In the Iranian city of Bam in 2003, Bam Citadel—one of the world’s largest adobe forts and a UNESCO World Heritage Site—was devastated by an earthquake. Not only was the citadel almost leveled, but more than 85 percent of the adobe brick houses in Bam were damaged. There are, of course, other vivid examples of heritage loss in recent years: the L’Aquila, Italy, earthquake of 2009; Haiti’s 2010 earthquake near Port-au-Prince; the 2011 Christchurch, New Zealand, earthquake with the loss of its namesake cathedral; monasteries and pagodas in Myanmar destroyed by the 2012 Shwebo earthquake; and severe damage to Spanish-era churches on the Philippine island of Bohol in 2013. Earthquakes continue to cause immense damage to built cultural heritage.
Built heritage is exposed to various natural hazards, but seismic events are unique in that—unlike floods, storms, and fires—there is no warning, and thus the loss of life can be staggering. And earthquakes cause damage not just from shaking but also from related hazards. During the 1964 Alaska earthquake there was significant damage from soil liquefaction. The Erwang Temple in Dujiangyan, China, was extensively damaged by a landslide caused by the 2008 Wenchuan earthquake. A tsunami following the Great East Japan earthquake of 2011 swept away entire villages. The fire that devastated historic neighborhoods of wooden houses following the 1995 Kobe earthquake, also in Japan, illustrates the increased vulnerability of cultural heritage due to the interruption of essential services following a major seismic event.
An overlay of World Heritage Sites on a map of earthquake hot spots of the world reveals that many of these sites are vulnerable to earthquakes. Therefore, effective measures must be taken to reduce seismic risks. The importance of thorough methodologies for assessing earthquake damage and of appropriate measures for their mitigation, preparedness, and recovery is recognized, but these measures have been insufficiently developed and implemented.
The 1987 publication Between Two Earthquakes: Cultural Property in Seismic Zones by Sir Bernard Feilden—one of the most respected conservation architects of his day—made a pioneering contribution in this area. This brief book is groundbreaking in its accessibility, speaking to multidisciplinary groups of practitioners in a way that is easy to grasp. And it focuses with prescience on the task of preserving cultural heritage in the face of earthquakes. The book’s title conveys the message that mitigation and preparedness are critical to earthquake risk reduction. Restoration and strengthening measures that follow an earthquake should serve to mitigate and prepare for the inevitable next quake.
Our built cultural heritage is increasingly vulnerable to earthquakes for many reasons. A major factor is poor or inadequate maintenance. Seismic events instantly expose the weaknesses in building structures. The oft-quoted adage “Earthquakes don’t kill people, buildings do” remains true, and its meaning is well illustrated by comparing the experiences of Haiti and Chile. Whereas the 7.0 magnitude earthquake in Haiti caused more than one hundred thousand deaths because of the poor conditions of buildings, a much stronger 8.8 magnitude earthquake in Chile caused fewer than six hundred casualties. Rampant termites in Port-au-Prince weakened normally resilient wood-framed Gingerbread houses; dilapidated unreinforced masonry structures also fared very poorly.
Vulnerability can also result from inappropriate repair and additions to heritage structures using incompatible materials and construction techniques that adversely impact structural integrity, causing damage during earthquakes. This dynamic was demonstrated at the Prambanan temple compound, a UNESCO World Heritage Site in Indonesia, where the previous addition of a concrete frame understructure affected the performance of the temples during the 2006 Java earthquake, leading to cracking, splitting, and dislodging of stone units. Haiti’s Gingerbread houses were damaged by post-1925 additions of concrete and concrete blocks, which oscillated differently from the wood frames and operated like battering rams.
A further cause of vulnerability is the tendency of engineers to maximize structural strength to ensure occupant safety. Overly high standards for strength can result in increased interventions that may actually contribute to a building’s destruction, as well as having the unfortunate side effect of sacrificing heritage values. Excessive strengthening may have the unintended effect of causing catastrophic brittle behavior, which counters the capacity of resilient heritage structures to absorb and dissipate forces and can lead to immediate collapse. Resilient structures may be damaged during a seismic event but allow time for people to evacuate before the structure collapses. Safety is important, and we want to save our heritage, but in some instances we need to protect this built heritage from well-intended but inadvertently destructive interventions.
This principle is demonstrated by the evolution of building practices leading to greater resiliency of some traditional structural types in earthquake-prone regions, as a result of knowledge accumulated through trial and error. Significant examples include dhajji dewari construction in Kashmir, himis construction in Anatolia and the Balkans, and circular bhungas in Gujarat in India. Making safety and heritage values paramount is a good strategy. It may allow for economic affordability as more structures can be strengthened using locally known and available resources rather than maximizing safety with more sophisticated and expensive techniques in relatively fewer structures. This is important in developing countries, where there is a severe scarcity of financial and skilled human resources.
The misconception that traditional constructions are inferior has become prevalent in the developing world. At the recently fire-damaged Wangduephodrang Dzong in Bhutan, the debate continues about whether to rebuild and retrofit using traditional methods and materials, including stone, clay mortar, and horizontal timbers, or to use concrete or steel, which are perceived as superior. On Majuli island in the state of Assam in India, traditional houses constructed on bamboo stilts performed well against earthquakes, in addition to being flood resistant. However, the recent use of concrete for the stilts has made these structures vulnerable to earthquakes.
Governmental agencies tend to apply the principles of professional engineering to the evaluation of vernacular structures. As a result, some heritage structures may be categorized as unsafe and worthy of demolition. This judgment has been made in many post-earthquake damage assessments conducted by national and international teams of experts, as in Bhutan following the 2009 earthquake.
Cultural heritage is not static, and we continue to add new kinds of built cultural heritage stock. The Industrial Revolution brought about the use of structural metals, concrete, cement mortars, and composite materials. Engineered structures utilized braced and moment frames that allowed the realization of tall buildings and trusses to cover immense interior spaces. This sophisticated built heritage poses unique challenges and is no less vulnerable to earthquakes.
Assessment and Post-Quake Intervention
Feilden’s book suggests a template for designing damage assessment forms. Experience has shown that such templates should be tailored to building materials, construction systems, relative building condition, geotechnical aspects, patterns of past repair efforts, and hazards to which building stock has been exposed, as well as to cultural aspects. Assessment should be seen as a continuum that includes:
- initial inspections immediately following the emergency, which are aimed at recording the damage level and undertaking immediate action, such as temporary shoring and bracing;
- more detailed assessments for determining the best options for recovery—restoration, rehabilitation, strengthening, and/or repair, along with a comprehensive work plan for implementation;
- linkage of these assessments to dynamic and comprehensive vulnerability and risk assessments undertaken during mitigation, preparedness, and response, as well as recovery stages. This would also help in the effective monitoring and maintenance of heritage buildings and sites.
Depending on the nature of the damage, post-earthquake treatments of heritage structures may include repairs, restoration, and retrofitting. The scope of these interventions may differ between heritage conservation and engineering perspectives. From the heritage conservation standpoint, treatment may imply reinstating the heritage values while, from the engineering perspective, treatment would lean toward reinstating original structural strength. Reconciling these differences presents a challenge in formulating appropriate post-quake interventions. It must be remembered that any intervention, no matter how slight, will cause loss of heritage fabric, and we should strive to minimize this loss.
Modern research has produced a great variety of techniques to retrofit existing buildings. The majority of these techniques are designed for modern buildings rather than traditional ones, which is not surprising since built heritage represents a small percentage of building stock. With the goal of saving heritage values in mind, there are compelling reasons to start with traditional materials and techniques to strengthen built heritage damaged by earthquakes.
The built heritage that is most vulnerable simply because of its material nature is unreinforced masonry. This includes bricks and stone, adobe, rammed earth, and unreinforced concrete. In North America, cement mortars are ubiquitous and are appropriate for the repair of industrial-era heritage, where cement was the original mortar ingredient. However, cement is a disastrous repair material for buildings constructed with clay, lime or gypsum mortars, plasters, and stuccos. Repair materials must be compatible with the original materials in hardness, density, porosity, permeability, elastic modulus, and moisture expansion. They must weather in the same way as the original materials. If newer, stronger materials are bonded to older, weaker ones, the stronger one will “win” during a seismic event. Another concern is that new materials must not introduce foreign salts that can cause efflorescence or subflorescence.
Buildings composed of soft masonry materials, such as low-fired brick in lime mortar, adobe, and rammed earth, can absorb seismic shock. In contrast, contemporary buildings are typically designed to resist seismic shock. Strengthening systems that are appropriate for contemporary buildings may therefore be damaging when applied to heritage structures realized in traditional materials. Strengthening systems must be compatible in stiffness, flexibility, and deformability. Traditional strengthening techniques should be studied at the regional level and accepted where they prove effective.
If traditional strengthening techniques are not effective, then modern techniques must be considered, either on their own or in conjunction with traditional techniques. Modern materials include metal, concrete, fiberglass, and carbon fibers. Some modern techniques are buttressing, bracing, and introducing moment or braced frames, shear walls, and energy dissipation devices. With unreinforced masonry, these systems should work to keep the masonry in the compression zone, since masonry does not perform in tension.
A word must be said about the “bad boy” of heritage conservation: reinforced concrete. This technology had made its debut by the beginning of the twentieth century. As a material it is plentiful, easy to transport, and relatively easy to install in fluid form. Like it or not, reinforced concrete is an icon of modern development, and the desire to use it in tandem with heritage structures in the developing world must be recognized. However, as Feilden wrote, “a blind use of reinforced concrete can be disastrous.” This statement implies that concrete is not the culprit, but the inappropriate use of it is.
Seismic base isolation is a potentially attractive seismic strengthening technique. With base isolation, a major part of the earthquake energy that would have been transferred into the building is absorbed at the base level. Consequently, deformability demand on the structure is reduced; displacement is strictly controlled by an appropriate amount of damping within the base isolation system, and the frequency of the isolated structure is decreased to a value below that which dominates in a typical earthquake. This may eliminate the need for intervention above the level of isolation. However, this approach has two disadvantages: it is costly, and it requires significant intervention on the cultural layer of the surrounding soil. The technique has been used extensively in the western United States, where neither of these disadvantages is overriding.
Other factors affecting the selection of seismic retrofits include affordability and availability of feasible options. These factors are illustrated by the following range of interventions that were suggested for retrofitting following the 1995 Kobe earthquake: (a) additions using traditional materials and traditional techniques—for example, reinforcement by palm tree rope; (b) additions using traditional techniques and modern materials—for example, reinforcement by carbon fiber sheet; (c) additions using modern techniques and modern materials—for example, burden share by iron frame; and (d) replacement using modern techniques and modern materials—for example, introduction of a base isolator.
Built heritage is exposed to more than one hazard. Therefore, reduction in vulnerability to earthquakes may, in fact, increase the vulnerability of cultural heritage to other hazards. For example, during the March 2011 disaster that struck Japan, construction of light wooden houses made them resistant to earthquakes but increased their vulnerability to tsunami waves. As the ultimate goal is to compromise as little of the heritage fabric as possible, any post-earthquake intervention should be undertaken with a multi-hazard perspective.
Benjamin Franklin famously said, “An ounce of prevention is worth a pound of cure.” This axiom was clearly illustrated in Feilden’s book. Since then, we have developed at least a pound of cure—and reacting to earthquakes is addressed in more than one annual conference on the topic. But it is only recently that we have begun to seriously focus on that ounce of prevention. Rather than the expenditure of resources solely for post-earthquake interventions on built cultural heritage, there needs to be more emphasis on pre-earthquake mitigation and preparedness that takes into consideration multiple hazards, minimal loss of heritage values, and affordability. Most importantly, adequate maintenance and monitoring are keys to reducing earthquake risks to cultural heritage.
Stephen Kelley, FAIA, a heritage conservation specialist in the United States, was president from 2008 to 2014 of the International Scientific Committee on the Analysis and Restoration of Structures of Architectural Heritage. Rohit Jigyasu, a conservation architect currently at Ritsumeikan University in Japan, is president of the International Scientific Committee on Risk Preparedness.