# 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.

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 S _{1}= 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 S _{1}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}T _{0}= Period at which the constant acceleration and constant velocity regions of the design spectrum intersect for 0.2T

_{0}< T < T_{0}(3-5)

for T

_{0}> T_{0}(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.

SITE CLASS | S_{s} | ||||

< 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.

SITE CLASS | S_{1} | ||||

< 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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