Seismic safety assessment

Seismic safety assessment

Seismic safety assessment

Seismic safety assessment

The seismic safety assessment of existing buildings has to be carried out according to the procedures highlighted by codes and guidelines.

They require the implementation of preliminary operations aimed at achieving a proper level of knowledge of the parameters necessary to develop a reliable structural model of the building. These activities can be listed as follows: identification of the construction, functional characterization of the building and of its use, geometric survey of the whole building and of the construction details, historical analysis of relevant events during construction and of the use of the building, survey of the construction material and of their conservation state, mechanical analysis of materials and geotechnical analysis of the soils.

Depending on the level of knowledge achieved, it is possible to define specific confidence factors to be used in the following assessment as additional safety factors. It is intuitive that the deepening of the model parameters knowledge is useful in obtaining a higher reliability of data; this allows the model results to be closer to the actual structural behaviour, as well as saving money for further strenghtening works

The safety assessment of a building must be developed through a structural analysis; this is aimed at translating the determined mechanical behavior of the building in quantitative terms. This analysis is performed by defining different interpretative models, with different degrees of accuracy that can represent the whole structure or parts of it. Starting from a model obtained with a minimum level of knowledge, which is based on a limited amount of information, it is possible to define more refined interpretation models in which reliability is evaluated and calibrated through the deepening of knowledge, with an “iterative” mechanism that achieves the most reliable model for the construction.


Structural modelling

The mathematical model of a structure can be seen as a tool which assesses the safety of the buildings and which is part of a wide context of analysis aimed at the acquisition of the knowledge of the structural behavior and state of the building. It should be noted that there are not mathematical theories which fully reproduce the objective reality of the physical phenomena, so it should be stressed that mathematical models provide only a useful aid in the understanding of physical phenomena; they do not directly provide the understanding of them. It should be noted also that there is not only one model for a given structure.

The complexity of models must be strictly related to the level of a material’s mechanical knowledge and the structure through diagnosis analysis carried out.

To assess the reliability of a model it is necessary that it complies with the following requirements: that the analytical model for a certain class of problems provides a good description of the phenomenon, and that different numerical techniques should give slightly different results if implemented with the same data.

It is also noted that an analytical model that well reproduces the behavior of a real structure can be considered reliable for correct predictions only within the scope of its relevance. The calculation codes solve the equations of the theory on which the mathematical models are based. From this point of view, we can distinguish two different assessment criteria: the one related to the assessment of the correctness of the solution procedure (validation) and the one related to the assessment of the adequacy of the model to reproduce the real behavior of the analyzed structures (calibration). The complexity of the models that are set-up have to be closely related with the level of knowledge of the characteristics of materials and structures achieved through the diagnostic surveys performed.

From a seismic risk analysis perspective, with a regular plan or height, a building can be evaluated using a global integrated linear static model, testing micro-elements that analyse the main localised movement damage; should the building be irregular, it can be analysed using the push-over technique which also tests for mixed material structures like masonry or reinforced concrete.

Buildings in reinforced concrete or steel are tested through linear dynamic checks with the emphasis on vibration modality or through push-over testing.

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