3103F.4.2.4 Simplified Evaluation of Site Amplification Effects

When the MOT site class is different from the Site Classes B to C boundary, site amplification effects shall be incorporated in peak ground accelerations and spectral accelerations. This may be accomplished using a simplified method or a site-specific evaluation (Section 3103F.4.2.5).

For a given site class, the following procedure from Chapter 1 of FEMA 356 [3.1] presents a simplified method that may be used to incorporate the site amplification effects for peak ground acceleration and spectral acceleration computed for the Site Classes B and C boundary.

1. Calculate the spectral acceleration values at 0.20 and 1.0 second period:

   (3-1)

   (3-2)

   where:

   Fa	=	site coefficient obtained from Table 31F-3-3
   Fv	=	site coefficient obtained from Table 31F-3-4
   SS	=	short period (usually at 0.20 seconds) spectral acceleration value (for the boundary of Site Classes B and C) obtained using Section 3103F.4.2.2, or at the period corresponding to the peak in spectral acceleration values when obtained from Section 3103F.4.2.3
   S1	=	spectral acceleration value (for the boundary of Site Classes B and C) at 1.0 second period
   SXS	=	spectral acceleration value obtained using the short period Ss and factored by Table 31F-3-3 for the site class under consideration.
   SX1	=	spectral acceleration value obtained using the 1.0 second period S1 and factored by Table 31F-3-4 for the site class under consideration.

2. (3-3)

   where:

   PGAX

   =

   peak ground acceleration corresponding to the site class under consideration.

   When the value of PGA_X is less than the peak ground acceleration obtained following Section 3103F.4.2.2 or Section 3103F.4.2.3, an explanation of the results shall be provided.

3. PGA_X, S_XS, and S_X1 constitute three spectral acceleration values for the site class under consideration corresponding to periods of 0, S_S (usually 0.2 seconds), and 1.0 second, respectively.

4. The final response spectra, without consideration for near-fault directivity effects, values of S_a for the site class under consideration may be obtained using the following equations (for 5 percent critical damping):

   (3-4)

   where:

   T	=	Period corresponding to calculated Sa
   T0	=	Period at which the constant acceleration and constant velocity regions of the design spectrum intersect

   for 0.2T₀ < T < T₀

   (3-5)

   for T₀ > T₀

   (3-6)

   where:

   (3-7)

   The resulting PGA_X is the DPGA. However, the S_a shall be modified for near-fault directivity effects, per Section 3103F.4.2.6 to obtain the final DSAs.