Proposed Change 1896

The requirements of Article 4.1.8.4. of Division B of the NBC may not apply to ground profiles that differ significantly from the generic ground profiles assumed in the models that underpin the NBC provisions. Therefore, the estimation of seismic hazard in accordance with Article 4.1.8.4. may not be appropriate for sites with such non-conforming ground profiles.

The NBC does not currently alert Code users to this important precondition for the application of the requirements of Article 4.1.8.4. The application of these requirements to sites with non-conforming ground profiles may lead to incorrect and inadequate design, which may increase the risk to life safety and the risk of injury beyond the risk levels that are currently acceptable in the NBC for a design-level earthquake event.

Justification

A wide range of ground profiles are found on construction sites. The models used to generate the estimated seismic hazard values for the NBC are based on generic ground profiles. Therefore, these estimates are not valid for sites with ground profiles that are significantly different from the generic profiles, such as sites with ground profiles having strong impedance contrast and sites above deep sedimentary basins. Use of the NBC provisions to calculate the seismic hazard for such sites would be inappropriate and may lead to incorrect results. Code users must be alerted to the limitations in the scope of Article 4.1.8.4.

Since it is not possible to codify all variations of ground profiles, an explanatory Note is proposed to inform Code users that certain ground profiles are not covered by the NBC. This information will prevent any increase (beyond the currently acceptable level in the NBC) in the risk to life safety and the risk of injury from the application of Code requirements to non-conforming ground profiles. A few examples of non-conforming ground profiles are provided in the Note. Additional guidance is proposed to be included in the Commentary entitled Design for Seismic Effects in the "Structural Commentaries (User's Guide – NBC 2025: Part 4 of Division B)."

PROPOSED CHANGE

[4.1.8.4.] 4.1.8.4.Site Properties

[1] 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] a)except as provided in Sentence (4), be determined in accordance with Subsection 1.1.3., and

[b] b)except as provided in Article 4.1.8.23., correspond to a 2% probability of exceedance in 50 years.

[2] 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] 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] 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)PROPOSED CHANGE A-4.1.8.4.(2) and (3).)

Table [4.1.8.4.-A] 4.1.8.4.-A
Exceptions for Site Designation Using V_s30 Calculated from In Situ Measurements
Forming Part of Sentence [4.1.8.4.] 4.1.8.4.([2] 2)
Ground Profile Characteristics	Site Designation
V_s30 > 760	Ground profile contains more than 3 m of softer materials between rock and the underside of footing or mat foundations	X₇₆₀
V_s30 > 140	Ground profile contains more than 3 m of soil with all the following characteristics: plasticity index, PI > 20, moisture content, w ≥ 40%, and undrained shear strength, s_u < 25 kPa	X_E
V_s30 > 140	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, 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	X_F
V_s30 ≤ 140	n/a	X_F

[3] 3)Where V_s30 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,

\bar{N}

₆₀, or the average undrained shear strength,

{\overset{─}{s}}_{u}

, in accordance with Table 4.1.8.4.-B,

\bar{N}

₆₀ and

{\overset{─}{s}}_{u}

being calculated based on rational analysis. (See Notes A-4.1.8.4.(3)PROPOSED CHANGE A-4.1.8.4.(3) and A-4.1.8.4.(2) and (3)PROPOSED CHANGE A-4.1.8.4.(2) and (3).)

Table [4.1.8.4.-B] 4.1.8.4.-B
Site Classes, S, for Site Designation X_S
Forming Part of Sentence [4.1.8.4.] 4.1.8.4.([3] 3)
Site Class, S	Ground Profile	Ground Profile Characteristics
A	Hard rock^{PROPOSED CHANGE Table 4.1.8.4.B. Footnote (2)}	V_s30 > 1 500	n/a	n/a
B	Rock^{PROPOSED CHANGE Table 4.1.8.4.B. Footnote (2)}	760 < V_s30 ≤ 1 500	n/a	n/a
C	Very dense soil and soft rock	360 < V_s30 ≤ 760	$\bar{N}$ ₆₀ > 50	${\overset{─}{s}}_{u}$ > 100
D	Stiff soil	180 < V_s30 ≤ 360	15 < $\bar{N}$ ₆₀ ≤ 50	50 < ${\overset{─}{s}}_{u}$ ≤ 100
E	Soft soil	140 < V_s30 ≤ 180	10 < $\bar{N}$ ₆₀ ≤ 15	40 < ${\overset{─}{s}}_{u}$ ≤ 50
Any ground profile other than Site Class F that contains more than 3 m of soil with all the following characteristics: plasticity index, PI > 20, moisture content, w ≥ 40%, and undrained shear strength, s_u < 25 kPa
F	Other soils^{PROPOSED CHANGE Table 4.1.8.4.B. Footnote (3)}	V_s30 ≤ 140	$\bar{N}$ ₆₀ ≤ 10	${\overset{─}{s}}_{u}$ ≤ 40
Any ground profile that contains liquefiable soil, quick and highly sensitive clay, collapsible weakly cemented soil, or other 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

[5] 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.

Table [4.1.8.4.-C] 4.1.8.4.-C
Design Spectral Acceleration
Forming Part of Sentence [4.1.8.4.] 4.1.8.4.([6] 6)
Period, T, in s	Design Spectral Acceleration, S(T)
≤ 0.2	S_a(0.2,X) or S_a(0.5,X), whichever is greater
0.5	S_a(0.5,X)
1.0	S_a(1.0,X)
2.0	S_a(2.0,X)
5.0	S_a(5.0,X)
10.0	S_a(10.0,X)

[7] 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).

Note A-4.1.8.4. Site Properties.

The provisions of Article 4.1.8.4. may not be sufficient for determining the seismic hazard values for sites with ground profiles that differ significantly from the ground profiles implicit in the ground motion models (GMMs) used to establish the NBC seismic hazard values. A few examples of such sites are sites with ground profiles having strong impedance contrast, sites above deep sedimentary basins, sites near active faults, sloping bedrock sites, sites with ground profiles that are highly variable across the building footprint, and sites with ground improvements.

Information on the ground profiles used in the GMMs, as well as general guidance for sites with ground profiles that differ from those used in the GMMs, can be found in the Commentary entitled Design for Seismic Effects in the "Structural Commentaries (User's Guide – NBC 2020: Part 4 of Division B)".

Impact analysis

The proposed change is a clarification and does not add any new requirements. The impact would be neutral in terms of cost and positive in terms of facilitating the correct application of the Code.

Enforcement implications

The proposed change is a clarification of the scope of Article 4.1.8.4. It would help enforcement staff to properly understand the requirements of this Article and their application. No difficulties are expected to result from the proposed change.