API Std 650 10th Ed. Add.3 - Seismic design of storage tanks
Seismic zone factor Z
The seismic zone factor (horizontal seismic acceleration) is determined by the purchaser or the appropriate government authority that has jurisdiction. The seismic zone maps of Figure E-1 in the standard, the seismic zone tabulation for areas outside the United States (see Table E-1 below), or the Supplement to National Building Code of Canada may be used as an aid to determine the seismic zone. Table E-2 below can be used to determine the seismic zone factor, Z.
Table E-1 - Seismic Zone Tabulation for Areas Outside the United States
Atlantic Ocean areaAzores2B
CaribbeanBahama Islands1
Canal Zone2B
Leeward Islands3
Puerto Rico3
Trinidad Island2B
North AmericaGreenland1
North AmericaIcelandKeflavik3
Pacific Ocean areaCaroline IslandsKoror,Paulau2B
Johnston Island1
Mariana IslandGuam3
Marcus Island1
Philippine Islands3
Samoa Islands3
Wake Island0
Table E-2 - Seismic Zone Factor (horizontal acceleration)
Seismic FactorSeismic Zone Factor
Importance Factor I
I=1.0 for all tanks unless a larger importance factor is specified by the purchaser. The I factor should not exceed 1.25, and this maximum value should be applied only to tanks that must provide emergency post-earthquake service or to tanks that store toxic or explosive substances in areas where an accidental release of product would be considered to be dangerous to the safety of the general public.
Table E-3 - Site Coefficients (See Note)
TypeDescriptionS Factor
S1 A soil profile with either a) a rock-like material characterized by a shear wave velocity greater than 760 m/sec (2500 ft/sec) or by other suitable means of classification or b) stiff or dense soil conditions where the soil depth is less than 60 m (200 ft)1.0
S2 A soil profile with stiff or dense soil conditions where the soil depth exceeds 60 m (200 ft)1.2
S3 A soil profile 12 m (40 ft) or more in depth containing more than 6 m (20 ft) of soft to medium stiff clay but more than 12 m (40 ft) of soft clay1.5
S4 A soil profile containing more than 12 m (40 ft) of soft clay2.0
Note: The site factor shall be established from properly substantiated geotechnical data. In locations where the soil properties are not known in sufficient detail to determine the soil profile type, soil profile S3 shall be used. Soil profile S4 need not be assumed unless the building official determines that soil profile S4 may be present at the site or in the event that soil profile S4 is established by geotechnical data.