Seismic assessment of existing masonry buildings
Prof. Sergio Lagomarsino
Professor of Structural Engineering, the University of Genoa, Italy
Christchurch Seminar: Tuesday 18th February
Seismic vulnerability of ancient masonry structures: post-earthquake actions and preventive mitigation strategies
The last earthquakes in Italy have highlighted the high vulnerability of ancient masonry structures, including ordinary and heritage buildings. In order to mitigate their seismic risk effective actions are needed including: effective damage assessment and provisional interventions during the emergency and post-event; assessment procedures and retrofitting interventions pre-event.
After a brief classification of European ancient masonry structures and the description of their seismic behaviour, some examples of observed damage for different types of construction (residential buildings, churches, towers, fortresses) will be presented. The damage interpretation, by subdivision of the structure into macroelements and related collapse mechanisms, can support the definition of damage survey procedures and the execution of provisional interventions. Some examples from the last earthquake in Italy will be shown.
Assessment procedures for existing masonry buildings are needed for ensuring safety and planning preventive interventions, pre-event. Reliable nonlinear models are necessary for an accurate evaluation of single buildings, through a displacement-based approach (static pushover or incremental dynamic analysis). Simplified analytical models can be effectively used on a regional scale, for seismic risk analysis. They are based on few geometric and mechanical variables and are able to represent in a consistent way the influence of each relevant parameter, as well as the dynamic interaction with seismic hazard (in terms of spectral ordinates). Observational vulnerability models, although less accurate, are very valuable being implicitly validated.
An innovative method for developing fragility functions from both observational and mechanical vulnerability models will be presented.
Auckland Seminar: Wednesday 19th February
Seismic assessment of existing masonry buildings
Recent earthquakes in Italy have shown the high vulnerability of existing masonry buildings and the urgent need for reliable seismic assessment procedures for ensuring safety and for planning preventative interventions. Many masonry buildings are subject to damage during low-intensity earthquakes and, if connections are inadequate, local failure mechanisms can occur. However, the collapse limit state can be prevented even for medium to high intensity earthquakes, if the building has a significant displacement capacity. Therefore, nonlinear modelling is unavoidable for accurate assessment of existing buildings and can be performed by static pushover or incremental dynamic analyses.
This seminar will present modelling strategies, target performances and acceptance criteria for the seismic assessment. The displacement-based approach is adopted both for the global and the local seismic behaviour. In particular, the basic hypotheses of the TREMURI program are discussed, which is based on the discretization of masonry walls by piers and spandrel beams (equivalent frame approach). For the analysis of local failure mechanisms a macro-block approach is proposed, which is based on limit equilibrium analysis; the displacement capacity is obtained by considering geometric nonlinear effects.
The proposed procedures are validated by application to some case studies, taking also advantage of observed seismic damage and shaking table tests.
Wellington Seminar: Thursday 20th February
Preservation of cultural heritage masonry structures in seismic areas: displacement-based assessment procedures for single monuments and vulnerability models for risk analysis on territorial scale
Preservation of cultural heritage assets requires the assessment of their capacity against decay, natural hazards and accidental events, as well as the adoption of strengthening interventions without losing their authenticity. Earthquakes are the main cause of damage for ancient masonry constructions and in order to reduce their vulnerability with compatible and light interventions, accurate models for the seismic risk analysis on a territorial scale and the assessment of single buildings are needed.
The seismic risk analysis of a large number of monumental buildings at territorial scale is aimed to forecast the impact of a given earthquake on the cultural heritage and to single out those at higher risk, in order to plan mitigation interventions. Fragility functions for different types of masonry historical constructions were developed by macroseismic observational approach, taking advantage of post-earthquake damage survey, in particular for churches and palaces. Analytical models, based on few geometric and mechanical parameters, can also be very useful; even if simplified, they are able to catch in a consistent way the influence of most relevant parameters.
With reference to the detailed evaluation of single buildings, the performance-based assessment procedure developed within PERPETUATE research project (www.perpetuate.eu), funded by the European Commission, is presented. Architectonic assets are classified and proper modeling strategies are defined; moreover, immovable artistic assets are also considered in the assessment. Nonlinear models and the displacement-based approach are adopted, because masonry structures crack even for low intensity earthquakes and can survive in severe shocks only if they have a sufficient displacement capacity. Safety and conservation requirements are proposed by considering distinct sets of performance levels, related to use and safety of people, conservation of the building and of the artistic assets (if they are present). Within the fundamental knowledge phase, sensitivity analysis is proposed in order to address and optimize the in-situ investigations and to define proper confidence factors, aimed to consider epistemic and statistical uncertainties. Different nonlinear modeling approaches, methods of analysis and verification procedures are considered, depending on the characteristics of the construction.
These presentations are part of a 2-year project funded by the Earthquake Commission Biennial Grants Programme 2014 titled “Vulnerability analysis of unreinforced masonry churches” (reference 14/660)
Prof. Sergio Lagomarsino has been a Professor of Structural Engineering at the University of Genoa since 2000. He is the author of more than 250 papers on numerical modelling of masonry structures, seismic risk analysis, preservation of monumental buildings and historical centres. He has coordinated many research projects, in particular the European FP7 project PERPETUATE (www.perpetuate.eu). He has served on the drafting panel of the new Italian seismic code, for the chapters “Design of masonry buildings” and “Assessment and retrofit of existing masonry buildings” and the drafting panel, formed by the Ministry of Cultural Heritage and the Civil Protection Department, which prepared the “Guidelines for the safety and conservation of monumental buildings in seismic areas”. He has developed, together with co-workers, the program TREMURI for the static and dynamic nonlinear analysis of masonry buildings and has developed the survey form for seismic damage assessment of ancient churches, used by the Italian Civil Protection Department.