4.1.8.17 — Site Stability

(1) The peak ground acceleration (PGA), peak ground velocity (PGV) and the 5% damped spectral response acceleration values, Sa(T), for the reference ground conditions (Site Class C in Table 4.1.8.4.A.) for periods T of 0.2 s, 0.5 s, 1.0 s, 2.0 s, 5.0 s and 10.0 s, shall be determined in accordance with Subsection 1.1.2. and are based on a 2% probability of exceedance in 50 years. (2) Site classifications for ground shall conform to Table 4.1.8.4.A. and shall be determined using V̄_S30 or, where V̄_S30 is not known, using Sentence (3). (3) If average shear wave velocity, V̄_S30, is not known, Site Class shall be determined from energy-corrected Average Standard Penetration Resistance, N̄_60, or from soil average undrained shear strength, su, as noted in Table 4.1.8.4.A., N̄_60 and su being calculated based on rational analysis. (4) For the purpose of determining the values of F(T) to be used in the calculation of design spectral acceleration, S(T), in Sentence (9), and the values of F(PGA) and F(PGV), the value of PGAref to be used with Tables 4.1.8.4.B. to 4.1.8.4.I. shall be taken as, (a) 0.8 PGA, where the ratio Sa(0.2)/PGA < 2.0, and (b) 1 PGA, in all other cases. (5) The values of the site coefficient for design spectral acceleration at period T, F(T), and of similar coefficients F(PGA) and F(PGV) shall conform to Tables 4.1.8.4.B. to 4.1.8.4.I. using linear interpolation for intermediate values of PGAref. (6) Site-specific evaluation is required to determine F(T), F(PGA) and F(PGV) for Site Class F. (7) For all applications in Subsection 4.1.8., Fa = F(0.2) and Fv = F(1.0). (8) For structures with a fundamental period of vibration equal to or less than 0.5 s that are built on liquefiable soils, Site Class and the corresponding values of F(T) may be determined as described in Tables 4.1.8.4.A., 4.1.8.4.B., and 4.1.8.4.C. by assuming that the soils are not liquefiable. (9) The design spectral acceleration values of S(T) shall be determined as follows, using linear interpolation for intermediate values of T: S(T) =F(0.2)Sa(0.2) or F(0.5)Sa(0.5), whichever is larger, for T ≤ 0.2 s =F(0.5)Sa(0.5) for T = 0.5 s =F(1.0)Sa(1.0) for T = 1.0 s =F(2.0)Sa(2.0) for T = 2.0 s =F(5.0)Sa(5.0) for T = 5.0 s =F(10.0)Sa(10.0) for T ≥ 10.0 s