Dear reader we were discussing about determination of shear wave velocity for earthquake design according to International Building Code. We have learnt about site class A, class B and class C. we know site class F is not dependent on shear wave velocity, this mostly include collapsible, sensitive, liquefiable and very plastic soils.
In determining site class A, where soil profile shows a continuation of hard rock up to 100 ft (30480 mm), we can determine shear wave velocity by extrapolation from surficial shear wave velocity.
IBC also prohibit to classify site as class A or class B, when there have soil layer of more than 3048mm between bottom of mat foundation or spread footing and rock surface; it should keep in mind that site as class A or class B are considered rock category.
Dear reader in site classification for site class C, class D and class E where soft rock to soft soil profile exist, v̅S, N̅, N̅ chand s̄u used as per code, considered for upper 30480mm (100ft)of site profile. In such cases, 100ft site profile have several distinctive layer of different soil or rock and they are subdivided into respective layers and designated as ranges of number form 1-n. n is the bottom layer and n distinct soil or rock layer are considered in upper 30480 mm profile.
i represents a layer between 1 and n
Vsi=shear wave velocity (ft/sec)
di= thickness of a layer within upper 30480 mm profile