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1613.5 Seismic Ground Motion Values
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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 1second period (S_{1}) shall be 0.073 g. The mapped longperiod 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.
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:
 
F  —  Any profile containing soils having one or more of the following characteristics:

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.
The MCE_{R} spectral response acceleration parameters for short periods, S_{MS}, and at 1second period, S_{M1}, adjusted for site class effects shall be determined by Equations 1647 and 1648, respectively:
where:
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}
TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT F_{v} AS A FUNCTION OF SITE CLASS AND MAPPED SPECTRAL RESPONSE ACCELERATION AT 1SECOND PERIOD (S_{1})^{a}
where:
F_{a}  =  Site coefficient defined in Table 1613.5.3(1). 
F_{v}  =  Site coefficient defined in Table 1613.5.3(2). 
S_{S}  =  The mapped MCE_{R} spectral accelerations for short periods as determined in Section 1613.5.1. 
S_{1}  =  The mapped MCE_{R} spectral accelerations for a 1second 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  F_{a} 
A  0.80 
B  1.00 
C  1.20 
D  1.57 
E  2. 37 
F  Note a 
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 1613.5.5.
TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT F_{v} AS A FUNCTION OF SITE CLASS AND MAPPED SPECTRAL RESPONSE ACCELERATION AT 1SECOND PERIOD (S_{1})^{a}
SITE CLASS  F_{v} 
A  0.80 
B  1.00 
C  1.70 
D  2.40 
E  3.50 
F  Note a 
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 1613.5.5.
Fivepercent damped design spectral response acceleration at short periods, S_{DS}, and at 1second period, S_{D1}, shall be determined from Equation 1649 and 1650, respectively:
where:
where:
S_{MS}  =  The MCE_{R} spectral response accelerations for short period as determined in Section 1613.5.3. 
S_{M1}  =  The MCE_{R} spectral response accelerations for 1second 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:
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:
where:
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}
where:
v_{si}  =  The shear wave velocity in feet per second (m/s). 
d_{i}  =  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 d_{i} and N_{i} for cohesionless soil layers only in Equation 1642. 
d_{s}  =  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). 
S_{ui}  =  The undrained shear strength in psf (kPa), not to exceed 5,000 psf (240 kPa), ASTM D 2166 or D 2850. 
where:
d_{c}  =  The total thickness (100d_{s}) (For SI: 30480ds) 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/m^{2}. 
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). 
 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 according to ASCE 710 and the requirements of Section 1813.
 Check for the existence of a total thickness of soft clay > 10 feet (3048 mm) where a soft clay layer is defined by: < 500 psf (24 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 and computed in all cases as specified.
 for the top 100 feet (30 480 mm) ( method).
 for the top 100 feet (30 480 mm) ( method).
 for cohesionless soil layers (PI < 20) in the top 100 feet (30 480 mm) and average, , for cohesive soil layers (PI > 20) in the top 100 feet (30 480 mm) ( method).
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 sitespecific procedures of ASCE 710. 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 SHORTPERIOD (0.2 SECOND) RESPONSE ACCELERATIONS
TABLE 1613.5.6(2) SEISMIC DESIGN CATEGORY BASED ON 1SECOND PERIOD RESPONSE ACCELERATION
TABLE 1613.5.6(1) SEISMIC DESIGN CATEGORY BASED ON SHORTPERIOD (0.2 SECOND) RESPONSE ACCELERATIONS
VALUE OF S_{DS}  RISK CATEGORY^{a}  
I & 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^{a}  D^{a}  D^{a} 
a. Requirements for Seismic Design Categories E and F have been eliminated from the New York City Building Code as such categories do not apply in New York City. References to these Seismic Design Categories can be found in ASCE 710.
TABLE 1613.5.6(2) SEISMIC DESIGN CATEGORY BASED ON 1SECOND PERIOD RESPONSE ACCELERATION
VALUE OF S_{D1}  RISK CATEGORY^{a}  
I & II  III  IV  
S_{D1 }< 0.067_{g}  A  A  A 
0.067g < S_{D1} < 0.133_{g}  B  B  C 
0.133g < S_{D1} < 0.20_{g}  C  C  D 
0.20g ≤ S_{D1}  D^{a}  D^{a}  D^{a} 
a. Requirements for Seismic Design Categories E and F have been eliminated from the New York City Building Code as such categories do not apply in New York City. References to these Seismic Design Categories can be found in ASCE 710.
The seismic design category is permitted to be determined from Table 1613.5.6(1) alone when all of the following apply:
 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 710, is less than 0.8 T_{s}, as determined in accordance with Section 11.4.5 of ASCE 710.
 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 12.82 of ASCE 710 is used to determine the seismic response coefficient, C_{s}.
 The diaphragms are rigid, as defined in Section 12.3.1 of ASCE 710 or, for diaphragms that are flexible, the distances between the vertical elements of the seismicforceresisting system do not exceed 40 feet (12 192 mm).
Where the alternate simplified design procedure of ASCE 710 is used, the seismic design category shall be determined in accordance with ASCE 710.
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