Seismic risk classification and improvement of a 60s reinforced concrete structure

The Seismic Risk Classification.

In 2017 the Italian government launched a strong state incentive (Sissabonus) for the mitigation of seismic damage, thus encouraging the private and productive sectors to invest in the seismic renovation of buildings that do not meet the requirements of the Technical Standards for Construction, NTC.
With the Ministerial Decree n.24 of 9/1/2020, the Ministry of Infrastructure and Transport (MIT) defined the Guidelines for the classification of the seismic risk of buildings, defining 7 seismic risk classes.

The seismic risk classes have been defined in order to obtain adjustment or improvement seismic of existing buildings, therefore, the seismic risk classification should be carried out to evaluate the previous and subsequent state of the building, leading to better seismic performance.

How is the Seismic Risk class calculated?

The seismic risk class of a building can be calculated using two methods:

  1. simplified method (with limitations on its application);
  2. conventional method (applicable to any structural type and using the analysis methods required by the NTC), which is defined by calculating two parameters defined in the guidelines: the Average Expected Annual Loss (MAP) and the PGA demand of the structure for the Safety Limit State Life (SLV).

Seismic risk classification and improvement of a reinforced concrete structure of the 60s, the Firm.

Even if the technical guidelines and the NTC application circular are available to help the professional community in the classification of the seismic risk of buildings, practical and public examples with explicit calculation and quantification procedures are still lacking. So professionals, in practice, entrust most of the responsibility for these crucial steps to commercial software, according to an approach black box, to perform the structural analysis and automatically classify the seismic risk by setting the structural model.

For this reason we have decided to document, and describe the steps to perform a seismic risk classification for a building. The study was developed as follows:

  • La section 2 is a description of the selected case study building and the parameters of the numerical model, including the type of numerical analysis and the results.
  • La section 3 is dedicated to the detailed explanation of the seismic risk classification procedure and the presentation of the results for the case study.
  • La section 4 introduces the technology of I-Pro 1 on the building to obtain the seismic improvement of the building. It is an active control mass damper connected to sensors positioned on the structure that feed a computer with acceleration data due to microtremors (in normal operating state) or earthquakes (in the event of a seismic event) to obtain the dynamic identification of the building and provide feedback to the mass damper control system to reduce building sway.

Seismic Risk Classification and improvement, a practical example.

We calculated the class of seismic risk and improvement (thanks to the application of I-Pro 1) of a full-scale building built on the occasion of the research project Assessment and Retrofitting of Full-Scale Models of Existing RC frames of the European Laboratory for Structural Evaluation (ELSA).
This particular building was built according to building design practices common in the late 50s and 60s of the last century, that is, which provide a low seismic coefficient (8%), C16 / 20 concrete, S235 steel for the shape bars round, typical overlap joints, bent brackets, absence of shear reinforcements in beam-column joints and plastic behavior of the Strong Beam-Weak Column type. This type of construction has been adopted for most of the reinforced concrete buildings currently existing in Italy.

What are the technical steps to determine the seismic risk class of a building? How to define the seismic improvement obtained? What are the results obtained from our study?

Register and download the ISAAC Doc dedicated to the study carried out
on a reinforced concrete structure from the 60s.

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