In Belgian regulations, the zones considered to be zones with very low seismicity are the zones for which the product a
gS (with a
g = γ
I . a
gr) does not exceed 0.06g (0.59
m/s2). Here is the summary:
Zone/Ground type | A | B | C | D | E |
Zone 0 | No |
Zone 1 | No | Yes (except I and II) | Yes (except I) |
Zone 2 | Yes (except I and II) | Yes |
Zone 3 | Yes |
Zone 4 | Yes |
The soil factor S is depending of the stratigraphic profile of the ground and the type of recommended elastic response spectra with respect to the surface-wave magnitude.
Ground type | Description of stratigraphic profile | S parameter for spectrum type 2 |
A | Rock or other rock-like geological formation, including at most 5m of weaker material at the surface. | 1,0 |
B | Deposits of very dense sand, gravel, or very stiff clay, at least several tens of metres in thickness, characterised by a gradual increase of mechanical properties with depth. | 1,35 |
C | Deep deposits of dense or medium-dense sand, gravel or stiff clay with thickness from several tens to many hundreds of metres. | 1,5 |
D | Deposits of loose-to-medium cohesionless soil (with or without some soft cohesive layers), or of predominantly soft-to-firm cohesive soil. | 1,8 |
E | A soil profile consisting of a surface alluvium layer with Vs values of type C or D and thickness varying between about 5m and 20m, underlain by stiffer material with Vs>800m/s. | 1,6 |
Buildings are classified in 4 importance classes, depending on the consequences of collapse for human life, on their importance for public safety and civil protection in the immediate post-earthquake period, and on the social and economic consequences of collapse.
Each importance class is attached to an importance factor γ
I.
Importance class | Buildings | γ I |
I | Buildings of minor importance for public safety, e.g. agricultural buildings, etc. | 0.8 |
II | Ordinary buildings, not belonging in the other categories. | 1.0 |
III | Buildings whose seismic resistance is of importance in view of the consequences associated with a collapse, e.g. schools, assembly halls, cultural institutions etc. | 1.2 |
IV | Buildings whose integrity during earthquakes is of vital importance for civil protection, e.g. hospitals, fire stations, power plants, etc. | 1.4 |
NOTE: Importance classes I, II and III or IV correspond roughly to consequences classes CC1, CC2 and CC3, respectively, defined in the Annex B to Eurocode 0.