Seismic Assessment and Retrofit Strategies for Existing Reinforced Concrete Buildings

Description

Background and Motivation

The  crucial  need  for  strengthening  or  retrofitting  existing  structures  designed  with substandard details, in order to withstand seismic loads without collapsing and possibly with repairable damage, has been further emphasized in the last decade by the catastrophic effects of earthquake events (e.g. Turkey, Colombia and Taiwan, 1999; India 2001; China 2008;  Italy 2009; Chile 2010).

The  impact  of  the  Canterbury  earthquakes  sequence  in  2010-2011  has  dramatically
emphasized, if at all needed, the urgent need to develop and implement simple and cost- effective (repair and) retrofit solutions for existing reinforced concrete buildings, designed according to older (though, in some cases, relatively recent) seismic code provisions.

Even  prior  to  selecting  the  most  appropriate  retrofit  strategy  and  technique,  the assessment of the seismic vulnerability of the structure represents a crucial and very delicate step. Important knowledge in this complex area has been gained in the past recent years at both national and international level, with more reliable procedures and techniques to support both the assessment and the retrofit phases.

Based on recent lessons learned from past earthquakes and on extensive experimental and analytical-numerical investigations, it is becoming more and more evident that major and sometimes controversial issues can arise when, for example:

(a) deciding whether the retrofit is actually needed and, if so, in what proportions and to what extent;

(b) assessing and predicting the expected seismic response pre and post- intervention by relying upon alternative analytical/numerical tools and methods;

(c) evaluating the effects of the presence of infills, partitions or in general “non- structural” elements on the seismic response of the overall structure, typically and improperly evaluated considering only the “skeleton”;

(d) deciding, counter-intuitively, to “weaken” one of more structural components in order to “strengthen” the whole structure;

(e) adopt a selective upgrading to independently modify strength, stiffness or ductility capacity;

(f) relying upon the deformation capacity of an under-designed member to comply with the displacement compatibility issues imposed by the overall structure;

(g) defining a desired or acceptable level of damage that the retrofit structure should sustain after a given seismic event, i.e. targeting a specific performance level after the retrofit.

Finally, while, on one hand, issues related to the socio-economical consequences of
excessive damage and/or downtime should be a major if not the first priority, on the other hand, considerations of costs, invasiveness, disruption of working activities as well as architectural aesthetics contributes to further complicate such a complex decision-making process.