4.1.8.4.Site Properties

(1) For site designation X, as determined in accordance with Sentence (2) or (3), the peak ground acceleration, PGA(X), the peak ground velocity, PGV(X), and the 5%-damped spectral acceleration values, Sa(T,X), at periods T of 0.2 s, 0.5 s, 1.0 s, 2.0 s, 5.0 s and 10.0 s shall (a) except as provided in Sentence (4), be determined in accordance with Subsection 1.1.3., and (b) except as provided in Article 4.1.8.23., correspond to a 2% probability of exceedance in 50 years. (2) Except as provided in Sentence (3), the site designation referred to in Sentence (1) shall be determined using the average shear wave velocity, Vs30, calculated from in situ measurements of shear wave velocity, as follows: (a) for the ground profiles described in Table 4.1.8.4.-A, the site designation shall be determined in accordance with the Table, and (b) for all other ground profiles, the site designation shall be X V, where V is the value of Vs30. (See Note A-4.1.8.4.(2) and (3)) Table 4.1.8.4.-A Exceptions for Site Designation Using Vs30 Calculated from In Situ Measurements Forming Part of Sentence 4.1.8.4.(2) Ground Profile Characteristics Average Shear Wave Velocity in Top 30 m, Vs30, Site Designation Additional Characteristics Calculated from In Situ Measurements, in m/s Ground profile contains more than 3 m of softer materials between rock and the underside of footing or mat Vs30 > 760 foundations X760 Ground profile contains more than 3 m of soil with all the following characteristics: • plasticity index, PI > 20, Vs30 > 140 • moisture content, w ≥ 40%, and XE • undrained shear strength, su < 25 kPa Ground profile contains • liquefiable soil, quick and highly sensitive clay, collapsible weakly cemented soil, or other soil susceptible to failure or collapse under seismic loading, Vs30 > 140 • more than 3 m of peat and/or highly organic clay, XF • more than 8 m of highly plastic soil (with PI > 75), or more than 30 m of soft to medium-stiff clay Vs30 ≤ 140 n/a XF (3) Where Vs30 calculated from in situ measurements is not available, the site designation referred to in Sentence (1) shall be XS, where S is the Site Class determined using the energy-corrected average standard penetration resistance, N60 , or the average undrained shear strength, su , in accordance with Table 4.1.8.4.-B, N60 and su being calculated based on rational analysis. (See Notes A-4.1.8.4.(3) and A-4.1.8.4.(2) and (3)) (4) Site-specific geotechnical evaluation is required to determine the values of PGA(X F), PGV(XF) and Sa(T,XF) for site designation XF. (5) Where structures on liquefiable soils have a fundamental lateral period, Ta, of 0.5 s or less, the site designation X and the corresponding values of Sa(T,X) and PGA(X) are permitted to be determined in accordance with Sentence (1) by assuming that the soils are not liquefiable. (6) The design spectral acceleration, S(T), shall be determined in accordance with Table 4.1.8.4.-C, using log–log or linear interpolation for intermediate values of T. (See Note A-4.1.8.4.(6)) (7) Where required for the application of a standard referenced in this Subsection, the acceleration-based site coefficient, Fa, for site designation X shall be taken as S(0.2)/Sa(0.2,X450) and the velocity-based site coefficient, Fv, for site designation X shall be taken as S(1.0)/Sa(1.0,X450). Table 4.1.8.4.-B Site Classes, S, for Site Designation XS Forming Part of Sentence 4.1.8.4.(3) Ground Profile Characteristics Average Standard Site Class, S Ground Profile Average Shear Wave Average Undrained Shear Velocity in Top 30 m, Penetration Resistance in Strength in Top 30 m, Top 30 m, 𝑁60 , in su , in kPa Vs30, in m/s(1) Blows per 0.3 m A Hard rock(2) Vs30 > 1 500 n/a n/a B Rock(2) 760 < Vs30 ≤ 1 500 n/a n/a C Very dense soil and 360 < Vs30 ≤ 760 soft rock 𝑁60 > 50 su > 100 D Stiff soil 180 < Vs30 ≤ 360 15 < 𝑁60 ≤ 50 50 < su ≤ 100 140 < Vs30 ≤ 180 10 < 𝑁60 ≤ 15 40 < su ≤ 50 Any ground profile other than Site Class F that contains more than 3 m of soil with all the E Soft soil following characteristics: • plasticity index, PI > 20, • moisture content, w ≥ 40%, and • undrained shear strength, su < 25 kPa Vs30 ≤ 140 𝑁60 ≤ 10 su ≤ 40 Any ground profile that contains (3) • liquefiable soil, quick and highly sensitive clay, collapsible weakly cemented soil, or other F Other soils soil susceptible to failure or collapse under seismic loading, • more than 3 m of peat and/or highly organic clay, • more than 8 m of highly plastic soil (with PI> 75), or • more than 30 m of soft to medium-stiff clay Notes to Table 4.1.8.4.-B: (1) See Note A-4.1.8.4.(2) and (3). (2) Site designations XA and XB, corresponding to Site Classes A and B, are not to be used in cases where the ground profile contains more than 3 m of softer materials between rock and the underside of footing or mat foundations. The appropriate site designation for such cases is X760. (3) Site-specific geotechnical evaluation is required. Table 4.1.8.4.C. Design Spectral Acceleration Forming Part of Sentence 4.1.8.4.(6) Period, T, in s Design Spectral Acceleration, S(T) ≤ 0.2 Sa(0.2,X) or Sa(0.5,X), whichever is greater 0.5 Sa(0.5,X) 1.0 Sa(1.0,X) 2.0 Sa(2.0,X) 5.0 Sa(5.0,X) 10.0 Sa(10.0,X)