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Section 1613 Earthquake Loads

Building Code 2018 of Illinois > 16 Structural Design > 1613 Earthquake Loads

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1613.1 Scope

Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with Chapters 11, 12, 13, 15, 17 and 18 of ASCE 7, as applicable. The seismic design category for a structure is permitted to be determined in accordance with Section 1613 or ASCE 7.

Exceptions:

Detached one- and two-family dwellings, assigned to Seismic Design Category A, B or C, or located where the mapped short-period spectral response acceleration, S_S, is less than 0.4 g.
The seismic force-resisting system of wood-frame buildings that conform to the provisions of Section 2308 are not required to be analyzed as specified in this section.
Agricultural storage structures intended only for incidental human occupancy.
Structures that require special consideration of their response characteristics and environment that are not addressed by this code or ASCE 7 and for which other regulations provide seismic criteria, such as vehicular bridges, electrical transmission towers, hydraulic structures, buried utility lines and their appurtenances and nuclear reactors.
References within ASCE 7 to Chapter 14 shall not apply, except as specifically required herein.

1613.2 Seismic Ground Motion Values

Seismic ground motion values shall be determined in accordance with this section.

1613.2.1 Mapped Acceleration Parameters

The parameters S_S and S₁ 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 S₁ 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.

FIGURE 1613.2.1(1)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR THE CONTERMINOUS UNITED STATES OF 0.2-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(2)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR THE CONTERMINOUS UNITED STATES OF 1-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(3)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER) GROUND MOTION RESPONSE ACCELERATIONS FOR HAWAII OF 0.2- AND 1-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(4)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR ALASKA OF 0.2-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(5)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR ALASKA OF 1.0-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(6)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR PUERTO RICO AND THE UNITED STATES VIRGIN ISLANDS OF 0.2- AND 1-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(7)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR GUAM AND THE NORTHERN MARIANA ISLANDS OF 0.2- AND 1-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

FIGURE 1613.2.1(8)
RISK-TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCE_R) GROUND MOTION RESPONSE ACCELERATIONS FOR AMERICAN SAMOA OF 0.2- AND 1-SECOND SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING)

1613.2.2 Site Class Definitions

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).

1613.2.3 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 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.

1613.2.4 Design Spectral Response Acceleration Parameters

Five-percent damped design spectral response acceleration at short periods, S_DS, and at 1-second period, S_D1, shall be determined from Equations 16-38 and 16-39, respectively:

(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.

1613.2.5 Determination of Seismic Design Category

Structures classified as Risk Category I, II or III that are located where the mapped spectral response acceleration parameter at 1-second period, S₁, 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, S₁, 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, S_DS and S_D1, 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.

TABLE 1613.2.5(1)

SEISMIC DESIGN CATEGORY BASED ON SHORT-PERIOD (0.2 second) RESPONSE ACCELERATION

VALUE OF S_DS	RISK CATEGORY
VALUE OF S_DS	I or II	III	IV
S_DS< 0.167g	A	A	A
0.167g ≤ S_DS < 0.33g	B	B	C
0.33g ≤ S_DS < 0.50g	C	C	D
0.50g ≤ S_DS	D	D	D

TABLE 1613.2.5(2)

SEISMIC DESIGN CATEGORY BASED ON 1-SECOND PERIOD RESPONSE ACCELERATION

VALUE OF S_D1	RISK CATEGORY
VALUE OF S_D1	I or II	III	IV
S_D1< 0.067g	A	A	A
0.067g ≤ S_D1 < 0.133g	B	B	C
0.133g ≤ S_D1 < 0.20g	C	C	D
0.20g ≤ S_D1	D	D	D

1613.2.5.1 Alternative Seismic Design Category Determination

Where S₁ 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).

1613.2.5.2 Simplified Design Procedure

Where the alternate simplified design procedure of ASCE 7 is used, the seismic design category shall be determined in accordance with ASCE 7.

1613.3 Ballasted Photovoltaic Panel Systems

Ballasted, roof-mounted photovoltaic panel systems need not be rigidly attached to the roof or supporting structure. Ballasted non-penetrating systems shall be designed and installed only on roofs with slopes not more than one unit vertical in 12 units horizontal. Ballasted nonpenetrating systems shall be designed to resist sliding and uplift resulting from lateral and vertical forces as required by Section 1605, using a coefficient of friction determined by acceptable engineering principles. In structures assigned to Seismic Design Category C, D, E or F, ballasted nonpenetrating systems shall be designed to accommodate seismic displacement determined by nonlinear response-history or other approved analysis or shake-table testing, using input motions consistent with ASCE 7 lateral and vertical seismic forces for nonstructural components on roofs.