The base method requires a limited amount of information and can therefore be conveniently used if information on site conditions, especially regarding soil stratigraphy and geotechnical characteristics, is relatively scarce. If more detailed information is available, the advanced method can be applied.

The base method includes the following steps:

• Assessment of seismic hazard on rock, with probability of exceedance of 10% in 50 years (1.e, return period: 475 years), by accessing a database containing the results of analyses carried out throughout Italy;

• Evaluation of local site amplification effects via comparison between the attenuation laws for alluvium deposits and rock, and assessment of seismic hazard at the ground surface;

• Evaluation of the expected damage on the given structure typology by means of fragility curves.

Seismic Hazard
By selecting the base method the seismic hazard in rock is first established by querying a database containing the results of about 20000 analyses performed throughout the entire Italian territory. The results contain both peak ground acceleration values and spectral acceleration values computed at different oscillation frequencies. In such a way an elastic response spectrum is defined, so that structural damage can be assessed by considering the spectral acceleration estimated at the structure's fundamental frequency.
The following figure depicts the procedure adopted to generate the seismic hazard database used by the system.






Local Amplification
The acceleration values computed on rock are then modified by means of an amplification factor, where the latter is obtained by comparing attenuation laws for generic alluvium deposits against attenuation laws for rock. Magnitudes and epicentral distances  computed by a disaggregation procedure are used in the process. Importance factors and additional amplification factors, accounting for site specific topographic features, can also be applied if required by the user. 




Structural Damage
The structural damage is assessed by means of fragility curves that estimate the probability of exceeding a pre-defined damage threshold induced by a given spectral displacement. The fragility curves are specific for building typology and age.

The conditional probability of exceeding a given damage state is modeled as a cumulative lognormal distribution. For structural damage, given the spectral displacement, Sd, the probability of exceeding a damage state, ds, is modeled as:

where:
S_d,ds is the median value of spectral displacement at which the building reaches the threshold of the damage state, ds,
β_ds is the standard deviation of the natural logarithm of spectral displacement of damage state, ds, and
Φ is the standard normal cumulative distribution function.

Using the fragility curves, the probability of exceeding a given level of structural damage is established.
Four structural damage levels are defined: slight, moderate, extensive and complete.  As an example the four damage levels for the typology "Concrete Moment Frame" are defined as follows:

Slight Structural Damage: Flexural or shear type hairline cracks in some beams and columns near joints or within joints.

Moderate Structural Damage: Most beams and columns exhibit hairline cracks. In ductile frames some of the frame elements have reached yield capacity indicated by larger flexural cracks and some concrete spalling. Non ductile frames may exhibit larger shear cracks and spalling.

Extensive Structural Damage: Some of the frame elements have reached their ultimate capacity indicated in ductile frames by large flexural cracks, spalled concrete and buckled main reinforcement; non ductile frame elements may have suffered shear failures or bond failures at reinforcement splices, or broken ties or buckled main reinforcement in columns which may result in partial collapse.

Complete Structural Damage: Structure is collapsed or in imminent danger of collapse due to brittle failure of non ductile frame elements or loss of frame stability. Approximately 20% (low-rise), 15% (mid-rise) or 10% (high-rise) of the buildings with Complete damage is expected to be collapsed.

Similarly there are fragility curves corresponding to these damage definitions to be applied to all other structural typologies included in the system.  

The main reference adopted for the typological classification is the EMS-98 (European Macroseismic Scale, Grunthal, 1998), that provides a detailed description of the most common structural typologies in the european context.  In particular, it must be noticed that the EMS-98 scale analyses in depth the masonry buildings as they are often most vulnerable, especially in the Mediterranean area. This basic classification of masonry buildings was integrated based on the work by Giovinazzi e Lagomarsino (2001), whereas the information provided by FEMA 178 (BSSC, 1992) e 310 (ASCE, 1998) was applied for steel and reinforced concrete structures. The latter data refer to the American design and construction practice and therefore they are not completely satisfactory when applied to other countries. Possible improvements in this area are currently under investigation.

The system recommends performing more detailed off-line analyses (by showing the red light) in the case that a 50% probability of having a slight damage is exceeded.



Bibliography    

ASCE, 1998. FEMA 310: Handbook for the Seismic Evaluation of Buildings — A Pre-standard. Prepared by the American Society of Civil Engineers for the Federal Emergency Management Agency, Washington D.C.

BSSC, 1992. FEMA 178: NEHRP Handbook for the Seismic Evaluation of Existing Buildings. Prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency, Washington D.C.

Cattari, S., Curti, E., Giovinazzi, S., Lagomarsino, S., Parodi, S., Penna, A. 2005. Un modello meccanico per l’analisi di vulnerabilità del costruito in muratura a scala urbana. Atti del 11° Convegno Nazionale ANIDIS: L’ingegneria Sismica in Italia, Genova, Italia, in Italian.

De Marco, R., Martini, M.G., Di Pasquale, G., Fralleone, A., Pizza, A.G. 2000. La classificazione e la normativa sismica dal 1909 al 1984. Servizio Sismico Nazionale, Italia, in Italian.

Eurocode 8 – ENV 1998-1. Design of structures for earthquake resistance.

Giovinazzi, S., Lagomarsino, S. 2001. Una metodologia per l’analisi di vulnerabilità sismica del costruito. Atti del 10° Convegno Nazionale ANIDIS: L’ingegneria Sismica in Italia, Potenza, Italia, in Italian.

Grunthal, G. 1998. European Macroseismic Scale 1998. Chaiers du Centre Europèèn de Géodynamique et de Séismologie, Volume 15, Luxembourg.