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Section 1615 Earthquake Loads — Site Ground Motion
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Ground motion accelerations, represented by response spectra and coefficients derived from these spectra, shall be determined in accordance with the general procedure of Section 1615.1, or the sitespecific procedure of Section 1615.2. The sitespecific procedure of Section 1615.2 shall be used for structures on sites classified as Site Class F, in accordance with Section 1615.1.1.
The mapped maximum considered earthquake spectral response acceleration at short periods (SS) shall be 0.365g and at 1second period (S_{1}) shall be 0.071g.
The site class shall be determined in accordance with Section 1615.1.1. The maximum considered earthquake spectral response accelerations at short period and 1second period adjusted for site class effects, _{SMS}^{and}_{SM1},shall be determined in accordance with Section 1615.1.2. The design spectral response accelerations at short period, SDS,and at 1second period, SD1,shall be determined in accordance with Section 1615.1.3. The general response spectrum shall be determined in accordance with Section 1615.1.4
The mapped maximum considered earthquake spectral response acceleration at short periods (SS) shall be 0.365g and at 1second period (S_{1}) shall be 0.071g.
The site class shall be determined in accordance with Section 1615.1.1. The maximum considered earthquake spectral response accelerations at short period and 1second period adjusted for site class effects, _{SMS}^{and}_{SM1},shall be determined in accordance with Section 1615.1.2. The design spectral response accelerations at short period, SDS,and at 1second period, SD1,shall be determined in accordance with Section 1615.1.3. The general response spectrum shall be determined in accordance with Section 1615.1.4
The site shall be classified as one of the site classes defined in Table 1615.1.1. Where the soil shear wave velocity, v_{s}, is not known, site class shall be determined, as permitted in Table 1615.1.1, from standard penetration resistance, N, or from soil undrained shear strength, s_{u}, calculated in accordance with Section 1615.1.5. Where sitespecific data are not available to a depth of 100 feet (30 480 mm), appropriate soil properties are permitted to be estimated by the registered design professional preparing the soils report based on known geologic conditions. When the soil properties are not known in sufficient detail to determine the site class, Site Class D shall be used unless the commissioner determines that Site Class E or F soil is likely to be present at the site.
For SI: 1 foot = 304.8 mm, 1 square foot = 0.0929 m^{2},
1 pound per square foot = 0.0479 kPa. N/A = Not applicable
SITE CLASS 
SOIL PROFILE NAME 
AVERAGE PROPERTIES IN TOP 100 feet, AS PER SECTION 1615.1.5  

Soil shear wave velocity, V _{s},(ft/s) 
Standard penetration resistance, N 
Soil undrained shear strength, S_{u}, (psf) 

A  Hard rock  v_{s} > 5,000  N/A  N/A 
B  Rock  2,500 < V_{s} ≤ 5,000  N/A  N/A 
C  Very dense soil and soft rock  1,200 < v_{s} ≤ 2,500  N > 50  s_{u} ≥ 2,000 
D  Stiff soil profile  600 ≤ v_{s} ≤ 1,200  15 ≤ N ≤ 50  1,000 ≤ s_{u} ≤ 2,000 
E  Soft soil profile  v_{s} < 600  N < 15  s_{u} < 1,000 
E    Any profile with more than 10 feet of soil having the following characteristics:


F    Any profile containing soils having one or more of the following characteristics:

1 pound per square foot = 0.0479 kPa. N/A = Not applicable
The maximum considered earthquake spectral response acceleration for short periods, S_{MS}, and at 1second period, S_{M1},adjusted for site class effects, shall be determined by Equations 1638 and 1639, respectively:
where:
S_{MS} = F_{a}S_{s}
(Equation 1638)
S_{M1} = F_{v}S_{1}
(Equation 1639)
where:
F_{a}  =  Site coefficient defined in Table 1615.1.2(1). 
F_{v}  =  Site coefficient defined in Table 1615.1.2(2). 
S_{s}  =  The mapped spectral accelerations for short periods as determined in Section 1615.1. 
S_{1}  =  The mapped spectral accelerations for a 1second period as determined in Section 1615.1. 
SITE CLASS  F_{a} 

A  0.80 
B  1.00 
C  1.20 
D  1.51 
E  2.13 
F  Note a 
 Sitespecific geotechnical investigation and dynamic site response analyses shall be performed to determine appropriate values, except that for structures with periods of vibration equal or less than 0.5 second, values of F_{a} for liquefiable soils are permitted to be taken equal to the values for the site class determined without regard to liquefaction in Section 1615.1.5.1.
SITE CLASS  Fa 

A  0.80 
B  1.00 
C  1.70 
D  2.4 
E  3.5 
F  Note a 
 Sitespecific geotechnical investigation and dynamic site response analyses shall be performed to determine appropriate values, except that for structures with periods of vibration equal or less than 0.5 second, values of F_{v} for liquefiable soils are permitted to be taken equal to the values for the site class determined without regard to liquefaction in Section 1615.1.5.1.
Fivepercent damped design spectral response acceleration at short periods, SDS, and at 1second period, SD1, shall be determined from Equations 1640 and 1641, respectively:
where:
S_{DS} = (2/3)S_{MS}
(Equation 1640)
where:
S_{D1} = (2/3)S_{M1}
(Equation 1641)
S_{MS}  =  The maximum considered earthquake spectral response accelerations for short period as determined in Section 1615.1.2. 
S_{M1}  =  The maximum considered earthquake spectral response accelerations for 1 second period as determined in Section 1615.1.2. 
he general design response spectrum curve shall be developed as indicated in Figure 1615.1.4 and as follows:
where:
 For periods less than or equal to T_{O}, the design spectral response acceleration, S_{a}, shall be determined by Equation 1642.
 For periods greater than or equal to T_{O} and less than or equal to T_{S},the design spectral response acceleration, S_{a}, shall be taken equal to S_{DS}.
 For periods greater than T_{s}, the design spectral response acceleration, S_{a}, shall be determined by Equation 1643.
S_{a} = 0.6[(S_{DS}/ T_{o})T] + 0.4S_{DS}
(Equation 1642)
S_{a} = S_{DI} / T
(Equation 1643)
where:
S_{DS}  =  The design spectral response acceleration at short periods as determined in Section 1615.1.3. 
S_{D1}  =  The design spectral response acceleration at 1second period as determined in Section 1615.1.3. 
T  =  Fundamental period (in seconds) of the structure (see Section 9.5.5.3 of ASCE 7). 
T_{o} = 0.2 S_{D1} / S_{DS}  
T_{s} = S_{D1} / S_{DS} 
Site classification for Site Class C, D or E shall be determined from Table 1615.1.5.
The notations presented below apply to the upper 100 feet (30 480 mm) of the site profile. Profiles containing distinctly different soil 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:
N_{i} is the Standard Penetration Resistance (ASTM D1586) not to exceed 100 blow/foot (mm) as directly measured in the field without corrections.
where:
Use only d_{i} and N_{i} for cohesionless soils.
where:
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, Site Class A, category 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 (30480mm), surficial shear wave velocity measurements are permitted to be extrapolated to assess vs.
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.
For SI: 1 foot per second = 304.8 mm per second, 1 pound per square foot = 0.0479kN/m^{2}.
The notations presented below apply to the upper 100 feet (30 480 mm) of the site profile. Profiles containing distinctly different soil 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:
v_{si}  =  The shear wave velocity in feet per second (m/s). 
d_{1}  =  The thickness of any layer between 0 and 100 feet (30480 mm). 
(Equation 1644)
N_{i} is the Standard Penetration Resistance (ASTM D1586) not to exceed 100 blow/foot (mm) as directly measured in the field without corrections.
(Equation 1645)
(Equation 1646)
where:
Use only d_{i} and N_{i} for cohesionless soils.
d_{s}  =  The total thickness of cohesionless soil layers in the top 100 feet (30 480 mm). 
S_{ui}  =  The undrained shear strength in psg (kPa), not to exceed 5,000 psf (240 kPa), ASTM D 2166 or D 2850. 
(Equation 1646)
where:
d_{c}  =  The total thickness (100ds) (For SI:30480ds) of cohesive soil layers in the top 100 feet (30480 mm). 
PI  =  The plasticity index, ASTM D 4318. 
W  =  The moisture content in percent, ASTM D 2216. 
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, Site Class A, category 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 (30480mm), surficial shear wave velocity measurements are permitted to be extrapolated to assess vs.
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.
SITE CLASS  v_{s}  N or N_{ch}  s_{u} 

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 
 If the su method is used and the N_{ch} and s_{u} criteria differ, select the category with the softer soils (for example, use Site Class E instead of D).
 Check for the four categories of Site Class F requiring sitespecific evaluation. If the site corresponds to any of these categories, classify the site as Site Class F and conduct a sitespecific evaluation.
 Check for the existence of a total thickness of soft clay > 10 feet (3048 mm) where a soft clay layer is defined by:s_{u} < 500 psf (25 kPa), w ≥ 40 percent,and PI > 20. If these criteria are satisfied, classify the site as Site Class E.
 Categorize the site using one of the following three methods with v_{s}, N, and s_{u} computed in all cases as specified.
 v_{s} for the top 100 feet (30 480 mm)(v_{s} method).
 N for the top 100 feet (30 480 mm)(N method).
 N_{ch} for cohesionless soil layers(PI < 20) in the top 100 feet (30 480 mm) and average, s_{u}, for cohesive soil layers(PI >20) in the top 100 feet (30 480 mm) (s_{u} method).
A sitespecific study shall account for the regional seismicity and geology; the expected recurrence rates and maximum magnitudes of events on known faults and source zones; the location of the site with respect to these; near source effects if any and the characteristics of subsurface site conditions.
Where sitespecific procedures are used as required or permitted by Section 1615, the maximum considered earthquake ground motion shall be taken as that motion represented by an acceleration response spectrum having a 2percent probability of exceedance within a 50year period. The maximum considered earthquake spectral response acceleration at any period, S_{aM}, shall be taken from the 2percent probability of exceedance within a 50year period spectrum.
Exception: Where the spectral response ordinates at 0.2 second or 1 second for a 5percent damped spectrum having a 2percent probability of exceedance within a 50year period exceed the corresponding ordinates of the deterministic limit of Section 1615.2.2, the maximum considered earthquake ground motion spectrum shall be taken as the lesser of the probabilistic maximum considered earthquake ground motion or the deterministic maximum considered earthquake ground motion spectrum of Section 16 15.2.3, but shall not be taken as less than the deterministic limit ground motion of Section 1615.2.2.
The deterministic limit for the maximum considered earthquake ground motion shall be the response spectrum determined in accordance with Figure 1615.2.2, where site coefficients, F_{a} and F_{v}, are determined in accordance with Section 1615.1.2, with the value of the mapped shortperiod spectral response acceleration, SS, taken as 1.5g and the value of the mapped spectral response acceleration at 1 second, S_{1}, taken as 0.6g.
The deterministic maximum considered earthquake ground motion response spectrum shall be calculated as 150 percent of the median spectral response accelerations, S_{aM}, at all periods resulting from a characteristic earthquake on any known active fault within the region.
Where sitespecific procedures are used to determine the maximum considered earthquake ground motion response spectrum, the design spectral response acceleration, Sa, at any period shall be determined from Equation 1648:
and shall be greater than or equal to 80 percent of the design spectral response acceleration, Sa, determined by the general response spectrum in Section 1615.1.4.
S_{a} = 2/3 S_{aM}
(Equation 1648)
and shall be greater than or equal to 80 percent of the design spectral response acceleration, Sa, determined by the general response spectrum in Section 1615.1.4.
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