# 1613.2 Seismic Ground Motion Values

The parameters S_{S} and S_{1} shall be determined from the 0.2 and 1-second spectral response accelerations shown on Figures 1613.2.1(1) through 1613.2.1(8). Where *S _{1}* is less than or equal to 0.04 and S

_{S}is less than or equal to 0.15, the structure is permitted to be assigned Seismic Design Category A.

Based on the site soil properties, the site shall be classified as Site Class A, B, C, D, E or F in accordance with Chapter 20 of ASCE 7.

Where the soil properties are not known in sufficient detail to determine the site class, Site Class D, subjected to the requirements of Section 1613.2.3, shall be used unless the building official or geotechnical data determines that Site Class E or F soils are present at the site.

Where site investigations that are performed in accordance with Chapter 20 of ASCE 7 reveal rock conditions consistent with Site Class B, but site-specific velocity measurements are not made, the site coefficients F_{a} and F_{v} shall be taken at unity (1.0).

The maximum considered earthquake spectral response acceleration for short periods, *S _{MS}*, and at 1-second period,

*S*, adjusted for site class effects shall be determined by Equations 16-36 and 16-37, respectively:

_{M1}**(Equation 16-36)**

**(Equation 16-37)**

but *S _{MS}* shall not be taken less than

*S*except when determining the seismic design category in accordance with Section 1613.2.5.

_{M1}where:

*F _{a}* = Site coefficient defined in Table 1613.2.3(1).

*F _{v}* = Site coefficient defined in Table 1613.2.3(2).

*S _{S}* = The mapped spectral accelerations for short periods as determined in Section 1613.2.1.

*S _{1}* = The mapped spectral accelerations for a 1-second period as determined in Section 1613.2.1.

Where Site Class D is selected as the default site class per Section 1613.2.2, the value of F_{a} shall be not less than 1.2. Where the simplified design procedure of ASCE 7 Section 12.14 is used, the value of F_{a} shall be determined in accordance with ASCE 7 Section 12.14.8.1, and the values of F_{v}, S_{MS} and S_{M1} need not be determined.

VALUES OF SITE COEFFICIENT *F _{a}*

^{ a}

SITE CLASS | MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_{R}) SPECTRAL RESPONSE ACCELERATION PARAMETER AT SHORT PERIOD | |||||

S ≤ 0.25_{s} | S = 0.50_{s} | S = 0.75_{s} | S = 1.00_{s} | S = 1.25_{s} | S ≥ 1.5_{s} | |

A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |

B | 0.9 | 0.9 | 0.9 | 0.9 | 0.9 | 0.9 |

C | 1.3 | 1.3 | 1.2 | 1.2 | 1.2 | 1.2 |

D | 1.6 | 1.4 | 1.2 | 1.1 | 1.0 | 1.0 |

E | 2.4 | 1.7 | 1.3 | Note b | Note b | Note b |

F | Note b | Note b | Note b | Note b | Note b | Note b |

- Use straight-line interpolation for intermediate values of mapped spectral response acceleration at short period,
*S*._{s} - Values shall be determined in accordance with Section 11.4.8 of ASCE 7.

VALUES OF SITE COEFFICIENT *F _{V}^{a}*

SITE CLASS | MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_{R}) SPECTRAL RESPONSE ACCELERATION PARAMETER AT 1-SECOND PERIOD | |||||

S_{1} ≤ 0.1 | S_{1} = 0.2 | S_{1} = 0.3 | S_{1} = 0.4 | S_{1} = 0.5 | S_{1} ≥ 0.6 | |

A | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |

B | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |

C | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.4 |

D | 2.4 | 2.2^{c} | 2.0^{c} | 1.9^{c} | 1.8^{c} | 1.7^{c} |

E | 4.2 | 3.3^{c} | 2.8^{c} | 2.4^{c} | 2.2^{c} | 2.0^{c} |

F | Note b | Note b | Note b | Note b | Note b | Note b |

Five-percent damped design spectral response acceleration at short periods, *S _{DS}*, and at 1-second period,

*S*, shall be determined from Equations 16-38 and 16-39, respectively:

_{D1}**(Equation 16-38)**

**(Equation 16-39)**

where:

*S _{MS}* = The maximum considered earthquake spectral response accelerations for short period as determined in Section 1613.2.3.

*S _{M1}* = The maximum considered earthquake spectral response accelerations for 1-second period as determined in Section 1613.2.3.

Structures classified as Risk Category I, II or III that are located where the mapped spectral response acceleration parameter at 1-second period, *S _{1}*, is greater than or equal to 0.75 shall be assigned to Seismic Design Category E. Structures classified as Risk Category IV that are located where the mapped spectral response acceleration parameter at 1-second period,

*S*, is greater than or equal to 0.75 shall be assigned to Seismic Design Category F. Other structures shall be assigned to a seismic design category based on their risk category and the design spectral response acceleration parameters,

_{1}*S*and

_{DS}*S*, determined in accordance with Section 1613.2.4 or the site-specific procedures of ASCE 7. Each building and structure shall be assigned to the more severe seismic design category in accordance with Table 1613.2.5(1) or 1613.2.5(2), irrespective of the fundamental period of vibration of the structure, T.

_{D1}SEISMIC DESIGN CATEGORY BASED ON SHORT-PERIOD (0.2 second) RESPONSE ACCELERATION

VALUE OF S_{DS} | RISK CATEGORY | ||

I or II | III | IV | |

S< 0.167g_{DS } | A | A | A |

0.167g ≤ S < 0.33g_{DS } | B | B | C |

0.33g ≤ S < 0.50g_{DS } | C | C | D |

0.50g ≤ S_{DS } | D | D | D |

SEISMIC DESIGN CATEGORY BASED ON 1-SECOND PERIOD RESPONSE ACCELERATION

VALUE OF S_{D1} | RISK CATEGORY | ||

I or II | III | IV | |

S< 0.067g_{D1 } | A | A | A |

0.067g ≤ S < 0.133g_{D1 } | B | B | C |

0.133g ≤ S < 0.20g_{D1 } | C | C | D |

0.20g ≤ S_{D1 } | D | D | D |

Where S_{1} is less than 0.75, the seismic design category is permitted to be determined from Table 1613.2.5(1) alone where all of the following apply:

- In each of the two orthogonal directions, the approximate fundamental period of the structure, T
_{a}, in each of the two orthogonal directions determined in accordance with Section 12.8.2.1 of ASCE 7, is less than 0.8 T_{s}determined in accordance with Section 11.8.6 of ASCE 7. - In each of the two orthogonal directions, the fundamental period of the structure used to calculate the story drift is less than T
_{s}. - Equation 12.8-2 of ASCE 7 is used to determine the seismic response coefficient, C
_{s}. - The diaphragms are rigid or are permitted to be idealized as rigid in accordance with Section 12.3.1 of ASCE 7 or, for diaphragms permitted to be idealized as flexible in accordance with Section 12.3.1 of ASCE 7, the distances between vertical elements of the seismic force-resisting system do not exceed 40 feet (12 192 mm).

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