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1613.2.3 Site Coefficients and Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameters

Building Code 2018 of Illinois > 16 Structural Design > 1613 Earthquake Loads > 1613.2 Seismic Ground Motion Values > 1613.2.3 Site Coefficients and Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameters

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The maximum considered earthquake spectral response acceleration for short periods, S_MS, and at 1-second period, S_M1, adjusted for site class effects shall be determined by Equations 16-36 and 16-37, respectively:

(Equation 16-36)

(Equation 16-37)

but S_MS shall not be taken less than S_M1 except when determining the seismic design category in accordance with Section 1613.2.5.

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.

S₁ = 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.

TABLE 1613.2.3(1)

VALUES OF SITE COEFFICIENT F_a^a

SITE CLASS	MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) SPECTRAL RESPONSE ACCELERATION PARAMETER AT SHORT PERIOD
SITE CLASS	S_s ≤ 0.25	S_s = 0.50	S_s = 0.75	S_s = 1.00	S_s = 1.25	S_s ≥ 1.5
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.

TABLE 1613.2.3(2)

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
SITE CLASS	S₁ ≤ 0.1	S₁ = 0.2	S₁ = 0.3	S₁ = 0.4	S₁ = 0.5	S₁ ≥ 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

Use straight-line interpolation for intermediate values of mapped spectral response acceleration at 1-second period, S₁.
Values shall be determined in accordance with Section 11.4.8 of ASCE 7.
See requirements for site-specific ground motions in Section 11.4.8 of ASCE 7.

Related Code Sections

1613.2.3 Structural Design, Site Coefficients and Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameters

The maximum considered earthquake spectral response acceleration for short periods, S MS , and at 1-second period, S M1 , adjusted for site class ...

1603.1.5 Structural Design, Earthquake Design Data

Site class . Design spectral response acceleration parameters, S DS and SD 1 . Seismic design category . Basic seismic force-resisting ...

Building Code 2018 of Illinois > 16 Structural Design > 1603 Construction Documents > 1603.1 General > 1603.1.5 Earthquake Design Data