Historic masonry and earthen buildings, with their near-universal material availability and low-cost construction, represent a significant portion of the world’s built cultural heritage. The irreplaceable value of this heritage has long been recognized, as has our responsibility to preserve this heritage for future generations.
However, earthen sites are among the most seismically vulnerable building typologies. Damage to these sites from earthquakes has caused significant cultural and economic losses in addition to the loss of human life. During the twentieth century, 75% of fatalities attributed to earthquakes were caused by the failure of buildings, of which 60% were due to collapse of masonry and earthen buildings.
These buildings present some common seismic vulnerabilities: low mechanical properties, unfavorable geometrical layout, heavy weight, inappropriate floors and roofs, poor connections between structural parts, and an absence of concern for seismic issues at the time of their construction and subsequent alterations. In addition, deterioration and lack of maintenance can further contribute to reduced performance in an earthquake.
As a result, when subjected to seismic activity, these buildings respond by brittle collapses with out-of-plane mechanisms of isolated parts. These local mechanisms can cause irreparable damage and can compromise the overall stability of the building. Nevertheless, simple and appropriate remedial measures can dramatically change the situation. If the premature activation of out-of-plane failures is prevented, these structures can activate a global mechanism, reducing their vulnerability.
Reducing Seismic Vulnerability of Historic Buildings
When dealing with the conservation of historic buildings, a multidisciplinary approach is needed. As stated in recommendations from the ICOMOS International Scientific Committee for Analysis and Restoration of Structures of Architectural Heritage, this requires a methodology similar to one used in medicine: condition survey (anamnesis); identification of causes and decay (diagnosis); choice of remedial measures (therapy); and control of the efficiency of interventions (controls). According to modern conservation practices, interventions must respect basic principles, including authenticity, reduced invasiveness, reversibility, compatibility, and durability. Efficient and appropriate remedial measures must result from an iterative process that optimizes these parameters, while maintaining sufficient safety to limit collapse and unacceptable losses.
In this scenario, seismic assessment plays a crucial role in evaluating the need, extent, and efficiency of any intervention. However, the intrinsic complexity and inherent diversity of historic buildings are a great hindrance to a strict application of codified approaches. As a result, the reduction of the seismic vulnerability of earthen sites remains an open issue in modern building codes which demands a rational, yet simple, assessment based on models well supported and validated by experimental research.
Significant developments in the last few decades have permitted reliable engineering for safety assessment and design of efficient intervention measures for these buildings. Conservation and preservation of historic constructions require specialists with appropriate theoretical and practical experience. Therefore, training qualified professionals, promotion of scientific research, international exchange, and circulation of information are of essential importance in this field.
A specialist conference series running for three decades is being held this week in Cusco, Peru, to discuss these aspects. At the 11th International Conference on Structural Analysis of Historical Constructions, participants will have the opportunity to make a site visit to the Kuñotambo Church outside of Cusco. Here, a series of interventions have been proposed to preserve and strengthen this seventeenth-century church against future earthquakes as part of the Getty Conservation Institute’s Seismic Retrofitting Project.
The Seismic Retrofitting Project
In 2010 the Getty Conservation Institute initiated the collaborative Seismic Retrofitting Project (SRP) to provide efficient low-tech seismic retrofitting techniques and easy-to-implement maintenance programs for improving the seismic performance of historic earthen buildings while preserving their original fabric. If you are a regular reader of The Iris, you may recall a previous post about the project by my colleague and the project’s manager, Claudia Cancino.
Four building prototypes representative of the historic earthen heritage of Peru were selected as case studies for the project: Ica Cathedral (Ica, eighteenth century) damaged in the 2007 Pisco earthquake, the Kuñotambo Church (Acomayo, seventeenth century), Hotel del Comércio (Lima, nineteenth century), and Casa Arones (Cusco, seventeenth century). Using these case studies, the project aims to develop repairing and strengthening techniques, provide guidance for those responsible for their implementation (such as architects, engineers, and conservators), and work with local authorities to include the developments in the Peruvian National Building Code.
Computer modeling and structural analysis were developed in collaboration with the Institute for Sustainability and Innovation in Structural Engineering (ISISE) of the University of Minho. Sophisticated computer simulations allowed the project team to evaluate the seismic performance of the prototypes and the need for their strengthening.
Safety Assessment under Current Conditions
Deep insight into the intrinsic complexity of the four building prototypes was necessary before delving into any form of computer simulation of potential structural failure of the buildings, allowing safety assessment.
Historic research and onsite surveys, carried out by the Conservation Institute, provided information on the buildings’ histories, geometries, construction techniques, and conditions. Material properties assigned to the computer models were derived from literature, building standards, and laboratory tests performed by the Pontificia Universidad Católica del Perú (PUCP).
Data obtained from onsite and nondestructive dynamic identification tests performed by the University of Minho allowed team members to define the modal parameters of the structures (natural frequencies, mode shapes, and modal damping coefficient) and to calibrate the computer models.
For example, we see, in the video below, that only portions of the Ica Cathedral were mobilized while testing, meaning the cathedral’s complex vaulted roof system is poorly connected to its masonry walls due to the damage resulting from the Pisco earthquake of 2007.
Among the different types of analysis available to assess the safety of structures, nonlinear analyses should be primarily considered when dealing with masonry and earthen structures, as these are able to trace the complete structural response from the elastic range, through cracking and crushing, up to complete failure. This analysis simulates seismic actions by applying lateral loading to the model of the structure, which increases up to collapse.
The so-called capacity curves, or load factor vs. control node displacement curves, were obtained and failure mechanisms were defined. Then, seismic safety was evaluated, comparing the maximum load factor found in the computer simulations of the four structures with the demand as provided by the Peruvian code.
We concluded that two of the prototypes, Hotel del Comércio and Casa Arones, are potentially safe in the current building configuration, provided that missing parts are completed, heavily deteriorated parts are repaired, and conservation works are carried out in general, including careful analysis of the connections between structural components. We also concluded that the other two case studies, Ica Cathedral and the Kuñotambo Church, do not comply with the safety requirements in the current building configuration, and that repair and strengthening work is needed.
Safety Assessment after Strengthening
Interventions were therefore proposed for the Ica Cathedral and the Kuñotambo Church. Repairing and strengthening techniques were identified according to the state-of-the-art research by the Conservation Institute and the Pontificia Universidad Católica del Perú on effective low-tech methods historically used for earthen structures. These techniques were adopted for use on the buildings following a decision-making process that included the use of a principle-based matrix dichotomized with conservation principles (e.g., minimal intervention, reversibility, authenticity) and engineering criteria (e.g., safety, durability, feasibility, and cost).
Several interventions were proposed to improve the integral behavior of these two structures: wooden collar beams and anchoring systems were placed to enhance connection among structural parts, while earthen buttresses were reestablished or added to increase the building capacity.
Material consolidation was also proposed—of vital importance to avoid disintegration of the walls themselves during seismic events. For example, highly deteriorated adobe and base course masonry parts were replaced using compatible materials, keeping this as limited as possible.
More information on the repairing and strengthening techniques proposed for the Kuñotambo Church, as well as their ongoing implementation, is available on the project website.
Computer simulations of the two structures being subjected to repair and strengthening showed a significant increase in the seismic capacity and an important reduction of out-of-plane vulnerabilities of these prototype buildings after implementing the proposed interventions. The capacity of the two structures increased significantly and the cracking was less in extent and size, with a more distributed pattern, when compared to extensive diagonal and horizontal fragmentations observed under the current state.
In short, the computer simulations were combined with historical information, surveys, on-site testing, and laboratory testing. The simulations allowed the project team to assess the safety of the four prototypes and contributed to the definition of repair and strengthening measures currently being implemented in two of the prototypes. Thus, the successful combination of state-of-the-art techniques for surveying, testing, and simulation demonstrated the adequacy of the use of traditional repair and strengthening measures for historic earthen sites.
Advancing Conservation Practice
The methodology and outcomes of the Seismic Retrofitting Project have gained broad consensus in peer-review groups of international experts in the field of conservation of earthen heritage buildings. While focused in Peru, the project’s results are widely applicable to similar structures around the world, especially in developing countries.
Currently, the Conservation Institute is working on dissemination of project information and experience to professional and scientific communities. Publications providing guidance on the design and implementation of strengthening solutions for the conservation of earthen historic buildings will be made available on its website.
The Institute is also organizing workshops with theoretical and practical sessions for professionals, including engineers, architects, archaeologists, conservators, and project managers. Moreover, several papers will be presented at international conferences, such as this week’s 11th International Conference on Structural Analysis of Historical Constructions (SAHC 2018), to discuss and exchange insights concerning recent advances and challenges that we all face in research and professional practice in this field.