Keeping vernacular architecture earthquake safe
An accepted understanding of vernacular architecture is that it embraces those buildings that are not designed by specialists, lacking any architectural or engineering project. On the contrary, they are part of a process that involves many people over many generations, relying on their empirical knowledge and reflecting the tradition and life style of a community, as well as their bonding with the natural environment.
However, precisely because of its empirical and traditional nature, people nowadays tend to see vernacular architecture as an obsolete and unsafe way of building, only valued as a key-element for a region’s cultural identity. This leads to the eventual abandonment of vernacular buildings or their substitution with modern ones, homogenizing the way of building throughout the world by providing a type of architecture that can be observed in any geography.
Besides their progressive abandonment, the scarcity of resources in generally poor communities many times result in an inadequate overall structural layout of vernacular constructions, a lack of proper constructive details and a poor maintenance. All these factors contribute to increase the vulnerability of the vernacular heritage facing natural disasters, including earthquakes. That is why vernacular buildings typically respond quite poorly to seismic events and is common to see many of them collapsing after an earthquake (Figure 1). Earthquakes come unexpectedly, endangering in-use vernacular architecture and the population who inhabits it. The first steps towards an earthquake safe vernacular built environment consist of better understanding the seismic behavior of representative vernacular construction systems, finding ways to rapidly identify the most vulnerable elements at risk and investigating proper seismic strengthening interventions that are both effective and respectful with the authenticity of the vernacular heritage.
Understanding vernacular construction
Understanding the seismic vulnerability of vernacular architecture demands a proper understanding of its construction. Vernacular buildings are intimately associated with the place, the territory and its history. There are social key factors (ritual, economics, defensive, religious, cultural, etc.) that determine the vernacular form. At the same time, there are physical determinants, including the climate, the natural hazards, the locally available materials or the available technology of the community through time.
Vernacular architecture thus responds to local conditions and is extremely heterogeneous, even though there is certain homogeneity among vernacular buildings within the same region. In the end, the resulting vernacular form in a given region translates into a set of geometrical, structural, constructive and material characteristics that have a direct impact on the structural seismic behavior of a vernacular building (Figure 2).
Therefore, since vernacular construction practices and details determine the seismic response of the building, by carefully evaluating key parameters related to constructive characteristics we can obtain a first estimation of its seismic performance. For example, by studying the masonry fabric typology (type of material, shape and size of masonry units, masonry layout, type of mortar, etc.) we can understand the quality of the masonry (thus having a preliminary idea of its possible mechanical properties) or if the walls are prone to suffer delamination or localized damage and partial collapses.
By evaluating the quality of the wall-to-wall connections we can understand if the building is likely to develop out-of-plane failure mechanisms due to the separation of walls at the corners. The maximum distance without intermediate supports spanned by the walls, together with their slenderness also govern their out-of-plane response. There are many other parameters (e.g. the amount of the wall openings, the type of diaphragm, the roof structural system, the state of conservation, etc.) that can be evaluated on site and give us hints about the structural performance of the construction in the event of an earthquake.
Identifying the most vulnerable elements at risk
The methods intended to evaluate the seismic vulnerability of the built environment play an important role on risk mitigation because they can predict the damage that a structure would suffer after an earthquake of a given intensity. Among the many methods existing in the literature, more detailed and sophisticated approaches that demand a deep investigation of the structures are typically restricted for individual monumental buildings.
The lack of resources is thus also evident at the time of preservation of the vernacular heritage. When dealing with vernacular buildings, we usually cannot invest the same amount of resources we would invest to safeguard monumental buildings. The restoration process of vernacular buildings cannot follow the commonly accepted steps required to obtain a proper understanding of the structural behavior of the built heritage (historical investigation, material characterization, non-destructive evaluation, advanced structural analysis, monitoring, etc.). Most probably, visual inspection will be the only tool available to carry out a seismic vulnerability assessment of vernacular structures (Figure 2).
Therefore, easy-to-use methods that define the seismic vulnerability of a building as a function of few qualitative and simple quantitative key parameters that are assumed to be the most influential in its seismic behavior are more appropriate when targeting vernacular buildings. These methods are particularly well-suited for large scale analysis, comprising large numbers of buildings. We can conduct simple more expedited inspections because we rely on less detailed information to have a preliminary estimation of the seismic performance of buildings. In this way we can collect rapidly data from a large number of buildings and carry out a seismic vulnerability assessment at an urban or regional scale. Then, after defining a correlation between seismic vulnerability and damage, we can also estimate damage and losses for different earthquake scenarios (Figure 4).
That is why they have become a valuable tool for the preservation of the built heritage, since they allow identifying the most vulnerable elements at risk. As a result, we can define and optimize structural retrofitting strategies at an urban or regional level by highlighting those buildings where we should concentrate our efforts. This is particularly important for vernacular architecture because, as already highlighted by the ICOMOS Charter on the built vernacular heritage (1999), “the vernacular is only seldom represented by single structures, and it is best conserved by maintaining and preserving groups and settlements of a representative character, region by region”.
Lessons from the past
The final objective of any seismic vulnerability assessment is the definition of retrofitting strategies and the evaluation of their impact. In the case of vernacular architecture (as with any structural intervention on the built heritage) the strategy should avoid excessively intrusive interventions that lead to a loss of authenticity. Thus, we can ask ourselves which are the architectural solutions that the local communities developed to prevent or repair earthquake damage. Vernacular architecture is an outstanding inheritance that bears important lessons on hazard mitigation. There are many examples throughout the world of vernacular buildings located in seismic prone areas that have lasted until today, surviving several earthquakes. Due to the long-term exposure to earthquake hazard, the local populations were forced to adapt to this risk and protect their built-up environment.
As a result, and based on observation and intuition, they developed specific architectural elements to prevent or repair earthquake damage. Their eventual application and integration into current vernacular building practices can help to preserve surviving in-use examples without prejudice to their identity.
We can recognize seismic resilient local building practices throughout the world, which reveal the existence of a local seismic culture. Given the unchangeable nature of earthquakes, the most common traditional earthquake resistant techniques observed in different highly seismic regions across the world follow similar intuitive and straightforward resisting principles, despite the use of different materials and techniques more adapted to each local situation and practice. They may involve some basic structural members of the building or consist of an entire building structural system (Figure 5). Ordinary constructions and dwellings belonging to vernacular architecture had to make use of affordable and locally available materials. Local populations acknowledged that is not economically viable to construct every building to resist earthquakes without suffering deformation and damage. Thus, vernacular builders traditionally intended to improve the capacity of the structures to undergo deformation and damage while maintaining enough load-carrying capacity to prevent global collapse.
Validation of the traditional knowledge
Earthquake resistant techniques have become traditional because they have continually proven to be effective in resisting past seismic events. However, since they are typically the result of empirical knowledge transmitted along generations, the origin of the solution is sometimes lost and their possible beneficial effect in the seismic resistance of vernacular buildings should be properly evaluated. Thus, we must increase our knowledge of the efficiency of traditional earthquake resistant solutions to confirm their positive effect. For that matter, there is a need for experimental and numerical work that can provide a quantitative evaluation of their efficiency. The different solutions identified should be characterized in-situ and replicated at the laboratory for testing. Numerical analysis can help to further evaluate their influence in reducing the seismic vulnerability of vernacular architecture and to better understand their structural role under seismic loading (Figure 6).
The validation and eventual update of these techniques is important because it can help to reintroduce them in the vernacular building culture for the preservation of the vernacular heritage, which is a demand of modern societies. The use of traditional solutions is in accordance with the modern principles of preservation regarding compatibility and authenticity, since they use similar materials and techniques. Continuing with this research is justified and fundamental, since the findings have an important impact in the society by providing a safer built environment regarding in-use vernacular architecture, which is inhabited by a significant percentage of the planet’s population.
Correia M, Carlos G (2015) Cultura sísmica local em Portugal, Argumentum, Lisboa, Portugal: https://esg.pt/seismic-v/assets/uploads/2015/10/seismic_v_cultura_sismica_local.pdf?
ICOMOS (1999) Charter on the built vernacular heritage, International Council of Monuments and Sites (ICOMOS), ICOMOS 12th General Assembly, Mexico: https://www.icomos.org/images/DOCUMENTS/Charters/vernacular_e.pdf
Karanikoloudis G (2014) Canterbury Cathedral – Structural Analysis of the South Aisle, SAHC M.Sc. Thesis, University of Minho, Portugal: http://www.msc-sahc.org/upload/docs/new.docs/2014_GKaranikoloudis.pdf