1613.5 Seismic Ground Motion Values

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

The mapped maximum considered earthquake spectral response acceleration at short periods (S_S) shall be 0.281 g and at 1-second period (S₁) shall be 0.073 g. The mapped long-period transition period (T_L) shall l be 6 seconds.

Based on the site soil properties, the site shall be classified as either Site Class A, B, C, D, E or F in accordance with Table 1613.5.2. Where the soil properties are not known in sufficient detail to determine the site class, Site Class D shall be used unless the commissioner or geotechnical data determines that Site Class E or F soil is present at the site.

TABLE 1613.5.2
SITE CLASS DEFINITIONS

SITE CLASS	SOIL PROFILE NAME	AVERAGE PROPERTIES IN TOP 100 feet, AS PER SECTION 1615.1.5
		Soil shear wave velocity, , (ft/s)	Standard penetration resistance,	Soil undrained shear strength, , (psf)
A	Hard rock	> 5,000	N/A	N/A
B	Rock	2,500 <≤ 5,000	N/A	N/A
C	Very dense soil and soft rock	1,200 <≤ 2,500	> 50	≥ 2,000
D	Stiff soil profile	600 ≤≤ 1,200	15 ≤ ≤ 50	1,000 ≤≤ 2,000
E	Soft soil profile	< 600	< 15	< 1,000
E	—	Any profile with more than 10 feet of soil having the following characteristics: Plasticity index PI > 20, Moisture content w ≥ 40%, and Undrained shear strength < 500 psf.
F	—	Any profile containing soils having one or more of the following characteristics: Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, collapsible weakly cemented soils. Peats and/or highly organic clays (H > 10 feet of peat and/or highly organic clay where H = thickness of soil). Very high plasticity clays (H > 25 feet with plasticity index PI >75). Very thick soft/medium stiff clays (H > 120 feet).

The MCE_R spectral response acceleration parameters for short periods, S_MS, and at 1-second period, S_M1, adjusted for site class effects shall be determined by Equations 16-47 and 16-48, respectively:



where:

Fa	=	Site coefficient defined in Table 1613.5.3(1).
Fv	=	Site coefficient defined in Table 1613.5.3(2).
SS	=	The mapped MCER spectral accelerations for short periods as determined in Section 1613.5.1.
S1	=	The mapped MCER spectral accelerations for a 1-second period as determined in Section 1613.5.1.

TABLE 1613.5.3(1) VALUES OF SITE COEFFICIENT F_a AS A FUNCTION OF SITE CLASS AND MAPPED SPECTRAL RESPONSE ACCELERATION AT SHORT PERIODS (SS)^a

SITE CLASS	Fa
A	0.80
B	1.00
C	1.20
D	1.57
E	2. 37
F	Note a

TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT F_v AS A FUNCTION OF SITE CLASS AND MAPPED SPECTRAL RESPONSE ACCELERATION AT 1-SECOND PERIOD (S₁)^a

SITE CLASS	Fv
A	0.80
B	1.00
C	1.70
D	2.40
E	3.50
F	Note a

Five-percent damped design spectral response acceleration at short periods, S_DS, and at 1-second period, S_D1, shall be determined from Equation 16-49 and 16-50, respectively:



where:

SMS	=	The MCER spectral response accelerations for short period as determined in Section 1613.5.3.
SM1	=	The MCER spectral response accelerations for 1-second period as determined in Section 1613.5.3.

Site classification for Site Class C, D or E shall be determined from Table 1613.5.5. The notations presented below apply to only materials encountered above rock meeting Class 1a, 1b, or 1c as defined in Section 1804 or rock with shear wave velocity greater than 2500 feet per second (762 meters per second) to a maximum depth of 100 feet (30 480 mm). Profiles containing distinctly different soil and rock layers shall be subdivided into those layers designated by a number that ranges from 1 to n at the bottom where there is a total of n distinct layers in the upper 100 feet (30 480 mm). The symbol i then refers to any one of the layers between 1 and n.

where:

vsi	=	The shear wave velocity in feet per second (m/s).
di	=	The thickness of any layer between 0 and 100 feet (30 480 mm).

where:

N_i is the Standard Penetration Resistance (ASTM D1586) not to exceed 100 blows/foot (328 blows/m) as directly measured in the field without corrections. When refusal is met for a rock layer of Class 1d, N_i shall be less than or equal to 100 blows/foot (328 blows/m) provided that the extent of the Class 1d material is confirmed by a boring to a depth where Class 1c or better rock is determined, not to exceed 100 feet. Alternatively, if this boring is not performed, site classification should be based on all soil material that is above the Class 1d layer.



where:

Use di and Ni for cohesionless soil layers only in Equation 16-42.

ds	=	The total thickness of cohesionless soil layers in the top 100 feet (30 480 mm).
m	=	The number of cohesionless soil layers in the top 100 feet (30 480 mm).
Sui	=	The undrained shear strength in psf (kPa), not to exceed 5,000 psf (240 kPa), ASTM D 2166 or D 2850.

where:

dc	=	The total thickness (100-ds) (For SI: 30480-ds) of cohesive soil layers in the top 100 feet (30 480 mm).
k	=	The number of cohesive soil layers in the top 100 feet (30 480 mm).
PI	=	The plasticity index, ASTM D 4318.
w	=	The moisture content in percent, ASTM D 2216.

Where a site does not qualify under the criteria for Site Class F and there is a total thickness of soft clay greater than 10 feet (3048 mm) where a soft clay layer is defined by < 500 psf (24 kPa), w ≥ 40 percent, and PI > 20, it shall be classified as Site Class E. The shear wave velocity for rock, Site Class B, shall be either measured on site or estimated by a geotechnical engineer or engineering geologist/seismologist for competent rock with moderate fracturing and weathering. Softer and more highly fractured and weathered rock shall either be measured on site for shear wave velocity or classified as Site Class C. The hard rock category, Site Class A, shall be supported by shear wave velocity measurements either on site or on profiles of the same rock type in the same formation with an equal or greater degree of weathering and fracturing. Where hard rock conditions are known to be continuous to a depth of 100 feet (30 480 mm), surficial shear wave velocity measurements are permitted to be extrapolated to assess v_s. The rock categories, Site Classes A and B, shall not be used if there is more than 10 feet (3048 mm) of soil between the rock surface and the bottom of the spread footing or mat foundation.

TABLE 1613.5.5 SITE CLASSIFICATION^a

SITE CLASS		or
E	< 600 ft/s	< 15	< 1,000 psf
D	600 to 1,200 ft/s	15 to 50	1,000 to 2,000 psf
C	1,200 to 2,500 ft/s	> 50	> 2,000

For SI: 1 foot per second = 304.8 mm per second, 1 pound per square foot = 0.0479 kN/m2.

a. If the method is used and the and criteria differ, select the category with the softer soils (for example, use Site Class E instead of D).

All structures shall be assigned to a seismic design category based on their risk category determined in accordance with Table 1604.5 and the design spectral response acceleration parameters S_DS and S_D1, determined in accordance with Section 1613.5.4 or the site-specific procedures of ASCE 7-10. Each building and structure shall be assigned to the more severe seismic design category in accordance with Table 1613.5.6(1) or 1613.5.6(2), irrespective of the fundamental period of vibration of the structure, T.

TABLE 1613.5.6(1) SEISMIC DESIGN CATEGORY BASED ON SHORT-PERIOD (0.2 SECOND) RESPONSE ACCELERATIONS

VALUE OF SDS	RISK CATEGORYa
	I & II	III	IV
SDS < 0.167g	A	A	A
0.167g < SDS < 0.33g	B	B	C
0.33g < SDS < 0.50g	C	C	D
0.50g < SDS	Da	Da	Da

TABLE 1613.5.6(2) SEISMIC DESIGN CATEGORY BASED ON 1-SECOND PERIOD RESPONSE ACCELERATION

VALUE OF SD1	RISK CATEGORYa
	I & II	III	IV
SD1 < 0.067g	A	A	A
0.067g < SD1 < 0.133g	B	B	C
0.133g < SD1 < 0.20g	C	C	D
0.20g ≤ SD1	Da	Da	Da

The seismic design category is permitted to be determined from Table 1613.5.6(1) alone when all of the following apply:

1. In each of the two orthogonal directions, the approximate fundamental period of the structure, T_a, as determined in accordance with Section 12.8.2.1 of ASCE 7-10, is less than 0.8 T_s, as determined in accordance with Section 11.4.5 of ASCE 7-10.
2. In each of the two orthogonal directions, the fundamental period of the structure used to calculate the story drift is less than T_s.
3. Equation 12.8-2 of ASCE 7-10 is used to determine the seismic response coefficient, C_s.
4. The diaphragms are rigid, as defined in Section 12.3.1 of ASCE 7-10 or, for diaphragms that are flexible, the distances between the vertical elements of the seismic-force-resisting system do not exceed 40 feet (12 192 mm).

Where the alternate simplified design procedure of ASCE 7-10 is used, the seismic design category shall be determined in accordance with ASCE 7-10.