Hard rock Rock

Very firm soil or soft rock

Stiff soil


Special soils

>2000 psf

1000-2000 psf <1000 psf

Soils requiring site-specific evaluations:

1. Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils

2. Peats and/or highly organic clays (H > 10 ft of peat and/or highly organic clay where H = thickness of soil)

3. Very high plasticity clays (H> 25 ft [8 m] with PI > 75)

4. Very thick soft/medium stiff clays (H > 120 ft [36 m])

Note: vs, N, su represent the average value of the parameter over the top 30 m (100 ft) of soil. TABLE 19.4 Coefficient Fa as a Function of Site Class and Mapped Spectral Response Acceleration

Site class


maximum considered earthquake spectral response acceleration at short periods

SS = 0.25

SS = 0.50

SS = 0.75

SS = 1.00

Ss = 1.25

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