Seismic Retrofitting Techniques for Historic Adobe Buildings

INTRODUCTION

Research Context and Objectives

This study is the first extensive analysis of all seismic retrofitting techniques available for historic adobe buildings. While research has been conducted on individual techniques, most techniques were not developed specifically for historic structures and have not been evaluated for such a usage. The presented state of the art is based on a comprehensive review of published research about all techniques that could be applied to historic adobe buildings. These findings will help conservation practitioners select the most optimal approach to address challenges in the conservation of earthen heritage.

Católica de Perú, and the Peruvian Ministry of Culture in which in-depth analysis of four historical earthen buildings in Peru is being conducted based on elaborate material testing and numerical modeling. The final aim of this initiative is to establish ideal retrofitting plans for these prototype buildings and to provide general guidance for the implementation of effective and ideally low-tech retrofitting solutions that preserve the authenticity of buildings and can be applied in countries where advanced equipment, materials, and technical skills are not readily available.

Literature Review

The starting point for the presented literature review was the project bibliography of the SRP (Cancino et al. 2013). This bibliography compiles all published works known to the project members relating to earthen architecture in seismic areas worldwide. Reviewed literature includes, but is not limited to, the proceedings of a series of international conferences on the study and conservation of earthen heritage and the structural analysis of historical constructions (Terra, SIACOT, SAHC, STREMAH) and internationally published articles on the same subjects. This paper considers relevant works regarding retrofitting or structural stabilization of adobe and applies these works to historic buildings, also taking into consideration a series of recommendations of professionals at the 2013 SIACOT (Seminario Iberoamericano de Arquitectura y Construcción con Tierra) Conference in Valparaìso, Chile (August 28-30, 2103). In addition, policies, standards and guidelines related to earthen buildings and the strengthening of historic structures were analyzed to provide the theoretical framework in Section 1.3. While the reference list in this article is limited to the works that are directly cited, a series of publications and articles have contributed indirectly. All these can be found in the bibliography of the SRP-project (Cancino et al. 2013).

Guiding Principles for Seismic Retrofitting of Historic Adobe Buildings

Adobe buildings, typically classified as unreinforced masonry structures, are extremely vulnerable to earthquakes and can be subjected to sudden collapse during a seismic event-especially if the building lacks proper and regular 1059 Downloaded by [Princeton University] at 20:22 02 December 2015 maintenance. Therefore, historic earthen sites located in seismic areas are at high risk of being heavily damaged and even destroyed, necessitating the introduction of retrofit measures to protect them and their users.

The most widely recognized international charters 1 in the field of historic preservation establish guiding principles for the conservation of historic culture, but do not specifically address either adobe buildings or earthquake damage. Nonetheless, several principles established by these charters can be applied when designing seismic retrofitting for adobe structures.

For example, the ICOMOS-ISCARSAH (ICOMOS 2003) principles stress that: full understanding of the structural and material characteristics is required in conservation practice. Information is essential on the structure in its original and earlier states, on the techniques that were used in the construction, on the alterations and their effects, on the phenomena that have occurred, and, finally, on its present state. (ICOMOS 2003, Section 2.3) Indeed, before deciding upon any type of retrofit measure for an adobe building professionals must conduct an interdisciplinary analysis and structural assessment so that they are familiar with heritage values, construction techniques, and modes of failure.

If an intervention is deemed indispensable, established conservation conventions dictate that the integrity and authenticity of the building must be understood in its local and historical context, and any work must comply with the accepted principles of minimum intervention, re-treatability, compatibility and the usage of historical or the same material during repair. When considering which retrofitting technique to choose Article 10 of the Venice Charter 2 makes clear that the first priority should be to try to employ traditional techniques since they will most likely offer a cost-effective, compatible solution, in some cases using original material (ICOMOS 1965). If these techniques are insufficient, other methods can be implemented, as long as their suitability, the expected remaining life of the structure and its functionality have been considered.

The recently published Lima Declaration for Disaster Risk Management of Cultural Heritage addresses the aforementioned principles in the context of major environmental threats such as earthquakes (ICOMOS 2010). It emphasizes that earthquakes cause cumulative damage to structures. This consideration should therefore be taken into account during the initial assessment phase. The Lima Declaration also stresses the primary importance of saving human lives. Thus, conservation professionals should seek to maintain the cultural significance of a structure while at the same time balancing the important concern of human safety.

Strength-Based Versus Stability-Based Techniques

The Getty Seismic Adobe Project 3 research distinguished two main types of retrofitting design concepts: strength-and stability-based (Tolles, Kimbro, and Ginell 2002). In order to analyze these techniques, it is important to classify the different varieties of walls. The GSAP designates adobe walls according to their height-to-thickness ratio (S L ). Categories are thick (S L <6), moderate (S L =6-8) and thin (S L >8) walls. Thin adobe walls may become unstable soon after the appearance of cracks while a thick-walled adobe building is still far from losing its stability after the first cracks develop.

Strength-based design depends on the elastic properties of the material and can accurately predict when cracks will occur. It thus aims to increase the capacity of buildings materials or elements in the elastic range and delay the formation of cracks. More important than when cracks form, however, is the structural ductility of a building: its ability to maintain its loadcarrying capacity and deform safely after the elastic limit of the building material has been exceeded. Thick-walled adobe buildings can exhibit substantial structural ductility, even though their construction material itself is brittle.

Therefore, the second approach, stability-based design, is more useful because it employs the characteristics for postelastic performance of adobe. These retrofit measures can greatly enhance overall stability and act to limit the extent of damage in the form of large permanent offsets. Therefore, both retrofitting stategies are not mutually exclusive and can be complementary: the strength-based approach may address the elastic behavior while the stability-based approach may address the postelastic performance (which is usually more important) (Tolles, Kimbro, and Ginell 2002;ICOMOS 2010).

STABILITY-BASED TECHNIQUES

Techniques to Obtain Overall Stability

The goal of stability-based techniques is to stabilize the building as a whole. They typically consist of elements that connect continuously around the entire building to make all facets of the structure work together.

Introducing a bond beam (also ring beam or collar beam) is often regarded as one of the most effective stability-based techniques to retrofit earthen buildings. A continuous beam is placed on top of the walls to prevent overturning, providing out-of-plane strength and stiffness. Additionally, they establish in-plane continuity as a continuous element along the length of the wall (Tolles, Kimbro, and Ginell 2002).

Common types of bond beams are made of timber and concrete although the timber bond beam is more traditionally used. A relatively thin 4 wooden plate on top of the wall can suffice. It is crucial that the plate goes around the entire building and that the beams on perpendicular walls are well connected at the corners ( Figure 1). Moderate to thick adobe walls (S L <8), typical of historic adobe buildings, are more stable to out-of-plane movement and consequently can be stabilized by adding limited horizontal stiffness, allowing a subtle intervention. Since timber ring beams are rather flexible, initiation of cracking takes place for an amount of loading similar to that of an unreinforced structure. This technique will thus mainly contribute to the building's performance in the inelastic range. The connection of the beam to the wall requires special detailing and can be accomplished by means of vertical dowels, steel straps or center core pins.

Figure 1

Downloaded by [Princeton University] at 20:22 02 December 2015 FIG. 7. (a) Tie beams in the church of Kuño Tambo, one of the prototype buildings of the SRP (image by C. Cancino, 2010); (b) exterior view of connections of tie beams in Cuzco (image by S. Lardinois, 2012). (a,b) © J. Paul Getty Trust. Reproduced by permission of J. Paul Getty Trust. Permission to reuse must be obtained from the rightsholder.

The integration of wooden bond beams in historic adobe buildings is common practice. Wood is a material compatible with earth, it is easily available and has a good cost-benefit value, even in remote areas. Furthermore, this type of intervention fulfills the requirement of re-treatability. Adequate measures to keep the wood dry and thereby reduce the risk of wood deterioration are necessary, however. Another inconvenience is the fact that installing the bond beam might require the removal of the roof and thus cause a loss of historic fabric.

A continuous reinforced concrete bond beam is a noncompatible alternative that has been used in many retrofitting projects in California and Cusco during the 1970s and 1980s (Samanez Argumedo 1983; Earthen Building Technologies 1995; Webster 2006). Although the introduction of a concrete beam has similar effects on the structural response as that of wooden bond beams, the main difference is that a concrete beam is much stiffer. A very stiff bond beam can cause additional damage to the walls due to the rocking of the beam against the adobe during a seismic event and, in particular, to in-plane walls by transferring a very large portion of the lateral load from the out-of-plane walls 5 (Tolles, Kimbro, and Ginell 2002). Furthermore, a concrete ring beam has been determined to be an unsuitable intervention (D'Ayala and Benzoni 2012) because 1) it requires the removal of a substantial amount of original roof framing; 2) it is rarely reversible because the connections to the earthen walls have to be very rigid (Tolles, Kimbro and Ginell 2002); and 3) it is not compatible with adobe due to excessive stiffness, causing the failure of the connection between the beam and the masonry during moderate ground motions.

The purpose of another technique, the traditional diaphragm, is to transfer loads for a given direction of motion from the roof and out-of plane walls to in-plane walls ( Figure 2). This is a typical retrofitting measure for unreinforced brick masonry. This type of masonry has much more in-plane resistance, thus forces are directed towards these walls. This is accomplished by installing a load transfer system, usually made of wood or steel. Properly designed roof or floor systems are examples of this technique.

Figure 2

Diaphragms are not advisable in buildings with thick adobe walls. As mentioned previously, thick walls are very stable in the out-of-plane direction, but their in-plane capacity is low. Therefore, a stiff diaphragm can cause overloading of the perpendicular walls. For thin adobe walls a diaphragm can be more useful. Nevertheless, the calculation of the distribution of the loads between the out-of-plane and in-plane walls is a delicate part of the design.

A partial plywood diaphragm is a flexible variant of traditional rigid diaphragms. A 1.20-m width diaphragm of plywood is nailed along the top of the joists (Tolles, Kimbro, and Ginell 2002). Chord members should be provided similar to those used in standard and rigid diaphragm design. The advantage of having a partial and flexible diaphragm is its limited stiffness that avoids excessive load transfer to the perpendicular walls that might fail in shear. The chord members provide in-plane continuity along the length of the wall. This intervention only requires a connection at the top of the wall or at floor level and can be harmoniously integrated into the building's roof or floors.

Another technique researched during the GSAP consists of tying the upper zones of walls together using horizontal steel rods. They are wrapped around the building in a groove and can be slightly tightened to precompress the wall. A major advantage is that the removal of the roof is not required for installation. By providing interlocking between walls, the rods will inhibit overturning and thus increase resistance to horizontal loads (Barrow et al. 2009). As they create a continuous horizontal zone that works together, extra in-plane continuity is obtained as well. Since the rods add little extra stiffness to the structure, they might be combined with a low-stiffness diaphragm, for instance made of plywood ( Figure 3). This can be a subtle and cost-effective intervention that can be applied below the outer plaster in a minimally invasive fashion. Because steel properties are well understood precaution can easily be taken to prevent common problems such as corrosion. Relaxation due to thermal expansion can be an issue as well, but an accessible buckle to tighten the steel rod can solve this possible problem. One could argue, however, that the introduction of steel is less desirable in an earth-wood composite building leading to a potential loss of authenticity.

Figure 3

The GSAP project has also analyzed the application of a more elaborate system of steel or nylon rods and straps in which a combination of horizontal and vertical straps are wrapped around the walls. Pre-tensioning the cables is not required, but straps should be tightened to eliminate slippage. They should be connected to the roof, floor framing or foundation to provide continuity. Horizontal straps can be placed at the walls' exterior and are useful in the upper and lower zones. In the upper zone they have a similar effect as the previously described horizontal steel rods. In the lower section, they prevent kicking out in-plane along the length of the wall. Vertical straps can be very useful for thin walls because they increase their ductility. The straps should be attached to both the interior and exterior wall surfaces and should be combined with upper and lower wall elements such as ring beams or horizontal straps (Figure 4).

Figure 4

Thick adobe walls might be stabilized with a limited amount of cables, which promotes the reversibility of the intervention. Holes, however, need to be drilled through the walls and renderings might have to be replaced, leading to a loss of original fabric. The cost of this intervention is relatively high and material availability in remote regions might also be prohibitive.

Center-core rods present an alternative to vertical straps. They are used as reinforcing elements to prevent out-of-plane failure. Rods are grouted in oversized holes using an epoxy, polyester, or cementitious grout. The use of these vertical elements can greatly increase the ductility of especially thin adobe walls. In thicker walls, center-core elements tend to act as shear dowels, rather than as flexural reinforcement. The diameter of center-core rods can range from 12 to 25 mm. It is important that small-diameter rods and holes less than 50 mm are used because larger-diameter center-core elements may act as hard spots and cause a split in low-strength adobe walls. Installing center core rods does not interfere with any visual aspects of the building, but is a hardly reversible and difficult-toinspect intervention. Nevertheless, if the dowels are thin enough to avoid hard spots, steel dowels glued in with grout or epoxy are a durable solution since the grout or epoxy will protect the dowels from corrosion. Although this technique is expensive, detailed research about its application is available (Tolles, Kimbro, and Ginell 2002). It might be interesting to use more traditional, accessible and less expensive materials such as cane or wood as suggested by research conducted at PUCP (Blondet, Torrealva, and Villa Garcia 2002).

Another technique is the application of different types of meshes. These can be applied externally, internally or on both sides of walls. The entire wall can be covered, but the mesh can also be applied in several strips. Torrealva, Vargas Neumann, and Blondet (2006) suggests that a minimum of 50% of the wall area should be covered with mesh for the technique to be effective. The mesh can be applied in strips of about 45 cm wide, creating beams and columns of composite material (Torrealva, Vargas Neumann, and Blondet 2006). The goal is to create a tight mesh-like matrix providing restraint to out-of-plane rocking and avoiding the overturning of wall panels. Since the mesh creates continuous reinforced zones, it also provides in-plane continuity. The mesh is fixed to the wall with nails or soda caps and is most effective when the inner and outer meshes are connected with cross ties through the wall. Covering the wall with a rendering will increase the initial shear strength and stiffness of the wall. If the mesh is not covered it will only become active when the wall is cracked, limiting the relative displacement of the various parts formed by the cracks. It is vital though that the correct rendering is applied. Earthen renderings are strongly advised to address concerns of compatibility. Limebased renderings are adequate because of their permeability, but cement renderings are hazardous because they are excessively stiff and inhibit vapor transport.

Different materials can be used for the mesh. Research has been conducted on employing steel welded wire meshes, polymer meshes and the low cost combination of bamboo with steel welded wire (Torrealva, Cerrón, and Espinoza 2008;Dowling 2004Dowling , 2006. Polymer meshes ( Figure 5) have been researched thoroughly in Peru and have been applied for instance in the church of San Pedro de Esquiña in Chile (Heinsen et al. 2012). This type of mesh seems to have the most potential due to its great durability and compatibility with earth (Torrealva 2008;Torrealva, Vargas Neumann, and Blondet 2006). Applying meshes is an advantageous solution because it doesn't interfere with the original structural system, nor affect the general layout or perception of the building. Renderings, however, have to be removed and reapplied, which can be problematic when they contain valuable wall paintings. Material availability and costs are dependent on the material chosen for the mesh.

Figure 5

Another stability-based retrofitting technique that is noteworthy but unsuitable for heritage structures is the application of a mesh of rubber straps around the building. These straps possess a large ductility that allows the reinforced adobe to undergo significant horizontal displacements without collapsing (Charleson 2009). These straps, however, are so flexible that they do not reduce harm in low to moderate intensity shaking, allowing irreparable damage to the structure. Thus, buildings would need to be reconstructed after an earthquake, which is unacceptable. Yamin et al. (2004) present research that promotes the use of boundary wooden elements. Wooden plates with a section of 15 by 2 cm are placed internally and externally against the walls, connected with through bolts. The holes for the bolts are filled with cement mortar. The horizontal elements are connected around the corners of the horizontal walls with steel plates. The planks are nailed every 15 cm to the underlying earthen walls to create rough surfaces. The entirely new wooden frame adds significant stiffness and strength. It provides restraint against out-of-plane rocking and in-plane-continuity, while the steel connections provide good interlocking between walls. The technique might be less applicable to historic buildings, however, since not only are the renderings strongly affected, but the underlying fabric is also pierced. Additionally, the structural system of the building is changed so substantially that the building's authenticity can be greatly compromised. Lastly, the durability of the technique depends on the wood species selected.

Finally, base isolation is a complex retrofitting technique based on a simple principle: the building is disconnected from ground movement by introducing layers of isolation and thus relative displacements occur at the level of the isolation system. By lengthening the structure's period and increasing the damping, the seismic response decreases allowing the building to almost keep its undeformed state. Useful examples of base isolation incorporated in vernacular adobe architecture in Iran and Indonesia are available (Naderzadeh 2009;Pudjisuryadi, Lumantarna, and Lase 2007). In Lahijan, Iran for instance, the foundation of certain traditional houses is composed of timber elements that can roll on each other and dissipate earthquake induced energy ( Figure 6). The adobe walls are then constructed on top of the wooden foundation (Naderzadeh 2009). These examples demonstrate that base isolation can, and has been applied traditionally to historic earthen buildings. However, the technique was always incorporated in the original structure, not as a retrofitting technique on historic adobe buildings. Application as a retrofitting technique would be effective but high-tech and costly as it would require the uplifting of the entire building. The great advantage of the technique, however, is that it allows the building to remain virtually untouched above foundation level, except for the introduction of flexible systems to accommodate the relative displacement of the structure with respect to the ground.

Figure 6

Techniques to Stabilize Building Elements

All the techniques detailed in this section stabilize or support one building element, or connect walls together, in contrast to the previous methods that stabilize a building as a whole. Therefore, the usage of only one type of the following techniques will seldom be sufficient. Techniques to stabilize wall-to-wall connections are very common within this category and can be split into two types. The first type contains horizontal elements that connect parallel walls to enhance lateral support and forestall overturning. These elements are commonly applied in long spaces with few transversal walls. Typically wooden tie beams or steel tie rods are installed in historic adobe buildings either before or after earthquake damage. Tolles et al. (2000) stress that a crucial element is that these ties are anchored properly to the walls, otherwise they might induce local stress concentrations causing cracking. The installation of tie-rods can add needed support, but may also create unforeseen extended damage because the forces generated locally can be very large. Wooden tie beams have been applied in multiple buildings for example in the Andes (Figure 7). The fact that this intervention has been widely used, can be implemented in compatible wood and can be incorporated without affecting the roof, make it a valuable technique that fulfills the requirements for effective conservation. Research is currently being conducted by the SRP to improve understanding of this method.

Figure 7

The second type of wall-to-wall connections addresses the anchoring of perpendicular walls by means of regrouting and doweling. According to Toles, Kimbro, and Ginell (2002), the connections between perpendicular walls can have sufficient strength to withstand moderate ground motions if the original construction consists of overlapping bricks or contains overlapping reinforcements. Regrouting or doweling of existing cracks gives the structure some ability to withstand moderate seismic activity. Dowels can prevent separation to a certain degree, yet their efficacy can be limited during an earthquake due to the differences between in-plane and out-of-plane motions, especially of thick adobe walls. Thus, although regrouting or doweling may result in a significant benefit during moderate ground motions, they will have little effect during strong ground motions, when damage will occur at or near the junction of walls, beyond the location of the dowels. Toles, Kimbro, and Ginell (2002) therefore conclude that local anchorages could be an effective means of limiting damage but that they should be combined with a solution providing global stability. For instance, a bond beam or strapping could be employed so the complete set of measures could be effective during major seismic events. Regrouting is a technique that could be considered maintenance within the framework of conservation principles. Doweling requires only small holes to be drilled into the historic fabric, although grouted dowels are very hard to remove, making it difficult to reverse the introduction of less compatible steel.

Perpendicular walls can also be interconnected with a traditional technique by inserting wooden elements at different heights-corner keys or braces. When the joints between perpendicular walls crack during earthquakes, the braces supply reinforcement so the walls continue to work together (Figure 8). Angulo-Ibáñez et al. (2012) present an overview of different types of bracings that are traditionally used in historic adobe buildings. As in the previously described technique of doweling, the same concern of perpendicular walls behaving inherently differently applies, and similarly, walls might crack right next to the reinforcement. No detailed research was found on the behavior of these elements, but since the technique is relatively easy to implement, has been traditionally applied in Peru and uses compatible and readily available materials, it deserves further exploration. The Pontificia Universidad Católica del Perú, as part of the SRP, has performed tests to improve understanding of this technique and results will be available soon.

Figure 8

Illustration of corner keys. (a) Schematic view of corner keys. (b) Installation of corner keys in Casa Garci Holguin, Trujillo, Peru. © 1999 J. Paul Getty Trust. Reproduced by permission of J. Paul Getty Trust. Permission to reuse must be obtained from the rightsholder. FIG. 9. (a) Adobe buttresses at the church of Esquiña in Chile. (b) Stone buttresses at the church of Parinacota in Chile. (Images by T. Michiels, 2013). © J. Paul Getty Trust. Reproduced by permission of J. Paul Getty Trust. Permission to reuse must be obtained from the rightsholder.

The use of buttresses is another traditional reinforcing technique widely employed in the Andean region. These pier-like, massive local additions of masonry provide walls with extra resistance to lateral thrusts (Figure 9). During an earthquake they act as counter support to restrain out-of-plane movement and are most effective at intermediate locations in long walls. If they are not attached to the masonry, they act independently when subjected to seismic forces (Roselund 1995). In that case, they only provide restraint when walls move towards the buttress. Moreover, they impose extra load towards the wall and can rock against it during a seismic event. Therefore, it is important to tie buttresses to the walls. Another option is to attach the buttress to a tie-beam or diaphragm. The connection can be executed by cross ties. In practice it is not always clear if a buttress is connected to the masonry. In the Andean region buttresses became common after an earthquake in 1746 when a decree from the Spanish viceroyalty made them mandatory (Walker 2008). Thus, existing historic buttresses are often later additions to churches and may not have always been properly connected to the walls they were meant to support.

Figure 9

An advantage of buttresses is that they can be made of the same material as the walls and can easily be removed since they interfere little with the original structure. That feature makes them a sympathetic, very low-cost, and easily applied technique that does not require extensive technical knowledge. The connection to the original structure is crucial, however, and this aspect is difficult to inspect or maintain and as mentioned previously, might have been disregarded in earlier retrofitted buildings. Another noteworthy concern pointed out by Roselund (1995) is that the application of buttresses, usually on different foundations, might lead to differential settlements.

STRENGTH-BASED TECHNIQUES

A last group of techniques is dedicated to improving the strength of adobe masonry.

Cement renderings or shotcrete reinforced with steel wire, polymer fibers or glass fibers are sometimes applied onto adobe masonry. These renderings are most functional when affixed on both sides of the wall with the two layers interconnected. This system increases the strength of the adobe walls and tends to absorb the seismic forces. It fails brittle when its elastic limit is reached, however, which may lead to the sudden collapse of the building. This type of disastrous failure was reported by D' Ayala and Benzoni (2012) at the parish churches of Lolol and Curepto in Chile after the 2010 Maule earthquake. Moreover, the rendering hinders moist transport through the wall, trapping moisture inside which compromises the mechanical resistance of the adobe masonry. Since this technique proved ineffective, its application should be avoided and where possible reversed.

Adobe masonry is often cracked due to previous earthquakes, foundations settlements, lack of maintenance, or shrinkage effects. In order to recreate a monolithic behavior, small cracks can be filled by grouting, which restores the transfer of stresses through the cracks. Furthermore, crack repair also prevents further decay caused by other agents, such as water infiltration and plant growth (Silva, Schueremans, and Oliveira 2009). Liquid mud grouts can be injected and several research attempts have been made to find ideal grout consistencies (Silva, Schueremans, and Oliveira 2009;Silva et al. 2012;Vargas Neumann et al. 2011;Roselund 1990). The durability and compatibility of the intervention are important aspects to consider.

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A successful grout should be weaker than the cohesion of the masonry so the original material is uncompromised and a new treatment is possible by regrouting. The insertion of a mudgrout is hardly reversible, but since no original material needs to be removed, grouting is a non-invasive intervention using sympathetic materials.

Big cracks in adobe masonry can also be repaired by a technique that involves the ramming of stabilized earth into pre-cut chases, incorporating metallic or wood reinforcement: softstitching. Chases are cut both internally and externally around the cracks and filled with rammed mud in alternating lifts every 0.5 m. Keefe (1993), Hurd (2009) and Heinsen (2012) provide detailed and illustrated repair sequences. While, to the author's knowledge, the engineering performance of this intervention has never been technically examined, this technique has been applied over the centuries in seismic regions in Central Asia and Trans-Himalaya (Hurd 2009). Although the technique requires the removal of original fabric, it allows the reuse of the original materials, it fits harmoniously into the building, it does not require many resources and it has proven in practice to be long-lasting.

CONCLUSIONS

This article provides an overview of the wide range of available retrofitting techniques for earthen buildings. It considers important principles of conservation of architectural heritage such as reversibility, compatibility and the conflict often posed between the preservation of the authenticity of the structure and its seismic retrofitting. There is no standard solution, but this paper relies on the assumption that traditionally applied techniques using compatible and low-cost materials such as wood and earth are most likely the best solution for historic adobe sites.

Traditionally, extra stability was introduced by using wooden ring beams, wooden ties connecting parallel walls, corner keys or by the addition of buttresses. These retrofitting solutions can be combined in order to improve the overall seismic response of a historic construction. Each of these techniques requires specific attention to be functional: a ring beam needs to be connected all around the building; wooden ties require special care at their connections with the walls; and buttresses should be interconnected with walls.

While a successful retrofit might be achieved by combining the traditional techniques mentioned above, the use of other newer techniques might be necessary to meet certain performance criteria or to preserve certain values. In those cases, the least invasive techniques, such as introducing a plywood diaphragm, horizontal steel rods, a geomesh covered with mud-rendering, or a strapping system are most ideal. The introduction of these methods, however, will require specialized knowhow and a site-specific approach.

ACKNOWLEDGMENTS

This article was produced during a fellowship at the Getty Conservation Institute (GCI) within the framework of the Seismic Retrofitting Project and elaborates on a research paper by T. Michiels and F. Fonseca Ferreira presented in Spanish at the 13th SIACOT Conference in Valparaïso, Chile, in 2013 (Técnicas de Estabilización Sísmoresistente para Mejorar el Compartamiento de Edificios Históricos de Tierra). The author would like to acknowledge the work of the project partners at the University College London and the Pontificia Universidad Católica del Perú, who participated in meetings where a previous version of this manuscript was presented. The author would also like to recognize the work of his GCI supervisor, Claudia Cancino, who reviewed and edited the manuscript.