Improving the seismic resistance of cultural heritage buildings

The paper addresses a possible methodology to improve the seismic resistance of cultural heritage buildings (CHB). The ICOMOS recommendations are briefly reviewed and recent research issues are addressed, with a focus on: (a) Behavior of masonry components under cyclic loading (tension, compression and shear); (b) Behavior of stone masonry shear walls under cyclic and dynamic loading; (c) Behavior of dry masonry blocks and structures under dynamic loading; (d) Possibilities of numerical analysis at the laboratory and engineering levels; (e) Monastery of Jeronimos as a case study: An EC funded research project aiming at reducing seismic vulnerability of CHB. a number of challenges in conservation, diagnosis, analysis, monitoring and strengthening that limit the application of modern legal codes and building standards. Recommendations are desirable and necessary to both ensure rational methods of analysis and repair methods appropriate to the cultural context. Therefore, an international committee has prepared recommendations, intended to be useful to all those involved in conservation and restoration problems, Icomos (2001). These recommendations contain Principles, where the basic concepts of conservation are presented, and Guidelines, where the rules and methodology that a designer should follow are discussed. More comprehensive information on techniques and specific knowledge can be found, e.g. in Croci (1998), Giuffre (1993) and Meli (1998). 2.1 Principles and Guidelines The principles entail: General criteria; Research and diagnosis; and Remedial measures and controls. A multi-disciplinary approach is required and the peculiarity of heritage structures, with their complex history, requires the organization of studies and analysis in steps: condition survey, identification of the causes of damage and decay, choice of the remedial measures and control of the efficiency of the interventions. Understanding of the structural behavior and material characteristics is essential for any project related to architectural heritage. Diagnosis is based on historical information and qualitative and quantitative approaches. The qualitative approach is based on direct observation of the structural damage and material decay as well as historical and archaeological research, while the quantitative approach requires material and structural tests, monitoring and structural analysis. Often the application of the same safety levels used in the design of new buildings requires excessive, if not impossible, measures. In these cases other methods, appropriately justified, may allow different approaches to safety. Therapy should address root causes rather than symptoms. Each intervention should be in proportion to the safety objectives, keeping intervention to the minimum necessary to guarantee safety and durability and with the least damage to heritage values. The choice between “traditional” and “innovative” techniques should be determined on a case-by-case basis with preference given to those that are least invasive and most compatible with heritage values, consistent with the need for safety and durability. At times the difficulty of evaluating both the safety levels and the possible benefits of interventions may suggest “an observational method”, i.e. an incremental approach, beginning with a minimum level of intervention, with the possible adoption of subsequent supplementary or corrective measures. The characteristics of materials used in restoration work (in particular new materials) and their compatibility with existing materials should be fully established. This must include long-term effects, so that undesirable side effects are avoided. 3 EXPERIMENTAL ISSUES Masonry is a heterogeneous material that consists of units and joints. Units are such as bricks, blocks, ashlars, adobes, irregular stones and others. Mortar can be clay, bitumen, chalk, lime/cement based mortar, glue or other. The huge number of possible combinations generated by the geometry, nature and arrangement of units as well as the characteristics of mortars raises doubts about the accuracy of the term “masonry”. Nevertheless, most of the advanced experimental research carried out in the last decades has concentrated in brick / block masonry and its relevance for design. Accurate modeling requires a thorough experimental description of the material, see Lourenco (1998) and Cur (1997). 3.1 Properties of unit and mortar The properties of masonry are strongly dependent upon the properties of its constituents. Compressive strength tests are easy to perform and give a good indication of the general quality of the materials used. Experiments about the uniaxial post-peak behavior and about the biaxial behavior of bricks and blocks are less common in the literature, together with tests on cyclic behavior. Next, some results for clay bricks under uniaxial compression are briefly reviewed (Oliveira 2002). A series of unloading-reloading cycles were performed in clay specimens, particularly in the post-peak region, to acquire data about stiffness degradation and energy dissipation. The experimental set-up, testing conditions and typical stress-strain diagrams are illustrated in Figure 2. The need for circumferential displacement control is stressed, and the results shown are rather difficult to obtain due to very high strength and brittleness of the units used in the testing program. The response indicates an important and monotonic decrease in Young’s modulus in the post-peak regime, associated with damage growth in the material.