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3103F.4.2.4 Simplified Evaluation of Site Amplification Effects
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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.
- 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. (3-3)
where:PGAX = peak ground acceleration corresponding to the site class under consideration. When the value of PGAX 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.- PGAX, SXS, and SX1 constitute three spectral acceleration values for the site class under consideration corresponding to periods of 0, SS (usually 0.2 seconds), and 1.0 second, respectively.
- The final response spectra, without consideration for near-fault directivity effects, values of Sa 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 PGAX is the DPGA. However, the Sa shall be modified for near-fault directivity effects, per Section 3103F.4.2.6 to obtain the final DSAs.
SITE CLASS | Ss | ||||
< 0.25 | 0.5 | 0.75 | 1.0 | > 1.25 | |
A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
B | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 |
C | 1.2 | 1.2 | 1.1 | 1.0 | 1.0 |
D | 1.6 | 1.4 | 1.2 | 1.1 | 1.0 |
E | 2.5 | 1.7 | 1.2 | 0.9 | 0.9 |
F | * | * | * | * | * |
Note: Linear interpolation can he used to estimate values of Fa for intermediate values of Ss.
SITE CLASS | S1 | ||||
< 0.1 | 0.2 | 0.3 | 0.4 | > 0.5 | |
A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
B | 1.0 | 1.0 | 1.0 | 1.0 | 1.0 |
C | 1.7 | 1.6 | 1.5 | 1.4 | 1.3 |
D | 2.4 | 2.0 | 1.8 | 1.6 | 1.5 |
E | 3.5 | 3.2 | 2.8 | 2.4 | 2.4 |
F | * | * | * | * | * |
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