1613.3.4.1 Site classification for seismic design.
BC § 1613.3.4.1
Site classification for Site Class C, D or E shall be determined from Table 1613.3.4.1. The notations presented below apply to only materials encountered above rock meeting Class 1a, 1b, or 1c as defined in Section 1803 or rock with shear wave velocity greater than 2,500 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. For situations in which site investigations, 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 Fa , Fv , and F PGA shall be taken as unity (1.0). 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: 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). (Equation 16-48) where: N i is the Standard Penetration Resistance (ASTM D 1586) 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 (30 480 mm). Alternatively, if this boring is not performed, site classification should be based on all soil material that is above the Class 1d layer. (Equation 16-49) (Equation 16-50) where: Use only d i and N i for cohesionless soil layers only in Equation 16-50. 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 ASTM D 2850. where: d c = The total thickness (100-ds ) (For SI: 30 480-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 s u < 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 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. Table 1613.3.4.1Site Classification a Site ClassvsN or NchsvE< 600 ft/s< 15< 1,000 psfD600 to 1,200 ft/s15 to 501,000 to 2,000 psfC1,200 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 sv method is used and the Nch and sv criteria differ, select the category with the softer soils (for example, use Site Class E instead of D).
