How to Determine Shear Wave Velocity for Earthquake Design, IBC

In our last post we have learnt that IBC classify site as Class A, Class B, Class C, Class D, Class E and Class F for earthquake design. Of these except class F, all are depend on shear wave velocity. In other building code, shear wave velocity is also important to classify site depending on soil properties.

Site class A and class B are defined as rock (soil profile), class C also include soft rock and dense soil. At first we start with site class B. it is recommended in code that shear wave velocity to measure on site.

Alternatively, seismologist/ engineering geologist or geotechnical engineer can estimate shear wave velocity on competent rock having moderate weathering and fracturing.

Site class C can be determined by determining value of shear wave velocity. But the rocks that are relatively weathered and fractured highly and also softer, we can classify them as class C.

The soil profile representing site class A is hard rock. Here also shear wave velocity is recommended to measure on the site. Alternatively, a value, measured in soil having identical soil profile of same type of rock in identical formation having a same or more fracturing or weathering condition, is recommended in code.

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, v̅_S, N̅, N̅ _chand s̄_u 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.

Where

i represents a layer between 1 and n

V_si=shear wave velocity (ft/sec)

d_i= thickness of a layer within upper 30480 mm profile