensity earthquakes.

Comparing the results of the 1:4 scale tests of the present research with the 1:2 and 1:1 scale tests previously conducted by the authors, the accelerations of the prototypes are in the same order of magnitude. In particular, when comparing the maximum acceleration of the first and second floors of the reinforced system, both in the out-of-plane and in-plane directions, an average agreement of at least 96% was found between the data of the 1:4 scale model and the 1:2 scale model. This means that it is possible to estimate the maximum accelerations of RE building prototypes using reasonably small-scale models. Due to the scale of the wall material and the characteristics of the steel plate reinforcement system, it is recommended to characterize the seismic response of reinforced prototypes using models at scales not smaller than 1:4.

Author Contributions

Conceptualization: Natalia Barrera, Daniel M. Ruiz; Funding acquisition: Daniel M. Ruiz, Juan C. Reyes; Formal analysis: Natalia Barrera, Daniel M. Ruiz; Methodology: Natalia Barrera, Daniel M. Ruiz, Yezid A. Alvarado, Daniela Carrasco; Project administration: Daniel M. Ruiz; Supervision: Juan C. Reyes; Resources: Yezid A. Alvarado, Daniel M. Ruiz; Writing original – draft: Natalia Barrera, Daniel M. Ruiz; Visualization: Natalia Barrera, Daniel M. Ruiz; Writing – review & editing: Daniela Carrasco, Juan C. Reyes.

Funding

This research was funded by MinCiencias (Colombia) through Patrimonio Autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas, grant number 120385269649 MGI, proposal Id: 6988. The research was titled Rehabilitación sísmica de edificaciones en tierra (patrimoniales) de dos niveles, in accordance with 852_2019, and contract 80740_510_2020.

Data Availability

Ground data accelerations are available in the following link: https://livejaverianaedumy.sharepoint.com/:x:/g/personal/daniel_ruiz_javeriana_edu_co/EWhc8wRSEiFOjOO410aFC90BKy0gxpOk0m wWE7cS7G_ayQ?rtime=-4FEsMe020g.

Acknowledgments

The research was developed in the Structures Laboratory of Pontificia Universidad Javeriana. The authors thank the technical staff, in particular Miss Francia Abril and Jeisson Hurtado. The authors also acknowledge to Mr. Jaime Cruz and Mr. Dairo Bandera for building and retrofitting the earthen model.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

Xie, Liye, et al. "Architectural energetics for rammed-earth compaction in the context of Neolithic to early Bronze Age urban sites in Middle Yellow River Valley, China." Journal of Archaeological Science 126 (2021): 105303. [CrossRef]
D. Ruiz, C. Lopez, J.C. Rivera, Propuesta de normativa para la rehabilitación sísmica de edificaciones patrimoniales [Proposed regulations for seismic rehabilitation of earthen heritage buildings], Apuntes: Revista de Estudios Sobre Patrimonio Cultural - Journal of Cultural Heritage Studies 25(2 (2012) SE-). http://revistas.javeriana.edu.co/index.php/revApuntesArq/article/view/8767. 8767.
D. Gandreau & L. Delboy. UNESCO- CRAterre-ENSAG. World Heritage. Inventory of earthen architecture. 2012. https://whc.unesco.org/document/116577. 1165.
D.M. Ruiz, C. López, S. Unigarro, M. Domínguez, 2015. Seismic Rehabilitation of Sixteenth-and Seventeenth-Century RE–Built Churches in the Andean Highlands: Field and Laboratory Study. Journal of Performance of Constructed Facilities 29(6):04014144. [CrossRef]
M. Hall, R. Lindsay, M. Krayenhoff. Modern Earth Buildings: Materials, Engineering, Constructions and Applications. Woodhead Publishing Series in Energy. Chapter 4: H. Schroeder. Modern earth building codes, standards and normative development. In Modern Earth Buildings. Woodhead Publishing; 2012.
M. Blondet, G. Villa Garcia, S. Brzev, Á. Rubiños, Earthquake-resistant construction of adobe buildings: a tutorial, EERI/IAEE World Hous. Encycl. 56 (2011) 13–21. http://www.world-housing.net/wp-content/uploads/2011/06/Adobe_Tutorial.pdf.
D. D’Ayala, M. EERI, G. Benzoni, Historic and Traditional Structures during the 2010 Chile Earthquake: Observations, Codes, and Conservation Strategies, Earthquake Spectra. 28 (2012) 425–451. [CrossRef]
J.C. Reyes, R. Rincon, L.E. Yamin, J.F. Correal, J.G. Martinez, J.D. Sandoval, C, D. Gonzalez, C.C. Angel, Seismic retrofitting of existing earthen structures using steel plates, Construction and building materials, 230 (2020) 117039. [CrossRef]
D.M. Ruiz, J.C. Reyes, C. Bran, M. Restrepo, Y.A. Alvarado, N. Barrera, D. Suesca. Flexural behavior of rammed earth components reinforced with steel plates based on experimental, numerical, and analytical modeling. Construction and Building Materials. 320 (2022) 126231. [CrossRef]
T.-L. Bui, T.-T. Bui, Q.-B. Bui, X.-H. Nguyen, A. Limam, Out-of-plane behavior of rammed earth walls under seismic loading: Finite element simulation, Structures 24 (2020) 191–208. F: Limam, Out-of-plane behavior of rammed earth walls under seismic loading. [CrossRef]
Tavares, A. Costa, H. Varum, Common pathologies in composite adobe and reinforced concrete constructions, J. Perform. Constr. Facil 26 (4) (2012) 389–401. [CrossRef]
H. Varum, D. Silviera, C. Figeiredo, A. Costa, Structural Behaviour and Retrofitting of Adobe Masonry Buildings, Structural Rehabilitation of Old Buildings. Building Pathology and Rehabilitation. 2 (2014) 37-75. [CrossRef]
N. Tarque, M. Blondet, J. Vargas-Neumann, R. Yallico-Luque, Rope mesh as a seismic reinforcement for two-storey adobe buildings, Bulletin of Earthquake Engineering (2022) 1-26. [CrossRef]
R. Rincon, J.C Reyes, J. Carrillo J, A. Clavijo-Tocasuchyl, Empirical fragility assessment of adobe and rammed earth walls subjected to seismic actions, Earthquake Engineering & Structural Dynamics. (2022)1-25. https://onlinelibrary.wiley.com/doi/full/10.1002/eqe.3608. 3608.
Ruiz, D. M. , Barrera, N., Reyes, J. C., Restrepo, M., Alvarado, Y. A., Lozada, M., & Vacca, H. A. (2023). Strengthening of historical earthen constructions with steel plates: Full-scale test of a two-story wall subjected to in-plane lateral load. Construction and Building Materials, 363, 129877.
NTE, E. 080: Norma Técnica de Edificación. “Adobe Peruvian Code”. In: Spanish. Lima, Peru: 2000.
Standards Australia. The Australian Earth Building Handbook. Sydney, AU: 2002.
D Lhem. Lehmbau Regeln: Begriffe Baustoffe Bauteile. 2013.
AIS - Asociación Colombiana de Ingeniería Sísmica. Norma AIS-610-EP-2017: Evaluación e intervención de edificaciones patrimoniales de uno y dos pisos de Adobe y Tapia Pisada. 2017.
Bossio, S. , Blondet M., Rihal S. (2013). Seismic behavior and shaking direction influence on adobe wall structures reinforced with geogrid, Earthq. Spectra. 29. 59–84. 84. [CrossRef]
Yamín L., E. , Phillips C., Reyes J. C., and Ruiz D. (2007). Estudios de vulnerabilidad sísmica, rehabilitación y refuerzo de casas en adobe y tapia pisada. Apuntes: Revista de Estudios Sobre Patrimonio Cultural, 20(2), 286–303. https://repository.javeriana.edu.co/handle/10554/23002. 1055. [Google Scholar]
Blondet M., D. Torrealva, F. Ginocchio, J. Vargas, J. Velásquez (2006). Seismic reinforcement of adobe houses using external polymer mesh, in: 8th US Natl. Conf. Earthq. Eng. 2006, 2006, pp. 4223–4232.
Charleson, A. , Blondet M., (2012). Seismic reinforcement for adobe houses with straps from used car tires, Earthq. Spectra. 28. 511–530. [CrossRef]
Liu, K. , Wanga M., Wang Y.. (2015). Seismic retrofitting of rural RE buildings using externally bonded fibers. Construction and Building Materials 100 (2015) 91–101. [CrossRef]
Miccoli, L. , Müller U., Pospíšil S. (2017). RE walls strengthened with polyester fabric strips: experimental analysis under in-plane cyclic loading, Constr. Build. Mater. 149, 29–36. 36. [CrossRef]
Hamilton, H.R. , McBride J., Grill J. (2006). Cyclic testing of RE walls containing post-tensioned reinforcement, Earthq. Spectra. 22, 937–959. 22. [CrossRef]
Cassese, P. , Balestrieri, C., Fenu, L., Asprone, D., & Parisi, F. (2021). In-plane shear behaviour of adobe masonry wallets strengthened with textile reinforced mortar. Construction and Building Materials, 306, 124832.
López, C. , Ruiz, D., Jerez, S., Aguilar, S., Torres, J., and Alvarado, Y. (2020). Seismic behavior of RE buildings reinforced with wood elements and an upper concrete beam. Informes de la Construcción, Vol. 72, 559, e347. july-september 2020. [CrossRef]
Ruiz, D. , López C., Unigarro S., and Domínguez M. (2014), Seismic Rehabilitation of Sixteenth- and Seventeenth-Century RE–Built Churches in the Andean Highlands: Field and Laboratory Study. Journal of Performance of Constructed Facilities, 29(6): 04014144-1 - 04014144-17. [CrossRef]
Ruiz, D. M. , Barrera, N., Reyes, J. C., López, C., & Restrepo, M. (2022). An approach to Colombian Andean earthen architecture in urban environments: a case study of Bogotá Historic Centre. Journal of Architectural Conservation, 1-25.
Ruiz, D. M. , Barrera, N., Reyes, J. C., Alvarado, Y. A., Villalba-Morales, J. D., Gómez, I. D., Vacca H.A & Carrasco, D. (2023). Bi-axial shaking table tests to evaluate the seismic performance of two-story rammed-earth walls retrofitted with steel plates. Bulletin of Earthquake Engineering. [CrossRef]
ASTM-American Society for Testing and Materials. ASTM D422. Standard test method for particle-size analysis of soils. 2007.
ASTM-American Society for Testing and Materials. ASTM D4318. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. 2017.
ASTM-American Society for Testing and Materials. ASTM D698. Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort. 2021.
ASTM-American Society for Testing and Materials. ASTM A370-17. Standard test methods and definitions for mechanical testing of steel products. 2017a.
Avila F, Puertas E, Gallego R (2020) Characterization of the mechanical and physical properties of unstabilized RE: a review. Constr Build Mater 270:121435. [CrossRef]
Harris H, Sabnis G (1999) Structural modeling and experimental techniques, 1st edn. CRC Press, Boca Raton.
Reyes, J. C. , Galvis F., Yamín L., Gonzalez C., Sandoval J.D., Heresi P. (2019). Out-of-plane shaking table tests of full-scale historic adobe corner walls retrofitted with timber elements. Earthquake Engineering and Structural Dynamics. 48:888–909. [CrossRef]

Figure 1. a) Typical façade of an Andean heritage earthen house b) Reinforcement of the earthen wall with steel plates.

Figure 2. Typical Andean earthen house.

Figure 3. Two story scaled 1:4 earthen house tested.

Figure 5. Test Set Up. a) Unreinforced model; b) reinforced model. Link for QR code 360° Unreinforced model: https://pano.autodesk.com/pano.html?mono=jpgs/183016e4-f29c-4ede-95c9-bfe33a7024d3&version=2. Link for QR code 360° Reinforced model: https://pano.autodesk.com/pano.html?mono=jpgs/6d32568d-f123-4ce6-9d34-4675c3ce6dc1&version=2.

Figure 6. Location of accelerometers A to H.

Figure 7. Damage patterns of unreinforced (left) and reinforced specimens (right).

Figure 8. Acceleration in

y

-direction recorded in point D.

Figure 8. Acceleration in

y

-direction recorded in point D.

Figure 9. Un-reinforced prototype: a)

y

-acceleration profile, b)

x

-acceleration profile.

Figure 9. Un-reinforced prototype: a)

y

-acceleration profile, b)

x

-acceleration profile.

Figure 10. Reinforced prototype: a)

y

-acceleration profile, b)

x

-acceleration profile.

Figure 10. Reinforced prototype: a)

y

-acceleration profile, b)

x

-acceleration profile.

Figure 11. Main characteristics of the three wall models being compared.

Table 1. Physical characteristics of RE material.

Reference	Maximum dry density $γ_{d m a x}$ , $k N / m^{3}$	Optimum water content $w, %$	Atterberg’s limits, %			Passing sieve, %
Reference			LL	PL	PI	#200	#50	#4
Average present study	17	16	32	19	12	82	95	100
[21]	17	19	32	21	11	65	76	86
[8]	18	15	31	18	13	69	82	93
[9]	16	17	33	17	16	77	91	100
[15]	16	17	33	17	16	78	91	100

L L = Liquid Limit, P L = Plastic Limit, P I = Plasticity Index

Table 2. Compressive strength.

Test	Compressive strength, MPa
Average this research	2.07 (CV 16%)
[21]	0.55
[2]	0.50
[8]	1.11 (CV 8%)
[36]	Range from 15 references: 0.81 ~ 2.46
[9]	0.62 (CV 12%)
[15]	1.29

Table 3. Similarity relations (gravity forces neglected) for some physical characteristics for a 1:4 scale.

Physical variable	Similarity factor	Model (reduced-scale)	Prototype (scale 1:1)	Ratio for scaling
Spectral displacement, or length, or displacement	¼	$L_{m}$ $; ∆_{m}$ ; $S_{d m}$	$L_{p}$ $; ∆_{p}$ ; $S_{d p}$	$L_{m} = \frac{L_{p}}{4}$ $; ∆_{m} = \frac{∆_{p}}{4}$ ; $S_{d m} = \frac{S_{d p}}{4}$
Time	¼	$t_{m}$	$t_{p}$	$t_{m} = \frac{t_{p}}{4}$
Acceleration	4	$A_{m}$ ; $S_{a m}$	$A_{p}$ ; $S_{a p}$	$A_{m} = 4 A_{p}$ ; $S_{a m} = {4 S}_{a p}$
Frequency	4	$f_{m}; ω_{m}$	$f_{p}; ω_{p}$	$f_{m} = {4 f}_{p}; ω_{m} = {4 ω}_{p}$
Stress	1	$σ_{m}$	$σ_{p}$	$σ_{m} = σ_{p}$

Table 4. Protocol for the shaking table tests. Adapted from [8,31,38].

Step	Input record (Station name)	Exc. Prob. in 50 years	$M_{W}$ (Dist.)	$P G A$ ( $y$ -direction) $/ g$		$P G A$ ( $x$ -direction) $/ g$
Step	Input record (Station name)			Prototype	Model	Prototype	Model
1	Quetame 2008 (Bogota –Vitelma)	80%	5.9 (55 km)	0.14	0.56	0.11	0.44
2	Loma Prieta (scaled), 1989 San Francisco Int. Airport	20%	6.9 (59 km)	0.38	1.52	0.29	1.16
3	Loma Prieta (scaled), 1989 San Francisco Int. Airport	10%	6.9 (59 km)	0.43	1.72	0.34	1.36
4	Loma Prieta (scaled), 1989 San Francisco Int. Airport	2%	6.9 (59 km)	0.53	2.12	0.39	1.56
5	Loma Prieta (scaled), 1989 San Francisco Int. Airport	*	6.9 (59 km)	0.76	3.04	0.55	2.22
6	Loma Prieta (scaled), 1989 San Francisco Int. Airport	**	6.9 (59 km)	1.02	4.08	0.78	3.12

*This ground movement corresponds to 150% the record with a probability of exceedance of 2% in 50 years; **This ground movement corresponds to 200% the record with a probability of exceedance of 2% in 50 years

Table 5. Comparison of the average in-plane accelerations (first and second floor) of a 1:1 scale prototype estimated from the data of the present study and the reference [31].

$P G A$ in-plane, g	Level	Prototype (scale 1:1) acceleration, g		Ratio $\frac{R e f e r e n c e [31]}{Present study}$
$P G A$ in-plane, g	Level	Reference [31]	Present study	Ratio $\frac{R e f e r e n c e [31]}{Present study}$
0.55	Floor 2	0.82	0.83	99%
0.55	Floor 1	0.69	0.67	102%
0.39	Floor 2	0.78	0.75	104%
0.39	Floor 1	0.65	0.57	113%
0.34	Floor 2	0.66	0.69	96%
0.34	Floor 1	0.49	0.52	94%
0.29	Floor 2	0.52	0.60	87%
0.29	Floor 1	0.39	0.44	87%
0.11	Floor 2	0.23	0.25	92%
0.11	Floor 1	0.16	0.18	88%
Average ratio				96%

Table 6. Comparison of the average out-of-plane accelerations (first and second floor) of a 1:1 scale prototype estimated from the data of the present study and the references [31] and [8].

$P G A$ out-of-plane, g	Level	Prototype acceleration, g			Ratio $\frac{R e f e r e n c e [31]}{Present study}$
$P G A$ out-of-plane, g	Level	Reference [31]	Present study	Reference [8]	Ratio $\frac{R e f e r e n c e [31]}{Present study}$
0.76	Floor 2	1.33	1.07	1.65	125%
0.76	Floor 1	1.07	0.93	----	115%
0.53	Floor 2	0.92	0.90	1.15	102%
0.53	Floor 1	0.68	0.72	----	95%
0.43	Floor 2	0.76	0.78	0.83	98%
0.43	Floor 1	0.56	0.62	----	91%
0.38	Floor 2	0.62	0.71	0.83	88%
0.38	Floor 1	0.47	0.52	----	89%
0.14	Floor 2	0.25	0.29	0.35	87%
0.14	Floor 1	0.19	0.21	----	88%
Average ratio					98%

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