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3103F.4.2.4 Simplified Evaluation of Site Amplification Effects

California Building Code 2016 (Vol 1 & 2) > 31F [SLC] Marine Oil Terminals > 3103F Structural Loading Criteria > 3103F.4 Earthquake Loads > 3103F.4.2 Design Earthquake Motion Parameters > 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:

F_a	=	site coefficient obtained from Table 31F-3-3
F_v	=	site coefficient obtained from Table 31F-3-4
S_S	=	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
S_XS	=	spectral acceleration value obtained using the short period S_s and factored by Table 31F-3-3 for the site class under consideration.
S_X1	=	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:
PGA_X = 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.
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.
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 S_a
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.

TABLE 31F-3-3
VALUES OF F_a
SITE CLASS	S_s
SITE CLASS	< 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 F_a for intermediate values of S_s.

* Site-specific dynamic site response analysis shall be performed.

TABLE 31F-3-4
VALUES OF F_v
SITE CLASS	S₁
SITE CLASS	< 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	*	*	*	*	*

Note: Linear interpolation can he used to estimate values of F_v for intermediate values of S₁.

* Site-specific dynamic site response analysis shall be performed.

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