Cedimenti strutturali: un punto di vista italiano sulla mitigazione della vulnerabilità strutturale

Dal 23-25 2015, a GInevra in Svizzera si è svolto lo IABSE Conference: Structural Engineering: Providing Solutions to Global Challenges. RIprendiamo qui l'Abstract dell'articolo rimandando con un link al sito del congresso per leggere l'articolo in modo completo.

Changes in codes following structural failures: an Italian perspective on mitigation of structural vulnerability
Fabrizio Palmisano, Amedeo Vitone
Politecnico di Bari, Bari, Italy

Abstract
In the last twenty years, forensic investigations of failures occurred in Italy have clearly highlighted that the increasing demand in society for economic growth has favoured the blooming of less conservative and even hazardous design and construction works.
After a short review of the Italian codes of practice dealing with structural vulnerability, some examples of structural vulnerability assessment in Italy is given in the article, with reference to both the large scale (i.e. the scale of the building) and the small (territorial/regional) scale.
Keywords: forensic structural engineering; collapse; failure; structural vulnerability assessment.

1 The revision of the Italian code of practice

Up to 2003, Italian Codes of practice did not include obligatory protocols for the assessment of structural vulnerability of existing constructions, but only very few indications. The first important revision of the Italian technical standards was stimulated by the collapse of a school in San Giuliano di Puglia (CB, South of Italy) in which 27 children and a teacher lost their lives. Although it occurred during the Molise Earthquake (South of Italy) in 2002, forensic investigations proved that the earthquake was only the triggering factor, since the building had strong internal structural weaknesses. One year after, there was an important revision of the Italian technical standards [1] which opened the way, for the first time in Italy, to a Eurocode-based approach for both the design of new structures and the assessment of existing structures. Moreover, this code introduced some cases in which the vulnerability assessment and the structural retrofitting were mandatory.

Even though this code was applicable only for ‘seismic zones’ (i.e. where seismic design is mandatory), it became the starting point for the complete revision of the codes of practice.

The 2008 code ([2]; in force at present) is an evolution of the 2003 one. Firstly a complete revision of the Italian seismic map has been made and all the Italian territory was declared ‘seismic zone’. Secondly this code deals with the assessment and retrofitting of existing buildings also for actions different from the seismic ones (e.g. gravitational, wind, accidental actions).

According to the 2008 code, the structural vulnerability assessment of an existing construction is mandatory in case of:

  • evident reduction of resistant and/or deformation capacity of a structure or of a part of it due to: environmental actions (earthquake, wind, snow, temperature), significant degradation of the mechanical characteristics of materials, accidental
  • actions (impacts, fire, explosions), abnormal use, significant deformations due to soil settlement;
  • evident construction and/or design errors;
  • changing of the use of the construction or of a part of it with a significant variation of live loads;
  • non-structural interventions that change the structural behaviour (in terms of stiffness and/or resistance) of the construction or of a part of it.

The aim of the structural vulnerability assessment is to establish if:

  • the use of the construction can continue without any kind of intervention;
  • the use of the construction needs to be changed;
  • retrofitting is necessary.

The 2008 Code includes a mandatory protocol for the structural vulnerability assessment.
Moreover, according to the Code, retrofitting is mandatory in the following cases:

  • extra floor addition;
  • enlargement of the construction;
  • change of building use which implies an increment of the overall foundation load higher than 10%;
  • structural interventions that can change the structural behaviour (in terms of stiffness and/or resistance).

2 Vulnerability assessment at large scale: the ‘building register’
As discussed above, the 2008 Code includes mandatory structural vulnerability assessments only in some cases (mainly for evident deficiencies or in case of interventions/modifications).

Recent Italian collapses (Roma, 1998; Foggia, 1999; Napoli, 2001; Salerno, 2007) have revealed that ‘hidden’ deficiencies can be the cause of ruinous and apparently unpredictable failures. Moreover the most recent Italian experience on
vulnerability assessment has been showing that these cannot be treated as extraordinary cases. The wide-spreading of such hidden weaknesses within existing buildings is, first of all, the consequence of the less conservative and hazardous design/construction approach which was the practice during the economic growth period (i.e. ‘50s-‘70s), in order to save time and minimise construction costs. Secondly, in many cases, enlargement interventions to existing buildings have been made without complying with the regulations in force.

In order to face the latter problem, in 2002, the so-called ‘building register’ has been introduced in two Italian Regions. The building register was a database containing all the interventions and modifications (architectural, structural, plant) that the building has undergone after its construction. The aim of this register was to collect all the data useful for an eventual structural vulnerability assessment. This register was immediately declared illegal by both the Constitutional and the Administrative Court because the regional government cannot delegate, to private citizens, tasks and responsibilities that fall within the competence of public administration (e.g. the preservation of public and private safety).

At present many Italian Regions are trying to introduce again the building register but only for new constructions. However, it has to be observed that, if the aim is to mitigate the structural risk, in its present form the building register contains numerous deficiencies: (a) only citizen-declared interventions are included, (b) it is assumed that the building has been designed and constructed according to the regulation in force, (c) it does not contain any structural vulnerability assessment.

... CONTINUE ... : LINK