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, S_a (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, V_s30, 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 V_s30. (See Note A-4.1.8.4.(2) and (3)) Forming Part of Sentence 4.1.8.4.(2)

(3)

Where Vs₃₀ calculated from in situ measurements is not available, the site designation referred to in Sentence (1) shall be X_S, where S is the Site Class determined using the energy-corrected average standard penetration resistance,

, or the average undrained shear strength,

, in accordance with [[b-4.1.8.4.#^table-4184-bhplca|Table 4.1.8.4.-B]],

and

being calculated based on rational analysis. (See Notes [[b-4.1.8.4.(3)-appx|A-4.1.8.4.(3)]] and [[b-4.1.8.4.(2) and (3)-appx|A-4.1.8.4.(2) and (3)]])

(4)

Site-specific geotechnical evaluation is required to determine the values of PGA(X_F), PGV(X_F) and S_a (T,X_F) for site designation X_F.

(5)

Where structures on liquefiable soils have a fundamental lateral period, T_a, of 0.5 s or less, the site designation X and the corresponding values of S_a (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, F_a, for site designation X shall be taken as S(0.2)/S_a (0.2,X₄₅₀) and the velocity-based site coefficient, F_v, for site designation X shall be taken as S(1.0)/S_a (1.0,X₄₅₀). Forming Part of Sentence 4.1.8.4.(3)

(1) See Note A-4.1.8.4.(2) and (3).

(2) Site designations X and X, 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 X.

(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)