stefano.pampanin@uniroma1.it
L’urgenza di un piano di attuazione e implementazione di una strategia a medio-lungo termine di riduzione del rischio sismico a scala nazionale è sempre più evidente.
E’ indubbio che la valutazione della vulnerabilità sismica di edifici esistenti e la definizione di soluzioni di rinforzo appropriate, dunque strutturalmente efficaci, di facile applicazione, economicamente convenienti, possibilmente reversibili e rispettose della conservazione del patrimonio architettonico-culturale della struttura, nascondano un livello di complessità significativamente superiore rispetto alla progettazione di nuove strutture.
Eppure studi e valutazioni comparative sull’efficacia di una strategia di prevenzione sismica rispetto ad un approccio di reazione/cura post-evento ne mostrano chiaramente i vantaggi a lungo termine e a livello nazionale.
In questa serie di tre articoli si affronteranno problemi e soluzioni legati alla valutazione della vulnerabilità sismica di edifici esistenti in calcestruzzo armato e alla definizione di strategie e tecniche di rinforzo alternative per una riduzione del rischio sismico sia a livello di singolo edificio che di classi di edifici a scala territoriale. Si fornirà una panoramica sui recenti sviluppi procedurali e normativi sia a livello nazionale ed internazionale con particolare attenzione alle nuove linee guida neozelandesi (NZSEE2016) nelle quali viene focalizzata l’attenzione su un approccio analitico semplificato (i.e. a mano), per la valutazione della capacità (in termini di forze e spostamenti combinati) di elementi strutturali, connessioni, sottosistemi e sistemi sismo-resistenti, come imprescindibile punto di partenza di ogni studio di vulnerabilità/sicurezza.
Nel primo di questi contributi si riprenderanno i concetti di base di una progettazione sismica prestazionale e resilienza di comunità, evidenziando le ragioni di un profondo disallineamento tra le aspettative della società, convinta di ricevere un sistema ‘antisismico’, e le reali prestazioni simiche ‘da norma’ di un sistema ‘sismo-resistente’.
The urgency of a medium-long-term seismic retrofit and risk reduction strategy and implementation plan on a national scale is becoming increasingly evident.
With no doubt the assessment of the seismic vulnerability of existing buildings and the definition of appropriate, i.e. structurally effective, easy to apply, cost-effective, possibly reversible and respectful of the architectural, heritage and cultural conservation requirements, hides a level of significantly higher complexity than designing new structures.
Moreover, as if the technical complexity was not a sufficient deterrent, the constraint of economic resources for a national scale implementation and the lack of a prioritization plan based on risk considerations and cost-benefit analyses are often referred to, or blamed as, primary obstacles to the practical implementation of such a board and ambitious project.
Yet studies and comparative evaluations of the effectiveness of a seismic prevention strategy when opposite to a post-event reaction/repair/reconstruction approach clearly show its long-term and national benefits.
First of all, it is necessary to improve and standardize the tools and procedures ('protocols') for the 'diagnosis' and 'prognosis' of the seismic vulnerability and of the expected performance of existing buildings, based on state-of-the-art but simplified methodologies (analytical rather than numerical approaches) that could highlight the structural weaknesses of the building system while ensuring consistency of results and proper level of independently from the operators.
Similarly, suitable "therapeutic pathways" or appropriate retrofit strategies can be defined by comparing alternative options through a cost-benefit approach.
This series of three papers will address issues and solutions related to the seismic vulnerability assessment of existing reinforced concrete buildings and the definition of alternative retrofit strategies and techniques for the reduction of the seismic risk either at a single building level as well as at a territorial scale level.
An overview of recent developments at national and international level in terms of methodology and regulatory approaches will be provided, with particular attention to the new New Zealand Guidelines (NZSEE2016), which revalue the critical importance of an analytical approach (i.e by hand) for the evaluation of the capacities (in terms of combination of forces and displacements) of structural elements, connections, subassemblies and seismic-resistant systems as a fundamental starting point for any vulnerability/safety study.
It is a revamping of the importance of the big picture or holistic view, of the understanding and controlling of the design by the engineer, today often and unfortunately considered, at least in the views of non-technical customers and decision makers, to play the simplicistic role as a 'structural' calculator.
It is worth noting how these New Zealand Guidelines represent the key instrument for the implementation of a compulsory national (at the owners’ expenses) prevention plan that, following the recent and devastating 2010-2011 earthquake sequence in Christchurch, requires the assessment and the seismic rating of all existing (either public and private) buildings and the associated retrofit intervention to reach a minimum level of seismic risk (associated to a seismic rating schemes) defined by the code.
The latter comes with definite maximum time-schedule both for the evaluation phase and for the retrofit phase (5-10-15 years and 15-25-35 years) depending on three level of seismicity (high-medium-low).
In the first of these contributions, an overview of the basic concepts of a performance-based seismic design and community resilience will be provided, highlighting the reasons for a profound misalignment between the expectations of the society, convinced to receive an 'anti-seismic' system, and the actual seismic performance 'as per code’ of a 'seismic-resisting' system.
In the second contribution, a presentation on recent simplified analytical procedures (i.e. SlaMa approach, or 'Simple Lateral Mechanism Analysis') for the seismic vulnerability assessment and seismic rating of existing buildings will be given, with particular reference to the recent New Zealand NZSEE2016 Guidelines and the time-schedule and criteria adopted for the implementation plan, with suggestions for their possible extension and adoption within the Italian regulatory and construction environment.
The third and final contribution will illustrate alternative seismic retrofit strategies and techniques for reinforced concrete buildings, aiming at achieving predefined performance improvement objectives under different earthquake intensity level and thus at reducing the Expected Annual Losses (EAL).
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